3. 資料模型

3.1. 物件、數值和型別

物件 是 Python 為資料的抽象表示方式。一個 Python 程式當中的所有資料皆由物件或物件之間的關係來呈現。（某種意義上，同時符合 Von Neumann 對於「儲存程式型電腦」("stored program computer") 的模型，程式碼也是以物件呈現的。）

每個物件都有一個識別性、型別，和數值。物件的識別性在物件建立後永遠不會改變；你也可以把它想成是該物件在記憶體中的位址。is 運算子會比較兩個物件的識別性是否相同；id() 函式則會回傳代表一個該物件的識別性的整數。

CPython 實作細節： 在 CPython 當中，id(x) 就是 x 所儲存在的記憶體位址。

一個物件的型別決定了該物件所支援的操作（例如「它有長度嗎？」），也同時定義該型別的物件能夠擁有的數值。type() 函式會回傳一個物件的型別（而該型別本身也是一個物件）。如同它的識別性，一個物件的型別 (type) 也是不可變的。[1]

某些物件的數值可被改變，這種物件稱作「可變的」(mutable)；建立後數值不能變更的物件則稱作「不可變的」(immutable)。（不可變的容器物件中如果包含對於可變物件的參照，則後者的數值改變的時候前者的數值也會跟著一起改變；這種時候該容器仍會被當成是不可變的，因為它包含的物件集合仍然無法變更。因此可變或不可變嚴格說起並不等同於數值是否能被改變，它的定義有其他不明顯的細節。）一個物件是否為可變取決於它的型別；舉例來說，數字、字串和 tuple 是不可變的，而字典與串列則是可變的。

物件永遠不會被明示的摧毀；但當它們變得不再能夠存取的時候可能會被作為垃圾回收。每個實作都能延後垃圾回收或是乾脆忽略它 --- 垃圾回收如何進行完全取決於各個實作，只要沒有被回收的物件仍是可達的。

CPython 實作細節： CPython 目前使用一種參照計數的方案，並提供可選的循環連結垃圾延遲偵測，這個方案會在大部分物件變得不可存取時馬上回收它們，但不保證能夠回收包含循環參照的垃圾。關於控制循環垃圾回收的資訊請見 gc 模組的說明文件。其他實作的行為不會相同，CPython 也有可能改變，因此請不要仰賴物件在變得不可存取時能夠馬上被最終化（亦即你應該總是明確關閉檔案）。

請注意，使用一個實作的追蹤或除錯工具可能會讓原本能夠回收的物件被維持存活。也請注意，使用 try...except 陳述式來抓捕例外也可能會讓物件維持存活。

某些物件包含對於「外部」資源的參照，像是開啟的檔案或是視窗。基本上這些資源會在物件被回收時釋放，但因為垃圾回收不保證會發生，這種物件也會提供明確釋放外部資源的方式 --- 通常是 close() method。強烈建議各個程式明確關閉這種物件。try...finally 陳述式與 with 陳述式提供進行明確關閉的方便手段。

某些物件包含對於其他物件的參照；這種物件被叫做「容器」。容器的範例有 tuple、串列與字典。這些參照是容器的數值的一部分。通常當我們提到容器的數值的時候，我們指的是其中包含的物件的數值，而不是它們的識別性；但當我們提到容器是否可變的時候，我們指的是直接包含在其中的物件的識別性。因此，如果一個不可變的容器（像一個 tuple）包含對於可變物件的參照，該可變物件被變更時該容器的數值也會跟著變更。

型別幾乎影響物件行為的所有面向。就連物件識別性的重要性某種程度上也受型別影響：對於不可變的型別，計算新數值的操作可能其實會回傳一個某個相同型別且相同數值的現存物件的參照；對於可變型別這則不會發生。舉例來說，在進行 a = 1; b = 1 之後，a 和 b 可能會參照同一個物件，也可能不會，取決於所使用的實作。這是因為 int 是不可變的型別，因此 1 的參照可以重複利用。這個行為取決於所使用的實作，因此不應該依賴它，但在進行物件識別性測試的時候還是需要注意有這件事情。而在進行 c = []; d = [] 之後，c 和 d 則保證會參照兩個不同、獨特、且新建立的空白串列。（請注意，e = f = [] 則會將同一個物件同時指派給 e 和 f。）

3.2. 標準型別階層

Below is a list of the types that are built into Python. Extension modules (written in C, Java, or other languages, depending on the implementation) can define additional types. Future versions of Python may add types to the type hierarchy (e.g., rational numbers, efficiently stored arrays of integers, etc.), although such additions will often be provided via the standard library instead.

Some of the type descriptions below contain a paragraph listing 'special attributes.' These are attributes that provide access to the implementation and are not intended for general use. Their definition may change in the future.

3.2.1. None

這個型別只有一個數值。只有一個物件有這個數值。這個物件由內建名稱 None 存取。它用來在許多情況下代表數值不存在，例如沒有明確回傳任何東西的函式就會回傳這個物件。它的真值是 false。

3.2.2. NotImplemented

這個型別只有一個數值。只有一個物件有這個數值。這個物件由內建名稱 NotImplemented 存取。數字方法和 rich comparison 方法應該在沒有為所提供的運算元實作該操作的時候回傳這個數值。（直譯器接下來則會依運算子嘗試反轉的操作或是其他的後備方案。）它不應該在預期布林值的情境中被計算。

更多細節請見 實作算術操作。

在 3.9 版的變更: 在預期布林值的情境中計算 NotImplemented 的行為已棄用。雖然目前它會計算為 true，它也會同時發出 DeprecationWarning。從某個未來 Python 版本開始這將會引發 TypeError。

3.2.3. Ellipsis

這個型別只有一個數值。只有一個物件有這個數值。這個物件由文本 ... 或內建名稱 Ellipsis 存取。它的真值是 true。

3.2.4. numbers.Number

These are created by numeric literals and returned as results by arithmetic operators and arithmetic built-in functions. Numeric objects are immutable; once created their value never changes. Python numbers are of course strongly related to mathematical numbers, but subject to the limitations of numerical representation in computers.

The string representations of the numeric classes, computed by __repr__() and __str__(), have the following properties:

• They are valid numeric literals which, when passed to their class constructor, produce an object having the value of the original numeric.

• The representation is in base 10, when possible.

• Leading zeros, possibly excepting a single zero before a decimal point, are not shown.

• Trailing zeros, possibly excepting a single zero after a decimal point, are not shown.

• A sign is shown only when the number is negative.

Python distinguishes between integers, floating-point numbers, and complex numbers:

3.2.4.1. numbers.Integral

These represent elements from the mathematical set of integers (positive and negative).

備註

The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers.

There are two types of integers:

Integers (int)

These represent numbers in an unlimited range, subject to available (virtual) memory only. For the purpose of shift and mask operations, a binary representation is assumed, and negative numbers are represented in a variant of 2's complement which gives the illusion of an infinite string of sign bits extending to the left.

Booleans (bool)

These represent the truth values False and True. The two objects representing the values False and True are the only Boolean objects. The Boolean type is a subtype of the integer type, and Boolean values behave like the values 0 and 1, respectively, in almost all contexts, the exception being that when converted to a string, the strings "False" or "True" are returned, respectively.

3.2.4.2. numbers.Real (float)

These represent machine-level double precision floating-point numbers. You are at the mercy of the underlying machine architecture (and C or Java implementation) for the accepted range and handling of overflow. Python does not support single-precision floating-point numbers; the savings in processor and memory usage that are usually the reason for using these are dwarfed by the overhead of using objects in Python, so there is no reason to complicate the language with two kinds of floating-point numbers.

3.2.4.3. numbers.Complex (complex)

These represent complex numbers as a pair of machine-level double precision floating-point numbers. The same caveats apply as for floating-point numbers. The real and imaginary parts of a complex number z can be retrieved through the read-only attributes z.real and z.imag.

3.2.5. Sequences

These represent finite ordered sets indexed by non-negative numbers. The built-in function len() returns the number of items of a sequence. When the length of a sequence is n, the index set contains the numbers 0, 1, ..., n-1. Item i of sequence a is selected by a[i]. Some sequences, including built-in sequences, interpret negative subscripts by adding the sequence length. For example, a[-2] equals a[n-2], the second to last item of sequence a with length n.

Sequences also support slicing: a[i:j] selects all items with index k such that i <= k < j. When used as an expression, a slice is a sequence of the same type. The comment above about negative indexes also applies to negative slice positions.

Some sequences also support "extended slicing" with a third "step" parameter: a[i:j:k] selects all items of a with index x where x = i + n*k, n >= 0 and i <= x < j.

Sequences are distinguished according to their mutability:

3.2.5.1. Immutable sequences

An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be changed; however, the collection of objects directly referenced by an immutable object cannot change.)

The following types are immutable sequences:

字串 (String)

A string is a sequence of values that represent Unicode code points. All the code points in the range U+0000 - U+10FFFF can be represented in a string. Python doesn't have a char type; instead, every code point in the string is represented as a string object with length 1. The built-in function ord() converts a code point from its string form to an integer in the range 0 - 10FFFF; chr() converts an integer in the range 0 - 10FFFF to the corresponding length 1 string object. str.encode() can be used to convert a str to bytes using the given text encoding, and bytes.decode() can be used to achieve the opposite.

Tuple（元組）

The items of a tuple are arbitrary Python objects. Tuples of two or more items are formed by comma-separated lists of expressions. A tuple of one item (a 'singleton') can be formed by affixing a comma to an expression (an expression by itself does not create a tuple, since parentheses must be usable for grouping of expressions). An empty tuple can be formed by an empty pair of parentheses.

位元組

A bytes object is an immutable array. The items are 8-bit bytes, represented by integers in the range 0 <= x < 256. Bytes literals (like b'abc') and the built-in bytes() constructor can be used to create bytes objects. Also, bytes objects can be decoded to strings via the decode() method.

3.2.5.2. 可變序列

Mutable sequences can be changed after they are created. The subscription and slicing notations can be used as the target of assignment and del (delete) statements.

備註

The collections and array module provide additional examples of mutable sequence types.

There are currently two intrinsic mutable sequence types:

List（串列）

The items of a list are arbitrary Python objects. Lists are formed by placing a comma-separated list of expressions in square brackets. (Note that there are no special cases needed to form lists of length 0 or 1.)

位元組陣列

A bytearray object is a mutable array. They are created by the built-in bytearray() constructor. Aside from being mutable (and hence unhashable), byte arrays otherwise provide the same interface and functionality as immutable bytes objects.

3.2.6. Set（集合）型別

These represent unordered, finite sets of unique, immutable objects. As such, they cannot be indexed by any subscript. However, they can be iterated over, and the built-in function len() returns the number of items in a set. Common uses for sets are fast membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference.

For set elements, the same immutability rules apply as for dictionary keys. Note that numeric types obey the normal rules for numeric comparison: if two numbers compare equal (e.g., 1 and 1.0), only one of them can be contained in a set.

There are currently two intrinsic set types:

Set（集合）

These represent a mutable set. They are created by the built-in set() constructor and can be modified afterwards by several methods, such as add().

Frozen set（凍結集合）

These represent an immutable set. They are created by the built-in frozenset() constructor. As a frozenset is immutable and hashable, it can be used again as an element of another set, or as a dictionary key.

3.2.7. 對映

These represent finite sets of objects indexed by arbitrary index sets. The subscript notation a[k] selects the item indexed by k from the mapping a; this can be used in expressions and as the target of assignments or del statements. The built-in function len() returns the number of items in a mapping.

There is currently a single intrinsic mapping type:

3.2.7.1. 字典

These represent finite sets of objects indexed by nearly arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity, the reason being that the efficient implementation of dictionaries requires a key's hash value to remain constant. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (e.g., 1 and 1.0) then they can be used interchangeably to index the same dictionary entry.

Dictionaries preserve insertion order, meaning that keys will be produced in the same order they were added sequentially over the dictionary. Replacing an existing key does not change the order, however removing a key and re-inserting it will add it to the end instead of keeping its old place.

Dictionaries are mutable; they can be created by the {} notation (see section Dictionary displays).

The extension modules dbm.ndbm and dbm.gnu provide additional examples of mapping types, as does the collections module.

在 3.7 版的變更: Dictionaries did not preserve insertion order in versions of Python before 3.6. In CPython 3.6, insertion order was preserved, but it was considered an implementation detail at that time rather than a language guarantee.

3.2.8. 可呼叫型別

These are the types to which the function call operation (see section Calls) can be applied:

3.2.8.1. 自訂函式

A user-defined function object is created by a function definition (see section 函式定義). It should be called with an argument list containing the same number of items as the function's formal parameter list.

3.2.8.1.1. 特殊唯讀屬性

屬性	含義
function.__globals__	A reference to the dictionary that holds the function's global variables -- the global namespace of the module in which the function was defined.
function.__closure__	None or a tuple of cells that contain bindings for the names specified in the co_freevars attribute of the function's code object. A cell object has the attribute cell_contents. This can be used to get the value of the cell, as well as set the value.

3.2.8.1.2. 特殊可寫屬性

Most of these attributes check the type of the assigned value:

屬性	含義
function.__doc__	函式的文件字串，若不可用則為 None。
function.__name__	The function's name. See also: __name__ attributes.
function.__qualname__	The function's qualified name. See also: __qualname__ attributes. 在 3.3 版被加入.
function.__module__	The name of the module the function was defined in, or None if unavailable.
function.__defaults__	A tuple containing default parameter values for those parameters that have defaults, or None if no parameters have a default value.
function.__code__	代表編譯函式主體的程式碼物件。
function.__dict__	The namespace supporting arbitrary function attributes. See also: __dict__ attributes.
function.__annotations__	A dictionary containing annotations of parameters. The keys of the dictionary are the parameter names, and 'return' for the return annotation, if provided. See also: 註釋 (annotation) 最佳實踐.
function.__kwdefaults__	A dictionary containing defaults for keyword-only parameters.
function.__type_params__	A tuple containing the type parameters of a generic function. 在 3.12 版被加入.

Function objects also support getting and setting arbitrary attributes, which can be used, for example, to attach metadata to functions. Regular attribute dot-notation is used to get and set such attributes.

CPython 實作細節： CPython's current implementation only supports function attributes on user-defined functions. Function attributes on built-in functions may be supported in the future.

Additional information about a function's definition can be retrieved from its code object (accessible via the __code__ attribute).

3.2.8.2. 實例方法

An instance method object combines a class, a class instance and any callable object (normally a user-defined function).

特殊唯讀屬性：

method.__self__	Refers to the class instance object to which the method is bound
method.__func__	Refers to the original function object
method.__doc__	The method's documentation (same as method.__func__.__doc__). A string if the original function had a docstring, else None.
method.__name__	The name of the method (same as method.__func__.__name__)
method.__module__	The name of the module the method was defined in, or None if unavailable.

Methods also support accessing (but not setting) the arbitrary function attributes on the underlying function object.

User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined function object or a classmethod object.

When an instance method object is created by retrieving a user-defined function object from a class via one of its instances, its __self__ attribute is the instance, and the method object is said to be bound. The new method's __func__ attribute is the original function object.

When an instance method object is created by retrieving a classmethod object from a class or instance, its __self__ attribute is the class itself, and its __func__ attribute is the function object underlying the class method.

When an instance method object is called, the underlying function (__func__) is called, inserting the class instance (__self__) in front of the argument list. For instance, when C is a class which contains a definition for a function f(), and x is an instance of C, calling x.f(1) is equivalent to calling C.f(x, 1).

When an instance method object is derived from a classmethod object, the "class instance" stored in __self__ will actually be the class itself, so that calling either x.f(1) or C.f(1) is equivalent to calling f(C,1) where f is the underlying function.

It is important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this only happens when the function is an attribute of the class.

3.2.8.3. 產生器函式

A function or method which uses the yield statement (see section yield 陳述式) is called a generator function. Such a function, when called, always returns an iterator object which can be used to execute the body of the function: calling the iterator's iterator.__next__() method will cause the function to execute until it provides a value using the yield statement. When the function executes a return statement or falls off the end, a StopIteration exception is raised and the iterator will have reached the end of the set of values to be returned.

3.2.8.4. Coroutine（協程）函式

A function or method which is defined using async def is called a coroutine function. Such a function, when called, returns a coroutine object. It may contain await expressions, as well as async with and async for statements. See also the 協程物件 section.

3.2.8.5. 非同步產生器函式

A function or method which is defined using async def and which uses the yield statement is called a asynchronous generator function. Such a function, when called, returns an asynchronous iterator object which can be used in an async for statement to execute the body of the function.

Calling the asynchronous iterator's aiterator.__anext__ method will return an awaitable which when awaited will execute until it provides a value using the yield expression. When the function executes an empty return statement or falls off the end, a StopAsyncIteration exception is raised and the asynchronous iterator will have reached the end of the set of values to be yielded.

3.2.8.6. 內建函式

一個內建函式物件是一個 C 函式的 wrapper。內建函式的範例有 len() 和 math.sin()（math 是一個標準的內建模組）。內建函式的引數數量與其型別由其包裝的 C 函式所決定。特殊唯讀屬性：

• __doc__ 是函式的文件字串，若不可用則為 None。請見 function.__doc__。

• __name__ 是函式的名稱。請見 function.__name__。

• __self__ is set to None (but see the next item).

• __module__ is the name of the module the function was defined in or None if unavailable. See function.__module__.

3.2.8.7. 內建方法

This is really a different disguise of a built-in function, this time containing an object passed to the C function as an implicit extra argument. An example of a built-in method is alist.append(), assuming alist is a list object. In this case, the special read-only attribute __self__ is set to the object denoted by alist. (The attribute has the same semantics as it does with other instance methods.)

3.2.8.8. Classes

Classes are callable. These objects normally act as factories for new instances of themselves, but variations are possible for class types that override __new__(). The arguments of the call are passed to __new__() and, in the typical case, to __init__() to initialize the new instance.

3.2.8.9. 類別實例

Instances of arbitrary classes can be made callable by defining a __call__() method in their class.

3.2.9. 模組

Modules are a basic organizational unit of Python code, and are created by the import system as invoked either by the import statement, or by calling functions such as importlib.import_module() and built-in __import__(). A module object has a namespace implemented by a dictionary object (this is the dictionary referenced by the __globals__ attribute of functions defined in the module). Attribute references are translated to lookups in this dictionary, e.g., m.x is equivalent to m.__dict__["x"]. A module object does not contain the code object used to initialize the module (since it isn't needed once the initialization is done).

Attribute assignment updates the module's namespace dictionary, e.g., m.x = 1 is equivalent to m.__dict__["x"] = 1.

3.2.9.1. Import-related attributes on module objects

Module objects have the following attributes that relate to the import system. When a module is created using the machinery associated with the import system, these attributes are filled in based on the module's spec, before the loader executes and loads the module.

To create a module dynamically rather than using the import system, it's recommended to use importlib.util.module_from_spec(), which will set the various import-controlled attributes to appropriate values. It's also possible to use the types.ModuleType constructor to create modules directly, but this technique is more error-prone, as most attributes must be manually set on the module object after it has been created when using this approach.

警示

With the exception of __name__, it is strongly recommended that you rely on __spec__ and its attributes instead of any of the other individual attributes listed in this subsection. Note that updating an attribute on __spec__ will not update the corresponding attribute on the module itself:

>>> import typing
>>> typing.__name__, typing.__spec__.name
('typing', 'typing')
>>> typing.__spec__.name = 'spelling'
>>> typing.__name__, typing.__spec__.name
('typing', 'spelling')
>>> typing.__name__ = 'keyboard_smashing'
>>> typing.__name__, typing.__spec__.name
('keyboard_smashing', 'spelling')

module.__name__

The name used to uniquely identify the module in the import system. For a directly executed module, this will be set to "__main__".

This attribute must be set to the fully qualified name of the module. It is expected to match the value of module.__spec__.name.

module.__spec__

A record of the module's import-system-related state.

Set to the module spec that was used when importing the module. See 模組規格 for more details.

在 3.4 版被加入.

module.__package__

The package a module belongs to.

If the module is top-level (that is, not a part of any specific package) then the attribute should be set to '' (the empty string). Otherwise, it should be set to the name of the module's package (which can be equal to module.__name__ if the module itself is a package). See PEP 366 for further details.

This attribute is used instead of __name__ to calculate explicit relative imports for main modules. It defaults to None for modules created dynamically using the types.ModuleType constructor; use importlib.util.module_from_spec() instead to ensure the attribute is set to a str.

It is strongly recommended that you use module.__spec__.parent instead of module.__package__. __package__ is now only used as a fallback if __spec__.parent is not set, and this fallback path is deprecated.

在 3.4 版的變更: This attribute now defaults to None for modules created dynamically using the types.ModuleType constructor. Previously the attribute was optional.

在 3.6 版的變更: The value of __package__ is expected to be the same as __spec__.parent. __package__ is now only used as a fallback during import resolution if __spec__.parent is not defined.

在 3.10 版的變更: ImportWarning is raised if an import resolution falls back to __package__ instead of __spec__.parent.

在 3.12 版的變更: Raise DeprecationWarning instead of ImportWarning when falling back to __package__ during import resolution.

Deprecated since version 3.13, will be removed in version 3.15: __package__ will cease to be set or taken into consideration by the import system or standard library.

module.__loader__

The loader object that the import machinery used to load the module.

This attribute is mostly useful for introspection, but can be used for additional loader-specific functionality, for example getting data associated with a loader.

__loader__ defaults to None for modules created dynamically using the types.ModuleType constructor; use importlib.util.module_from_spec() instead to ensure the attribute is set to a loader object.

It is strongly recommended that you use module.__spec__.loader instead of module.__loader__.

在 3.4 版的變更: This attribute now defaults to None for modules created dynamically using the types.ModuleType constructor. Previously the attribute was optional.

Deprecated since version 3.12, will be removed in version 3.16: 在模組上設定 __loader__ 而沒有設定 __spec__.loader 的做法已被棄用。在 Python 3.16 中，引入系統或標準函式庫將不再設定或考慮 __loader__。

module.__path__

A (possibly empty) sequence of strings enumerating the locations where the package's submodules will be found. Non-package modules should not have a __path__ attribute. See __path__ attributes on modules for more details.

It is strongly recommended that you use module.__spec__.submodule_search_locations instead of module.__path__.

module.__file__

module.__cached__

__file__ and __cached__ are both optional attributes that may or may not be set. Both attributes should be a str when they are available.

__file__ indicates the pathname of the file from which the module was loaded (if loaded from a file), or the pathname of the shared library file for extension modules loaded dynamically from a shared library. It might be missing for certain types of modules, such as C modules that are statically linked into the interpreter, and the import system may opt to leave it unset if it has no semantic meaning (for example, a module loaded from a database).

If __file__ is set then the __cached__ attribute might also be set, which is the path to any compiled version of the code (for example, a byte-compiled file). The file does not need to exist to set this attribute; the path can simply point to where the compiled file would exist (see PEP 3147).

Note that __cached__ may be set even if __file__ is not set. However, that scenario is quite atypical. Ultimately, the loader is what makes use of the module spec provided by the finder (from which __file__ and __cached__ are derived). So if a loader can load from a cached module but otherwise does not load from a file, that atypical scenario may be appropriate.

It is strongly recommended that you use module.__spec__.cached instead of module.__cached__.

Deprecated since version 3.13, will be removed in version 3.15: Setting __cached__ on a module while failing to set __spec__.cached is deprecated. In Python 3.15, __cached__ will cease to be set or taken into consideration by the import system or standard library.

3.2.9.2. 模組物件的其他可寫入屬性

As well as the import-related attributes listed above, module objects also have the following writable attributes:

module.__doc__

模組的文件字串，若不可用則為 None。請見 __doc__ attributes。

module.__annotations__

A dictionary containing variable annotations collected during module body execution. For best practices on working with __annotations__, please see 註釋 (annotation) 最佳實踐.

3.2.9.3. 模組字典

Module objects also have the following special read-only attribute:

module.__dict__

The module's namespace as a dictionary object. Uniquely among the attributes listed here, __dict__ cannot be accessed as a global variable from within a module; it can only be accessed as an attribute on module objects.

CPython 實作細節： Because of the way CPython clears module dictionaries, the module dictionary will be cleared when the module falls out of scope even if the dictionary still has live references. To avoid this, copy the dictionary or keep the module around while using its dictionary directly.

3.2.10. Custom classes

Custom class types are typically created by class definitions (see section 類別定義). A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., C.x is translated to C.__dict__["x"] (although there are a number of hooks which allow for other means of locating attributes). When the attribute name is not found there, the attribute search continues in the base classes. This search of the base classes uses the C3 method resolution order which behaves correctly even in the presence of 'diamond' inheritance structures where there are multiple inheritance paths leading back to a common ancestor. Additional details on the C3 MRO used by Python can be found at The Python 2.3 Method Resolution Order.

When a class attribute reference (for class C, say) would yield a class method object, it is transformed into an instance method object whose __self__ attribute is C. When it would yield a staticmethod object, it is transformed into the object wrapped by the static method object. See section 實作描述器 for another way in which attributes retrieved from a class may differ from those actually contained in its __dict__.

Class attribute assignments update the class's dictionary, never the dictionary of a base class.

A class object can be called (see above) to yield a class instance (see below).

3.2.10.1. 特殊屬性

屬性	含義
type.__name__	The class's name. See also: __name__ attributes.
type.__qualname__	The class's qualified name. See also: __qualname__ attributes.
type.__module__	The name of the module in which the class was defined.
type.__dict__	A mapping proxy providing a read-only view of the class's namespace. See also: __dict__ attributes.
type.__bases__	A tuple containing the class's bases. In most cases, for a class defined as class X(A, B, C), X.__bases__ will be exactly equal to (A, B, C).
type.__doc__	The class's documentation string, or None if undefined. Not inherited by subclasses.
type.__annotations__	A dictionary containing variable annotations collected during class body execution. For best practices on working with __annotations__, please see 註釋 (annotation) 最佳實踐. 警示 Accessing the __annotations__ attribute of a class object directly may yield incorrect results in the presence of metaclasses. In addition, the attribute may not exist for some classes. Use inspect.get_annotations() to retrieve class annotations safely.
type.__type_params__	A tuple containing the type parameters of a generic class. 在 3.12 版被加入.
type.__static_attributes__	A tuple containing names of attributes of this class which are assigned through self.X from any function in its body. 在 3.13 版被加入.
type.__firstlineno__	The line number of the first line of the class definition, including decorators. Setting the __module__ attribute removes the __firstlineno__ item from the type's dictionary. 在 3.13 版被加入.
type.__mro__	The tuple of classes that are considered when looking for base classes during method resolution.

3.2.10.2. 特殊方法

In addition to the special attributes described above, all Python classes also have the following two methods available:

type.mro()

This method can be overridden by a metaclass to customize the method resolution order for its instances. It is called at class instantiation, and its result is stored in __mro__.

type.__subclasses__()

Each class keeps a list of weak references to its immediate subclasses. This method returns a list of all those references still alive. The list is in definition order. Example:

>>> class A: pass
>>> class B(A): pass
>>> A.__subclasses__()
[<class 'B'>]

3.2.11. 類別實例

A class instance is created by calling a class object (see above). A class instance has a namespace implemented as a dictionary which is the first place in which attribute references are searched. When an attribute is not found there, and the instance's class has an attribute by that name, the search continues with the class attributes. If a class attribute is found that is a user-defined function object, it is transformed into an instance method object whose __self__ attribute is the instance. Static method and class method objects are also transformed; see above under "Classes". See section 實作描述器 for another way in which attributes of a class retrieved via its instances may differ from the objects actually stored in the class's __dict__. If no class attribute is found, and the object's class has a __getattr__() method, that is called to satisfy the lookup.

Attribute assignments and deletions update the instance's dictionary, never a class's dictionary. If the class has a __setattr__() or __delattr__() method, this is called instead of updating the instance dictionary directly.

Class instances can pretend to be numbers, sequences, or mappings if they have methods with certain special names. See section Special method names.

3.2.11.1. 特殊屬性

object.__class__

The class to which a class instance belongs.

object.__dict__

A dictionary or other mapping object used to store an object's (writable) attributes. Not all instances have a __dict__ attribute; see the section on __slots__ for more details.

3.2.12. I/O objects (also known as file objects)

A file object represents an open file. Various shortcuts are available to create file objects: the open() built-in function, and also os.popen(), os.fdopen(), and the makefile() method of socket objects (and perhaps by other functions or methods provided by extension modules).

The objects sys.stdin, sys.stdout and sys.stderr are initialized to file objects corresponding to the interpreter's standard input, output and error streams; they are all open in text mode and therefore follow the interface defined by the io.TextIOBase abstract class.

3.2.13. Internal types

A few types used internally by the interpreter are exposed to the user. Their definitions may change with future versions of the interpreter, but they are mentioned here for completeness.

3.2.13.1. 程式碼物件

Code objects represent byte-compiled executable Python code, or bytecode. The difference between a code object and a function object is that the function object contains an explicit reference to the function's globals (the module in which it was defined), while a code object contains no context; also the default argument values are stored in the function object, not in the code object (because they represent values calculated at run-time). Unlike function objects, code objects are immutable and contain no references (directly or indirectly) to mutable objects.

3.2.13.1.1. 特殊唯讀屬性

codeobject.co_name	函式名稱
codeobject.co_qualname	The fully qualified function name 在 3.11 版被加入.
codeobject.co_argcount	The total number of positional parameters (including positional-only parameters and parameters with default values) that the function has
codeobject.co_posonlyargcount	The number of positional-only parameters (including arguments with default values) that the function has
codeobject.co_kwonlyargcount	The number of keyword-only parameters (including arguments with default values) that the function has
codeobject.co_nlocals	The number of local variables used by the function (including parameters)
codeobject.co_varnames	A tuple containing the names of the local variables in the function (starting with the parameter names)
codeobject.co_cellvars	A tuple containing the names of local variables that are referenced from at least one nested scope inside the function
codeobject.co_freevars	A tuple containing the names of free (closure) variables that a nested scope references in an outer scope. See also function.__closure__. Note: references to global and builtin names are not included.
codeobject.co_code	A string representing the sequence of bytecode instructions in the function
codeobject.co_consts	A tuple containing the literals used by the bytecode in the function
codeobject.co_names	A tuple containing the names used by the bytecode in the function
codeobject.co_filename	The name of the file from which the code was compiled
codeobject.co_firstlineno	The line number of the first line of the function
codeobject.co_lnotab	A string encoding the mapping from bytecode offsets to line numbers. For details, see the source code of the interpreter. 在 3.12 版之後被棄用: This attribute of code objects is deprecated, and may be removed in Python 3.15.
codeobject.co_stacksize	The required stack size of the code object
codeobject.co_flags	An integer encoding a number of flags for the interpreter.

The following flag bits are defined for co_flags: bit 0x04 is set if the function uses the *arguments syntax to accept an arbitrary number of positional arguments; bit 0x08 is set if the function uses the **keywords syntax to accept arbitrary keyword arguments; bit 0x20 is set if the function is a generator. See Code Objects Bit Flags for details on the semantics of each flags that might be present.

Future feature declarations (from __future__ import division) also use bits in co_flags to indicate whether a code object was compiled with a particular feature enabled: bit 0x2000 is set if the function was compiled with future division enabled; bits 0x10 and 0x1000 were used in earlier versions of Python.

Other bits in co_flags are reserved for internal use.

If a code object represents a function, the first item in co_consts is the documentation string of the function, or None if undefined.

3.2.13.1.2. 用於程式碼物件的方法

codeobject.co_positions()

Returns an iterable over the source code positions of each bytecode instruction in the code object.

The iterator returns tuples containing the (start_line, end_line, start_column, end_column). The i-th tuple corresponds to the position of the source code that compiled to the i-th code unit. Column information is 0-indexed utf-8 byte offsets on the given source line.

This positional information can be missing. A non-exhaustive lists of cases where this may happen:

• Running the interpreter with -X no_debug_ranges.

• Loading a pyc file compiled while using -X no_debug_ranges.

• Position tuples corresponding to artificial instructions.

• Line and column numbers that can't be represented due to implementation specific limitations.

When this occurs, some or all of the tuple elements can be None.

在 3.11 版被加入.

備註

This feature requires storing column positions in code objects which may result in a small increase of disk usage of compiled Python files or interpreter memory usage. To avoid storing the extra information and/or deactivate printing the extra traceback information, the -X no_debug_ranges command line flag or the PYTHONNODEBUGRANGES environment variable can be used.

codeobject.co_lines()

Returns an iterator that yields information about successive ranges of bytecodes. Each item yielded is a (start, end, lineno) tuple:

• start (an int) represents the offset (inclusive) of the start of the bytecode range

• end (an int) represents the offset (exclusive) of the end of the bytecode range

• lineno is an int representing the line number of the bytecode range, or None if the bytecodes in the given range have no line number

The items yielded will have the following properties:

• The first range yielded will have a start of 0.

• The (start, end) ranges will be non-decreasing and consecutive. That is, for any pair of tuples, the start of the second will be equal to the end of the first.

• No range will be backwards: end >= start for all triples.

• The last tuple yielded will have end equal to the size of the bytecode.

Zero-width ranges, where start == end, are allowed. Zero-width ranges are used for lines that are present in the source code, but have been eliminated by the bytecode compiler.

在 3.10 版被加入.

也參考

PEP 626 - Precise line numbers for debugging and other tools.

The PEP that introduced the co_lines() method.

codeobject.replace(**kwargs)

Return a copy of the code object with new values for the specified fields.

Code objects are also supported by the generic function copy.replace().

在 3.8 版被加入.

3.2.13.2. Frame objects

Frame objects represent execution frames. They may occur in traceback objects, and are also passed to registered trace functions.

3.2.13.2.1. 特殊唯讀屬性

frame.f_back	Points to the previous stack frame (towards the caller), or None if this is the bottom stack frame
frame.f_code	在這個 frame 中執行的程式碼物件 (code object)。存取這個屬性會引發一個附帶引數 obj 與 "f_code" 的稽核事件 object.__getattr__。
frame.f_locals	The mapping used by the frame to look up local variables. If the frame refers to an optimized scope, this may return a write-through proxy object. 在 3.13 版的變更: Return a proxy for optimized scopes.
frame.f_globals	The dictionary used by the frame to look up global variables
frame.f_builtins	The dictionary used by the frame to look up built-in (intrinsic) names
frame.f_lasti	The "precise instruction" of the frame object (this is an index into the bytecode string of the code object)

3.2.13.2.2. 特殊可寫屬性

frame.f_trace	If not None, this is a function called for various events during code execution (this is used by debuggers). Normally an event is triggered for each new source line (see f_trace_lines).
frame.f_trace_lines	Set this attribute to False to disable triggering a tracing event for each source line.
frame.f_trace_opcodes	Set this attribute to True to allow per-opcode events to be requested. Note that this may lead to undefined interpreter behaviour if exceptions raised by the trace function escape to the function being traced.
frame.f_lineno	The current line number of the frame -- writing to this from within a trace function jumps to the given line (only for the bottom-most frame). A debugger can implement a Jump command (aka Set Next Statement) by writing to this attribute.

3.2.13.2.3. Frame object methods

Frame objects support one method:

frame.clear()

This method clears all references to local variables held by the frame. Also, if the frame belonged to a generator, the generator is finalized. This helps break reference cycles involving frame objects (for example when catching an exception and storing its traceback for later use).

RuntimeError is raised if the frame is currently executing or suspended.

在 3.4 版被加入.

在 3.13 版的變更: Attempting to clear a suspended frame raises RuntimeError (as has always been the case for executing frames).

3.2.13.3. Traceback objects

Traceback objects represent the stack trace of an exception. A traceback object is implicitly created when an exception occurs, and may also be explicitly created by calling types.TracebackType.

在 3.7 版的變更: Traceback objects can now be explicitly instantiated from Python code.

For implicitly created tracebacks, when the search for an exception handler unwinds the execution stack, at each unwound level a traceback object is inserted in front of the current traceback. When an exception handler is entered, the stack trace is made available to the program. (See section The try statement.) It is accessible as the third item of the tuple returned by sys.exc_info(), and as the __traceback__ attribute of the caught exception.

When the program contains no suitable handler, the stack trace is written (nicely formatted) to the standard error stream; if the interpreter is interactive, it is also made available to the user as sys.last_traceback.

For explicitly created tracebacks, it is up to the creator of the traceback to determine how the tb_next attributes should be linked to form a full stack trace.

特殊唯讀屬性：

traceback.tb_frame	Points to the execution frame of the current level. 存取此屬性會引發一個附帶引數 obj 與 "tb_frame" 的稽核事件 object.__getattr__。
traceback.tb_lineno	Gives the line number where the exception occurred
traceback.tb_lasti	Indicates the "precise instruction".

The line number and last instruction in the traceback may differ from the line number of its frame object if the exception occurred in a try statement with no matching except clause or with a finally clause.

traceback.tb_next

The special writable attribute tb_next is the next level in the stack trace (towards the frame where the exception occurred), or None if there is no next level.

在 3.7 版的變更: This attribute is now writable

3.2.13.4. Slice objects

Slice objects are used to represent slices for __getitem__() methods. They are also created by the built-in slice() function.

Special read-only attributes: start is the lower bound; stop is the upper bound; step is the step value; each is None if omitted. These attributes can have any type.

Slice objects support one method:

slice.indices(self, length)

This method takes a single integer argument length and computes information about the slice that the slice object would describe if applied to a sequence of length items. It returns a tuple of three integers; respectively these are the start and stop indices and the step or stride length of the slice. Missing or out-of-bounds indices are handled in a manner consistent with regular slices.

3.2.13.5. Static method objects

Static method objects provide a way of defeating the transformation of function objects to method objects described above. A static method object is a wrapper around any other object, usually a user-defined method object. When a static method object is retrieved from a class or a class instance, the object actually returned is the wrapped object, which is not subject to any further transformation. Static method objects are also callable. Static method objects are created by the built-in staticmethod() constructor.

3.2.13.6. Class method objects

A class method object, like a static method object, is a wrapper around another object that alters the way in which that object is retrieved from classes and class instances. The behaviour of class method objects upon such retrieval is described above, under "instance methods". Class method objects are created by the built-in classmethod() constructor.

3.3. Special method names

A class can implement certain operations that are invoked by special syntax (such as arithmetic operations or subscripting and slicing) by defining methods with special names. This is Python's approach to operator overloading, allowing classes to define their own behavior with respect to language operators. For instance, if a class defines a method named __getitem__(), and x is an instance of this class, then x[i] is roughly equivalent to type(x).__getitem__(x, i). Except where mentioned, attempts to execute an operation raise an exception when no appropriate method is defined (typically AttributeError or TypeError).

Setting a special method to None indicates that the corresponding operation is not available. For example, if a class sets __iter__() to None, the class is not iterable, so calling iter() on its instances will raise a TypeError (without falling back to __getitem__()). [2]

When implementing a class that emulates any built-in type, it is important that the emulation only be implemented to the degree that it makes sense for the object being modelled. For example, some sequences may work well with retrieval of individual elements, but extracting a slice may not make sense. (One example of this is the NodeList interface in the W3C's Document Object Model.)

3.3.1. Basic customization

object.__new__(cls[, ...])

Called to create a new instance of class cls. __new__() is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument. The remaining arguments are those passed to the object constructor expression (the call to the class). The return value of __new__() should be the new object instance (usually an instance of cls).

Typical implementations create a new instance of the class by invoking the superclass's __new__() method using super().__new__(cls[, ...]) with appropriate arguments and then modifying the newly created instance as necessary before returning it.

If __new__() is invoked during object construction and it returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to the object constructor.

If __new__() does not return an instance of cls, then the new instance's __init__() method will not be invoked.

__new__() is intended mainly to allow subclasses of immutable types (like int, str, or tuple) to customize instance creation. It is also commonly overridden in custom metaclasses in order to customize class creation.

object.__init__(self[, ...])

Called after the instance has been created (by __new__()), but before it is returned to the caller. The arguments are those passed to the class constructor expression. If a base class has an __init__() method, the derived class's __init__() method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: super().__init__([args...]).

Because __new__() and __init__() work together in constructing objects (__new__() to create it, and __init__() to customize it), no non-None value may be returned by __init__(); doing so will cause a TypeError to be raised at runtime.

object.__del__(self)

Called when the instance is about to be destroyed. This is also called a finalizer or (improperly) a destructor. If a base class has a __del__() method, the derived class's __del__() method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance.

It is possible (though not recommended!) for the __del__() method to postpone destruction of the instance by creating a new reference to it. This is called object resurrection. It is implementation-dependent whether __del__() is called a second time when a resurrected object is about to be destroyed; the current CPython implementation only calls it once.

It is not guaranteed that __del__() methods are called for objects that still exist when the interpreter exits. weakref.finalize provides a straightforward way to register a cleanup function to be called when an object is garbage collected.

備註

del x doesn't directly call x.__del__() --- the former decrements the reference count for x by one, and the latter is only called when x's reference count reaches zero.

CPython 實作細節： It is possible for a reference cycle to prevent the reference count of an object from going to zero. In this case, the cycle will be later detected and deleted by the cyclic garbage collector. A common cause of reference cycles is when an exception has been caught in a local variable. The frame's locals then reference the exception, which references its own traceback, which references the locals of all frames caught in the traceback.

也參考

Documentation for the gc module.

警告

Due to the precarious circumstances under which __del__() methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to sys.stderr instead. In particular:

• __del__() can be invoked when arbitrary code is being executed, including from any arbitrary thread. If __del__() needs to take a lock or invoke any other blocking resource, it may deadlock as the resource may already be taken by the code that gets interrupted to execute __del__().

• __del__() can be executed during interpreter shutdown. As a consequence, the global variables it needs to access (including other modules) may already have been deleted or set to None. Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist, this may help in assuring that imported modules are still available at the time when the __del__() method is called.

object.__repr__(self)

Called by the repr() built-in function to compute the "official" string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned. The return value must be a string object. If a class defines __repr__() but not __str__(), then __repr__() is also used when an "informal" string representation of instances of that class is required.

This is typically used for debugging, so it is important that the representation is information-rich and unambiguous. A default implementation is provided by the object class itself.

object.__str__(self)

Called by str(object), the default __format__() implementation, and the built-in function print(), to compute the "informal" or nicely printable string representation of an object. The return value must be a str object.

This method differs from object.__repr__() in that there is no expectation that __str__() return a valid Python expression: a more convenient or concise representation can be used.

The default implementation defined by the built-in type object calls object.__repr__().

object.__bytes__(self)

Called by bytes to compute a byte-string representation of an object. This should return a bytes object. The object class itself does not provide this method.

object.__format__(self, format_spec)

Called by the format() built-in function, and by extension, evaluation of formatted string literals and the str.format() method, to produce a "formatted" string representation of an object. The format_spec argument is a string that contains a description of the formatting options desired. The interpretation of the format_spec argument is up to the type implementing __format__(), however most classes will either delegate formatting to one of the built-in types, or use a similar formatting option syntax.

See 格式規格 (Format Specification) 迷你語言 for a description of the standard formatting syntax.

回傳值必須是個字串物件。

The default implementation by the object class should be given an empty format_spec string. It delegates to __str__().

在 3.4 版的變更: The __format__ method of object itself raises a TypeError if passed any non-empty string.

在 3.7 版的變更: object.__format__(x, '') is now equivalent to str(x) rather than format(str(x), '').

object.__lt__(self, other)

object.__le__(self, other)

object.__eq__(self, other)

object.__ne__(self, other)

object.__gt__(self, other)

object.__ge__(self, other)

These are the so-called "rich comparison" methods. The correspondence between operator symbols and method names is as follows: x<y calls x.__lt__(y), x<=y calls x.__le__(y), x==y calls x.__eq__(y), x!=y calls x.__ne__(y), x>y calls x.__gt__(y), and x>=y calls x.__ge__(y).

A rich comparison method may return the singleton NotImplemented if it does not implement the operation for a given pair of arguments. By convention, False and True are returned for a successful comparison. However, these methods can return any value, so if the comparison operator is used in a Boolean context (e.g., in the condition of an if statement), Python will call bool() on the value to determine if the result is true or false.

By default, object implements __eq__() by using is, returning NotImplemented in the case of a false comparison: True if x is y else NotImplemented. For __ne__(), by default it delegates to __eq__() and inverts the result unless it is NotImplemented. There are no other implied relationships among the comparison operators or default implementations; for example, the truth of (x<y or x==y) does not imply x<=y. To automatically generate ordering operations from a single root operation, see functools.total_ordering().

By default, the object class provides implementations consistent with Value comparisons: equality compares according to object identity, and order comparisons raise TypeError. Each default method may generate these results directly, but may also return NotImplemented.

See the paragraph on __hash__() for some important notes on creating hashable objects which support custom comparison operations and are usable as dictionary keys.

There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, __lt__() and __gt__() are each other's reflection, __le__() and __ge__() are each other's reflection, and __eq__() and __ne__() are their own reflection. If the operands are of different types, and the right operand's type is a direct or indirect subclass of the left operand's type, the reflected method of the right operand has priority, otherwise the left operand's method has priority. Virtual subclassing is not considered.

When no appropriate method returns any value other than NotImplemented, the == and != operators will fall back to is and is not, respectively.

object.__hash__(self)

Called by built-in function hash() and for operations on members of hashed collections including set, frozenset, and dict. The __hash__() method should return an integer. The only required property is that objects which compare equal have the same hash value; it is advised to mix together the hash values of the components of the object that also play a part in comparison of objects by packing them into a tuple and hashing the tuple. Example:

def __hash__(self):
    return hash((self.name, self.nick, self.color))

備註

hash() truncates the value returned from an object's custom __hash__() method to the size of a Py_ssize_t. This is typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an object's __hash__() must interoperate on builds of different bit sizes, be sure to check the width on all supported builds. An easy way to do this is with python -c "import sys; print(sys.hash_info.width)".

If a class does not define an __eq__() method it should not define a __hash__() operation either; if it defines __eq__() but not __hash__(), its instances will not be usable as items in hashable collections. If a class defines mutable objects and implements an __eq__() method, it should not implement __hash__(), since the implementation of hashable collections requires that a key's hash value is immutable (if the object's hash value changes, it will be in the wrong hash bucket).

User-defined classes have __eq__() and __hash__() methods by default (inherited from the object class); with them, all objects compare unequal (except with themselves) and x.__hash__() returns an appropriate value such that x == y implies both that x is y and hash(x) == hash(y).

A class that overrides __eq__() and does not define __hash__() will have its __hash__() implicitly set to None. When the __hash__() method of a class is None, instances of the class will raise an appropriate TypeError when a program attempts to retrieve their hash value, and will also be correctly identified as unhashable when checking isinstance(obj, collections.abc.Hashable).

If a class that overrides __eq__() needs to retain the implementation of __hash__() from a parent class, the interpreter must be told this explicitly by setting __hash__ = <ParentClass>.__hash__.

If a class that does not override __eq__() wishes to suppress hash support, it should include __hash__ = None in the class definition. A class which defines its own __hash__() that explicitly raises a TypeError would be incorrectly identified as hashable by an isinstance(obj, collections.abc.Hashable) call.

備註

By default, the __hash__() values of str and bytes objects are "salted" with an unpredictable random value. Although they remain constant within an individual Python process, they are not predictable between repeated invocations of Python.

This is intended to provide protection against a denial-of-service caused by carefully chosen inputs that exploit the worst case performance of a dict insertion, O(n²) complexity. See http://ocert.org/advisories/ocert-2011-003.html for details.

Changing hash values affects the iteration order of sets. Python has never made guarantees about this ordering (and it typically varies between 32-bit and 64-bit builds).

另請參閱 PYTHONHASHSEED。

在 3.3 版的變更: Hash randomization is enabled by default.

object.__bool__(self)

Called to implement truth value testing and the built-in operation bool(); should return False or True. When this method is not defined, __len__() is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither __len__() nor __bool__() (which is true of the object class itself), all its instances are considered true.

3.3.2. Customizing attribute access

The following methods can be defined to customize the meaning of attribute access (use of, assignment to, or deletion of x.name) for class instances.

object.__getattr__(self, name)

Called when the default attribute access fails with an AttributeError (either __getattribute__() raises an AttributeError because name is not an instance attribute or an attribute in the class tree for self; or __get__() of a name property raises AttributeError). This method should either return the (computed) attribute value or raise an AttributeError exception. The object class itself does not provide this method.

Note that if the attribute is found through the normal mechanism, __getattr__() is not called. (This is an intentional asymmetry between __getattr__() and __setattr__().) This is done both for efficiency reasons and because otherwise __getattr__() would have no way to access other attributes of the instance. Note that at least for instance variables, you can take total control by not inserting any values in the instance attribute dictionary (but instead inserting them in another object). See the __getattribute__() method below for a way to actually get total control over attribute access.

object.__getattribute__(self, name)

Called unconditionally to implement attribute accesses for instances of the class. If the class also defines __getattr__(), the latter will not be called unless __getattribute__() either calls it explicitly or raises an AttributeError. This method should return the (computed) attribute value or raise an AttributeError exception. In order to avoid infinite recursion in this method, its implementation should always call the base class method with the same name to access any attributes it needs, for example, object.__getattribute__(self, name).

備註

This method may still be bypassed when looking up special methods as the result of implicit invocation via language syntax or built-in functions. See Special method lookup.

For certain sensitive attribute accesses, raises an auditing event object.__getattr__ with arguments obj and name.

object.__setattr__(self, name, value)

Called when an attribute assignment is attempted. This is called instead of the normal mechanism (i.e. store the value in the instance dictionary). name is the attribute name, value is the value to be assigned to it.

If __setattr__() wants to assign to an instance attribute, it should call the base class method with the same name, for example, object.__setattr__(self, name, value).

For certain sensitive attribute assignments, raises an auditing event object.__setattr__ with arguments obj, name, value.

object.__delattr__(self, name)

Like __setattr__() but for attribute deletion instead of assignment. This should only be implemented if del obj.name is meaningful for the object.

For certain sensitive attribute deletions, raises an auditing event object.__delattr__ with arguments obj and name.

object.__dir__(self)

Called when dir() is called on the object. An iterable must be returned. dir() converts the returned iterable to a list and sorts it.

3.3.2.1. Customizing module attribute access

Special names __getattr__ and __dir__ can be also used to customize access to module attributes. The __getattr__ function at the module level should accept one argument which is the name of an attribute and return the computed value or raise an AttributeError. If an attribute is not found on a module object through the normal lookup, i.e. object.__getattribute__(), then __getattr__ is searched in the module __dict__ before raising an AttributeError. If found, it is called with the attribute name and the result is returned.

The __dir__ function should accept no arguments, and return an iterable of strings that represents the names accessible on module. If present, this function overrides the standard dir() search on a module.

For a more fine grained customization of the module behavior (setting attributes, properties, etc.), one can set the __class__ attribute of a module object to a subclass of types.ModuleType. For example:

import sys
from types import ModuleType

class VerboseModule(ModuleType):
    def __repr__(self):
        return f'Verbose {self.__name__}'

    def __setattr__(self, attr, value):
        print(f'Setting {attr}...')
        super().__setattr__(attr, value)

sys.modules[__name__].__class__ = VerboseModule

備註

Defining module __getattr__ and setting module __class__ only affect lookups made using the attribute access syntax -- directly accessing the module globals (whether by code within the module, or via a reference to the module's globals dictionary) is unaffected.

在 3.5 版的變更: __class__ 模組屬性現在是可寫入的。

在 3.7 版被加入: __getattr__ 和 __dir__ 模組屬性。

也參考

PEP 562 - 模組 __getattr__ 和 __dir__

Describes the __getattr__ and __dir__ functions on modules.

3.3.2.2. 實作描述器

The following methods only apply when an instance of the class containing the method (a so-called descriptor class) appears in an owner class (the descriptor must be in either the owner's class dictionary or in the class dictionary for one of its parents). In the examples below, "the attribute" refers to the attribute whose name is the key of the property in the owner class' __dict__. The object class itself does not implement any of these protocols.

object.__get__(self, instance, owner=None)

Called to get the attribute of the owner class (class attribute access) or of an instance of that class (instance attribute access). The optional owner argument is the owner class, while instance is the instance that the attribute was accessed through, or None when the attribute is accessed through the owner.

This method should return the computed attribute value or raise an AttributeError exception.

PEP 252 specifies that __get__() is callable with one or two arguments. Python's own built-in descriptors support this specification; however, it is likely that some third-party tools have descriptors that require both arguments. Python's own __getattribute__() implementation always passes in both arguments whether they are required or not.

object.__set__(self, instance, value)

Called to set the attribute on an instance instance of the owner class to a new value, value.

Note, adding __set__() or __delete__() changes the kind of descriptor to a "data descriptor". See Invoking Descriptors for more details.

object.__delete__(self, instance)

Called to delete the attribute on an instance instance of the owner class.

Instances of descriptors may also have the __objclass__ attribute present:

object.__objclass__

The attribute __objclass__ is interpreted by the inspect module as specifying the class where this object was defined (setting this appropriately can assist in runtime introspection of dynamic class attributes). For callables, it may indicate that an instance of the given type (or a subclass) is expected or required as the first positional argument (for example, CPython sets this attribute for unbound methods that are implemented in C).

3.3.2.3. Invoking Descriptors

In general, a descriptor is an object attribute with "binding behavior", one whose attribute access has been overridden by methods in the descriptor protocol: __get__(), __set__(), and __delete__(). If any of those methods are defined for an object, it is said to be a descriptor.

The default behavior for attribute access is to get, set, or delete the attribute from an object's dictionary. For instance, a.x has a lookup chain starting with a.__dict__['x'], then type(a).__dict__['x'], and continuing through the base classes of type(a) excluding metaclasses.

However, if the looked-up value is an object defining one of the descriptor methods, then Python may override the default behavior and invoke the descriptor method instead. Where this occurs in the precedence chain depends on which descriptor methods were defined and how they were called.

The starting point for descriptor invocation is a binding, a.x. How the arguments are assembled depends on a:

直接呼叫

The simplest and least common call is when user code directly invokes a descriptor method: x.__get__(a).

Instance Binding

If binding to an object instance, a.x is transformed into the call: type(a).__dict__['x'].__get__(a, type(a)).

Class Binding

If binding to a class, A.x is transformed into the call: A.__dict__['x'].__get__(None, A).

Super Binding

A dotted lookup such as super(A, a).x searches a.__class__.__mro__ for a base class B following A and then returns B.__dict__['x'].__get__(a, A). If not a descriptor, x is returned unchanged.

For instance bindings, the precedence of descriptor invocation depends on which descriptor methods are defined. A descriptor can define any combination of __get__(), __set__() and __delete__(). If it does not define __get__(), then accessing the attribute will return the descriptor object itself unless there is a value in the object's instance dictionary. If the descriptor defines __set__() and/or __delete__(), it is a data descriptor; if it defines neither, it is a non-data descriptor. Normally, data descriptors define both __get__() and __set__(), while non-data descriptors have just the __get__() method. Data descriptors with __get__() and __set__() (and/or __delete__()) defined always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances.

Python methods (including those decorated with @staticmethod and @classmethod) are implemented as non-data descriptors. Accordingly, instances can redefine and override methods. This allows individual instances to acquire behaviors that differ from other instances of the same class.

The property() function is implemented as a data descriptor. Accordingly, instances cannot override the behavior of a property.

3.3.2.4. __slots__

__slots__ allow us to explicitly declare data members (like properties) and deny the creation of __dict__ and __weakref__ (unless explicitly declared in __slots__ or available in a parent.)

The space saved over using __dict__ can be significant. Attribute lookup speed can be significantly improved as well.

object.__slots__

This class variable can be assigned a string, iterable, or sequence of strings with variable names used by instances. __slots__ reserves space for the declared variables and prevents the automatic creation of __dict__ and __weakref__ for each instance.

Notes on using __slots__:

• When inheriting from a class without __slots__, the __dict__ and __weakref__ attribute of the instances will always be accessible.

• Without a __dict__ variable, instances cannot be assigned new variables not listed in the __slots__ definition. Attempts to assign to an unlisted variable name raises AttributeError. If dynamic assignment of new variables is desired, then add '__dict__' to the sequence of strings in the __slots__ declaration.

• Without a __weakref__ variable for each instance, classes defining __slots__ do not support weak references to its instances. If weak reference support is needed, then add '__weakref__' to the sequence of strings in the __slots__ declaration.

• __slots__ are implemented at the class level by creating descriptors for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by __slots__; otherwise, the class attribute would overwrite the descriptor assignment.

• The action of a __slots__ declaration is not limited to the class where it is defined. __slots__ declared in parents are available in child classes. However, instances of a child subclass will get a __dict__ and __weakref__ unless the subclass also defines __slots__ (which should only contain names of any additional slots).

• If a class defines a slot also defined in a base class, the instance variable defined by the base class slot is inaccessible (except by retrieving its descriptor directly from the base class). This renders the meaning of the program undefined. In the future, a check may be added to prevent this.

• TypeError will be raised if nonempty __slots__ are defined for a class derived from a "variable-length" built-in type such as int, bytes, and tuple.

• Any non-string iterable may be assigned to __slots__.

• If a dictionary is used to assign __slots__, the dictionary keys will be used as the slot names. The values of the dictionary can be used to provide per-attribute docstrings that will be recognised by inspect.getdoc() and displayed in the output of help().

• __class__ assignment works only if both classes have the same __slots__.

• Multiple inheritance with multiple slotted parent classes can be used, but only one parent is allowed to have attributes created by slots (the other bases must have empty slot layouts) - violations raise TypeError.

• If an iterator is used for __slots__ then a descriptor is created for each of the iterator's values. However, the __slots__ attribute will be an empty iterator.

3.3.3. Customizing class creation

Whenever a class inherits from another class, __init_subclass__() is called on the parent class. This way, it is possible to write classes which change the behavior of subclasses. This is closely related to class decorators, but where class decorators only affect the specific class they're applied to, __init_subclass__ solely applies to future subclasses of the class defining the method.

classmethod object.__init_subclass__(cls)

This method is called whenever the containing class is subclassed. cls is then the new subclass. If defined as a normal instance method, this method is implicitly converted to a class method.

Keyword arguments which are given to a new class are passed to the parent class's __init_subclass__. For compatibility with other classes using __init_subclass__, one should take out the needed keyword arguments and pass the others over to the base class, as in:

class Philosopher:
    def __init_subclass__(cls, /, default_name, **kwargs):
        super().__init_subclass__(**kwargs)
        cls.default_name = default_name

class AustralianPhilosopher(Philosopher, default_name="Bruce"):
    pass

The default implementation object.__init_subclass__ does nothing, but raises an error if it is called with any arguments.

備註

The metaclass hint metaclass is consumed by the rest of the type machinery, and is never passed to __init_subclass__ implementations. The actual metaclass (rather than the explicit hint) can be accessed as type(cls).

在 3.6 版被加入.

When a class is created, type.__new__() scans the class variables and makes callbacks to those with a __set_name__() hook.

object.__set_name__(self, owner, name)

Automatically called at the time the owning class owner is created. The object has been assigned to name in that class:

class A:
    x = C()  # 自動呼叫：x.__set_name__(A, 'x')

If the class variable is assigned after the class is created, __set_name__() will not be called automatically. If needed, __set_name__() can be called directly:

class A:
   pass

c = C()
A.x = c                  # The hook is not called
c.__set_name__(A, 'x')   # Manually invoke the hook

更多細節請見 Creating the class object。

在 3.6 版被加入.

3.3.3.1. Metaclasses

By default, classes are constructed using type(). The class body is executed in a new namespace and the class name is bound locally to the result of type(name, bases, namespace).

The class creation process can be customized by passing the metaclass keyword argument in the class definition line, or by inheriting from an existing class that included such an argument. In the following example, both MyClass and MySubclass are instances of Meta:

class Meta(type):
    pass

class MyClass(metaclass=Meta):
    pass

class MySubclass(MyClass):
    pass

Any other keyword arguments that are specified in the class definition are passed through to all metaclass operations described below.

When a class definition is executed, the following steps occur:

• MRO entries are resolved;

• the appropriate metaclass is determined;

• the class namespace is prepared;

• the class body is executed;

• the class object is created.

3.3.3.2. Resolving MRO entries

object.__mro_entries__(self, bases)

If a base that appears in a class definition is not an instance of type, then an __mro_entries__() method is searched on the base. If an __mro_entries__() method is found, the base is substituted with the result of a call to __mro_entries__() when creating the class. The method is called with the original bases tuple passed to the bases parameter, and must return a tuple of classes that will be used instead of the base. The returned tuple may be empty: in these cases, the original base is ignored.

也參考

types.resolve_bases()

Dynamically resolve bases that are not instances of type.

types.get_original_bases()

Retrieve a class's "original bases" prior to modifications by __mro_entries__().

PEP 560

Core support for typing module and generic types.

3.3.3.3. Determining the appropriate metaclass

The appropriate metaclass for a class definition is determined as follows:

• if no bases and no explicit metaclass are given, then type() is used;

• if an explicit metaclass is given and it is not an instance of type(), then it is used directly as the metaclass;

• if an instance of type() is given as the explicit metaclass, or bases are defined, then the most derived metaclass is used.

The most derived metaclass is selected from the explicitly specified metaclass (if any) and the metaclasses (i.e. type(cls)) of all specified base classes. The most derived metaclass is one which is a subtype of all of these candidate metaclasses. If none of the candidate metaclasses meets that criterion, then the class definition will fail with TypeError.

3.3.3.4. Preparing the class namespace

Once the appropriate metaclass has been identified, then the class namespace is prepared. If the metaclass has a __prepare__ attribute, it is called as namespace = metaclass.__prepare__(name, bases, **kwds) (where the additional keyword arguments, if any, come from the class definition). The __prepare__ method should be implemented as a classmethod. The namespace returned by __prepare__ is passed in to __new__, but when the final class object is created the namespace is copied into a new dict.

If the metaclass has no __prepare__ attribute, then the class namespace is initialised as an empty ordered mapping.

也參考

PEP 3115 - Metaclasses in Python 3000

Introduced the __prepare__ namespace hook

3.3.3.5. Executing the class body

The class body is executed (approximately) as exec(body, globals(), namespace). The key difference from a normal call to exec() is that lexical scoping allows the class body (including any methods) to reference names from the current and outer scopes when the class definition occurs inside a function.

However, even when the class definition occurs inside the function, methods defined inside the class still cannot see names defined at the class scope. Class variables must be accessed through the first parameter of instance or class methods, or through the implicit lexically scoped __class__ reference described in the next section.

3.3.3.6. Creating the class object

Once the class namespace has been populated by executing the class body, the class object is created by calling metaclass(name, bases, namespace, **kwds) (the additional keywords passed here are the same as those passed to __prepare__).

This class object is the one that will be referenced by the zero-argument form of super(). __class__ is an implicit closure reference created by the compiler if any methods in a class body refer to either __class__ or super. This allows the zero argument form of super() to correctly identify the class being defined based on lexical scoping, while the class or instance that was used to make the current call is identified based on the first argument passed to the method.

CPython 實作細節： In CPython 3.6 and later, the __class__ cell is passed to the metaclass as a __classcell__ entry in the class namespace. If present, this must be propagated up to the type.__new__ call in order for the class to be initialised correctly. Failing to do so will result in a RuntimeError in Python 3.8.

When using the default metaclass type, or any metaclass that ultimately calls type.__new__, the following additional customization steps are invoked after creating the class object:

1. The type.__new__ method collects all of the attributes in the class namespace that define a __set_name__() method;

2. Those __set_name__ methods are called with the class being defined and the assigned name of that particular attribute;

3. The __init_subclass__() hook is called on the immediate parent of the new class in its method resolution order.

After the class object is created, it is passed to the class decorators included in the class definition (if any) and the resulting object is bound in the local namespace as the defined class.

When a new class is created by type.__new__, the object provided as the namespace parameter is copied to a new ordered mapping and the original object is discarded. The new copy is wrapped in a read-only proxy, which becomes the __dict__ attribute of the class object.

也參考

PEP 3135 - New super

Describes the implicit __class__ closure reference

3.3.3.7. Uses for metaclasses

The potential uses for metaclasses are boundless. Some ideas that have been explored include enum, logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization.

3.3.4. Customizing instance and subclass checks

The following methods are used to override the default behavior of the isinstance() and issubclass() built-in functions.

In particular, the metaclass abc.ABCMeta implements these methods in order to allow the addition of Abstract Base Classes (ABCs) as "virtual base classes" to any class or type (including built-in types), including other ABCs.

type.__instancecheck__(self, instance)

Return true if instance should be considered a (direct or indirect) instance of class. If defined, called to implement isinstance(instance, class).

type.__subclasscheck__(self, subclass)

Return true if subclass should be considered a (direct or indirect) subclass of class. If defined, called to implement issubclass(subclass, class).

Note that these methods are looked up on the type (metaclass) of a class. They cannot be defined as class methods in the actual class. This is consistent with the lookup of special methods that are called on instances, only in this case the instance is itself a class.

也參考

PEP 3119 - Introducing Abstract Base Classes

Includes the specification for customizing isinstance() and issubclass() behavior through __instancecheck__() and __subclasscheck__(), with motivation for this functionality in the context of adding Abstract Base Classes (see the abc module) to the language.

3.3.5. Emulating generic types

When using type annotations, it is often useful to parameterize a generic type using Python's square-brackets notation. For example, the annotation list[int] might be used to signify a list in which all the elements are of type int.

也參考

PEP 484 - Type Hints

Introducing Python's framework for type annotations

泛型別名型別

Documentation for objects representing parameterized generic classes

泛型, user-defined generics and typing.Generic

Documentation on how to implement generic classes that can be parameterized at runtime and understood by static type-checkers.

A class can generally only be parameterized if it defines the special class method __class_getitem__().

classmethod object.__class_getitem__(cls, key)

Return an object representing the specialization of a generic class by type arguments found in key.

When defined on a class, __class_getitem__() is automatically a class method. As such, there is no need for it to be decorated with @classmethod when it is defined.

3.3.5.1. The purpose of __class_getitem__

The purpose of __class_getitem__() is to allow runtime parameterization of standard-library generic classes in order to more easily apply type hints to these classes.

To implement custom generic classes that can be parameterized at runtime and understood by static type-checkers, users should either inherit from a standard library class that already implements __class_getitem__(), or inherit from typing.Generic, which has its own implementation of __class_getitem__().

Custom implementations of __class_getitem__() on classes defined outside of the standard library may not be understood by third-party type-checkers such as mypy. Using __class_getitem__() on any class for purposes other than type hinting is discouraged.

3.3.5.2. __class_getitem__ versus __getitem__

Usually, the subscription of an object using square brackets will call the __getitem__() instance method defined on the object's class. However, if the object being subscribed is itself a class, the class method __class_getitem__() may be called instead. __class_getitem__() should return a GenericAlias object if it is properly defined.

Presented with the expression obj[x], the Python interpreter follows something like the following process to decide whether __getitem__() or __class_getitem__() should be called:

from inspect import isclass

def subscribe(obj, x):
    """Return the result of the expression 'obj[x]'"""

    class_of_obj = type(obj)

    # If the class of obj defines __getitem__,
    # call class_of_obj.__getitem__(obj, x)
    if hasattr(class_of_obj, '__getitem__'):
        return class_of_obj.__getitem__(obj, x)

    # Else, if obj is a class and defines __class_getitem__,
    # call obj.__class_getitem__(x)
    elif isclass(obj) and hasattr(obj, '__class_getitem__'):
        return obj.__class_getitem__(x)

    # Else, raise an exception
    else:
        raise TypeError(
            f"'{class_of_obj.__name__}' object is not subscriptable"
        )

In Python, all classes are themselves instances of other classes. The class of a class is known as that class's metaclass, and most classes have the type class as their metaclass. type does not define __getitem__(), meaning that expressions such as list[int], dict[str, float] and tuple[str, bytes] all result in __class_getitem__() being called:

>>> # list has class "type" as its metaclass, like most classes:
>>> type(list)
<class 'type'>
>>> type(dict) == type(list) == type(tuple) == type(str) == type(bytes)
True
>>> # "list[int]" calls "list.__class_getitem__(int)"
>>> list[int]
list[int]
>>> # list.__class_getitem__ returns a GenericAlias object:
>>> type(list[int])
<class 'types.GenericAlias'>

However, if a class has a custom metaclass that defines __getitem__(), subscribing the class may result in different behaviour. An example of this can be found in the enum module:

>>> from enum import Enum
>>> class Menu(Enum):
...     """A breakfast menu"""
...     SPAM = 'spam'
...     BACON = 'bacon'
...
>>> # Enum classes have a custom metaclass:
>>> type(Menu)
<class 'enum.EnumMeta'>
>>> # EnumMeta defines __getitem__,
>>> # so __class_getitem__ is not called,
>>> # and the result is not a GenericAlias object:
>>> Menu['SPAM']
<Menu.SPAM: 'spam'>
>>> type(Menu['SPAM'])
<enum 'Menu'>

也參考

PEP 560 - Core Support for typing module and generic types

Introducing __class_getitem__(), and outlining when a subscription results in __class_getitem__() being called instead of __getitem__()

3.3.6. Emulating callable objects

object.__call__(self[, args...])

Called when the instance is "called" as a function; if this method is defined, x(arg1, arg2, ...) roughly translates to type(x).__call__(x, arg1, ...). The object class itself does not provide this method.

3.3.7. Emulating container types

The following methods can be defined to implement container objects. None of them are provided by the object class itself. Containers usually are sequences (such as lists or tuples) or mappings (like dictionaries), but can represent other containers as well. The first set of methods is used either to emulate a sequence or to emulate a mapping; the difference is that for a sequence, the allowable keys should be the integers k for which 0 <= k < N where N is the length of the sequence, or slice objects, which define a range of items. It is also recommended that mappings provide the methods keys(), values(), items(), get(), clear(), setdefault(), pop(), popitem(), copy(), and update() behaving similar to those for Python's standard dictionary objects. The collections.abc module provides a MutableMapping abstract base class to help create those methods from a base set of __getitem__(), __setitem__(), __delitem__(), and keys(). Mutable sequences should provide methods append(), count(), index(), extend(), insert(), pop(), remove(), reverse() and sort(), like Python standard list objects. Finally, sequence types should implement addition (meaning concatenation) and multiplication (meaning repetition) by defining the methods __add__(), __radd__(), __iadd__(), __mul__(), __rmul__() and __imul__() described below; they should not define other numerical operators. It is recommended that both mappings and sequences implement the __contains__() method to allow efficient use of the in operator; for mappings, in should search the mapping's keys; for sequences, it should search through the values. It is further recommended that both mappings and sequences implement the __iter__() method to allow efficient iteration through the container; for mappings, __iter__() should iterate through the object's keys; for sequences, it should iterate through the values.

object.__len__(self)

Called to implement the built-in function len(). Should return the length of the object, an integer >= 0. Also, an object that doesn't define a __bool__() method and whose __len__() method returns zero is considered to be false in a Boolean context.

CPython 實作細節： In CPython, the length is required to be at most sys.maxsize. If the length is larger than sys.maxsize some features (such as len()) may raise OverflowError. To prevent raising OverflowError by truth value testing, an object must define a __bool__() method.

object.__length_hint__(self)

Called to implement operator.length_hint(). Should return an estimated length for the object (which may be greater or less than the actual length). The length must be an integer >= 0. The return value may also be NotImplemented, which is treated the same as if the __length_hint__ method didn't exist at all. This method is purely an optimization and is never required for correctness.

在 3.4 版被加入.

備註

Slicing is done exclusively with the following three methods. A call like

a[1:2] = b

is translated to

a[slice(1, 2, None)] = b

and so forth. Missing slice items are always filled in with None.

object.__getitem__(self, key)

Called to implement evaluation of self[key]. For sequence types, the accepted keys should be integers. Optionally, they may support slice objects as well. Negative index support is also optional. If key is of an inappropriate type, TypeError may be raised; if key is a value outside the set of indexes for the sequence (after any special interpretation of negative values), IndexError should be raised. For mapping types, if key is missing (not in the container), KeyError should be raised.

備註

for loops expect that an IndexError will be raised for illegal indexes to allow proper detection of the end of the sequence.

備註

When subscripting a class, the special class method __class_getitem__() may be called instead of __getitem__(). See __class_getitem__ versus __getitem__ for more details.

object.__setitem__(self, key, value)

Called to implement assignment to self[key]. Same note as for __getitem__(). This should only be implemented for mappings if the objects support changes to the values for keys, or if new keys can be added, or for sequences if elements can be replaced. The same exceptions should be raised for improper key values as for the __getitem__() method.

object.__delitem__(self, key)

Called to implement deletion of self[key]. Same note as for __getitem__(). This should only be implemented for mappings if the objects support removal of keys, or for sequences if elements can be removed from the sequence. The same exceptions should be raised for improper key values as for the __getitem__() method.

object.__missing__(self, key)

Called by dict.__getitem__() to implement self[key] for dict subclasses when key is not in the dictionary.

object.__iter__(self)

This method is called when an iterator is required for a container. This method should return a new iterator object that can iterate over all the objects in the container. For mappings, it should iterate over the keys of the container.

object.__reversed__(self)

Called (if present) by the reversed() built-in to implement reverse iteration. It should return a new iterator object that iterates over all the objects in the container in reverse order.

If the __reversed__() method is not provided, the reversed() built-in will fall back to using the sequence protocol (__len__() and __getitem__()). Objects that support the sequence protocol should only provide __reversed__() if they can provide an implementation that is more efficient than the one provided by reversed().

The membership test operators (in and not in) are normally implemented as an iteration through a container. However, container objects can supply the following special method with a more efficient implementation, which also does not require the object be iterable.

object.__contains__(self, item)

Called to implement membership test operators. Should return true if item is in self, false otherwise. For mapping objects, this should consider the keys of the mapping rather than the values or the key-item pairs.

For objects that don't define __contains__(), the membership test first tries iteration via __iter__(), then the old sequence iteration protocol via __getitem__(), see this section in the language reference.

3.3.8. Emulating numeric types

The following methods can be defined to emulate numeric objects. Methods corresponding to operations that are not supported by the particular kind of number implemented (e.g., bitwise operations for non-integral numbers) should be left undefined.

object.__add__(self, other)

object.__sub__(self, other)

object.__mul__(self, other)

object.__matmul__(self, other)

object.__truediv__(self, other)

object.__floordiv__(self, other)

object.__mod__(self, other)

object.__divmod__(self, other)

object.__pow__(self, other[, modulo])

object.__lshift__(self, other)

object.__rshift__(self, other)

object.__and__(self, other)

object.__xor__(self, other)

object.__or__(self, other)

These methods are called to implement the binary arithmetic operations (+, -, *, @, /, //, %, divmod(), pow(), **, <<, >>, &, ^, |). For instance, to evaluate the expression x + y, where x is an instance of a class that has an __add__() method, type(x).__add__(x, y) is called. The __divmod__() method should be the equivalent to using __floordiv__() and __mod__(); it should not be related to __truediv__(). Note that __pow__() should be defined to accept an optional third argument if the ternary version of the built-in pow() function is to be supported.

If one of those methods does not support the operation with the supplied arguments, it should return NotImplemented.

object.__radd__(self, other)

object.__rsub__(self, other)

object.__rmul__(self, other)

object.__rmatmul__(self, other)

object.__rtruediv__(self, other)

object.__rfloordiv__(self, other)

object.__rmod__(self, other)

object.__rdivmod__(self, other)

object.__rpow__(self, other[, modulo])

object.__rlshift__(self, other)

object.__rrshift__(self, other)

object.__rand__(self, other)

object.__rxor__(self, other)

object.__ror__(self, other)

These methods are called to implement the binary arithmetic operations (+, -, *, @, /, //, %, divmod(), pow(), **, <<, >>, &, ^, |) with reflected (swapped) operands. These functions are only called if the left operand does not support the corresponding operation [3] and the operands are of different types. [4] For instance, to evaluate the expression x - y, where y is an instance of a class that has an __rsub__() method, type(y).__rsub__(y, x) is called if type(x).__sub__(x, y) returns NotImplemented.

Note that ternary pow() will not try calling __rpow__() (the coercion rules would become too complicated).

備註

If the right operand's type is a subclass of the left operand's type and that subclass provides a different implementation of the reflected method for the operation, this method will be called before the left operand's non-reflected method. This behavior allows subclasses to override their ancestors' operations.

object.__iadd__(self, other)

object.__isub__(self, other)

object.__imul__(self, other)

object.__imatmul__(self, other)

object.__itruediv__(self, other)

object.__ifloordiv__(self, other)

object.__imod__(self, other)

object.__ipow__(self, other[, modulo])

object.__ilshift__(self, other)

object.__irshift__(self, other)

object.__iand__(self, other)

object.__ixor__(self, other)

object.__ior__(self, other)

These methods are called to implement the augmented arithmetic assignments (+=, -=, *=, @=, /=, //=, %=, **=, <<=, >>=, &=, ^=, |=). These methods should attempt to do the operation in-place (modifying self) and return the result (which could be, but does not have to be, self). If a specific method is not defined, or if that method returns NotImplemented, the augmented assignment falls back to the normal methods. For instance, if x is an instance of a class with an __iadd__() method, x += y is equivalent to x = x.__iadd__(y) . If __iadd__() does not exist, or if x.__iadd__(y) returns NotImplemented, x.__add__(y) and y.__radd__(x) are considered, as with the evaluation of x + y. In certain situations, augmented assignment can result in unexpected errors (see Why does a_tuple[i] += ['item'] raise an exception when the addition works?), but this behavior is in fact part of the data model.

object.__neg__(self)

object.__pos__(self)

object.__abs__(self)

object.__invert__(self)

Called to implement the unary arithmetic operations (-, +, abs() and ~).

object.__complex__(self)

object.__int__(self)

object.__float__(self)

Called to implement the built-in functions complex(), int() and float(). Should return a value of the appropriate type.

object.__index__(self)

Called to implement operator.index(), and whenever Python needs to losslessly convert the numeric object to an integer object (such as in slicing, or in the built-in bin(), hex() and oct() functions). Presence of this method indicates that the numeric object is an integer type. Must return an integer.

If __int__(), __float__() and __complex__() are not defined then corresponding built-in functions int(), float() and complex() fall back to __index__().

object.__round__(self[, ndigits])

object.__trunc__(self)

object.__floor__(self)

object.__ceil__(self)

Called to implement the built-in function round() and math functions trunc(), floor() and ceil(). Unless ndigits is passed to __round__() all these methods should return the value of the object truncated to an Integral (typically an int).

The built-in function int() falls back to __trunc__() if neither __int__() nor __index__() is defined.

在 3.11 版的變更: The delegation of int() to __trunc__() is deprecated.

3.3.9. With 陳述式的情境管理器

A context manager is an object that defines the runtime context to be established when executing a with statement. The context manager handles the entry into, and the exit from, the desired runtime context for the execution of the block of code. Context managers are normally invoked using the with statement (described in section The with statement), but can also be used by directly invoking their methods.

Typical uses of context managers include saving and restoring various kinds of global state, locking and unlocking resources, closing opened files, etc.

For more information on context managers, see 情境管理器型別. The object class itself does not provide the context manager methods.

object.__enter__(self)

Enter the runtime context related to this object. The with statement will bind this method's return value to the target(s) specified in the as clause of the statement, if any.

object.__exit__(self, exc_type, exc_value, traceback)

Exit the runtime context related to this object. The parameters describe the exception that caused the context to be exited. If the context was exited without an exception, all three arguments will be None.

If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method.

Note that __exit__() methods should not reraise the passed-in exception; this is the caller's responsibility.

也參考

PEP 343 - "with" 陳述式

The specification, background, and examples for the Python with statement.

3.3.10. Customizing positional arguments in class pattern matching

When using a class name in a pattern, positional arguments in the pattern are not allowed by default, i.e. case MyClass(x, y) is typically invalid without special support in MyClass. To be able to use that kind of pattern, the class needs to define a __match_args__ attribute.

object.__match_args__

This class variable can be assigned a tuple of strings. When this class is used in a class pattern with positional arguments, each positional argument will be converted into a keyword argument, using the corresponding value in __match_args__ as the keyword. The absence of this attribute is equivalent to setting it to ().

For example, if MyClass.__match_args__ is ("left", "center", "right") that means that case MyClass(x, y) is equivalent to case MyClass(left=x, center=y). Note that the number of arguments in the pattern must be smaller than or equal to the number of elements in __match_args__; if it is larger, the pattern match attempt will raise a TypeError.

在 3.10 版被加入.

也參考

PEP 634 - Structural Pattern Matching

The specification for the Python match statement.

3.3.11. Emulating buffer types

The buffer protocol provides a way for Python objects to expose efficient access to a low-level memory array. This protocol is implemented by builtin types such as bytes and memoryview, and third-party libraries may define additional buffer types.

While buffer types are usually implemented in C, it is also possible to implement the protocol in Python.

object.__buffer__(self, flags)

Called when a buffer is requested from self (for example, by the memoryview constructor). The flags argument is an integer representing the kind of buffer requested, affecting for example whether the returned buffer is read-only or writable. inspect.BufferFlags provides a convenient way to interpret the flags. The method must return a memoryview object.

object.__release_buffer__(self, buffer)

Called when a buffer is no longer needed. The buffer argument is a memoryview object that was previously returned by __buffer__(). The method must release any resources associated with the buffer. This method should return None. Buffer objects that do not need to perform any cleanup are not required to implement this method.

在 3.12 版被加入.

也參考

PEP 688 - Making the buffer protocol accessible in Python

Introduces the Python __buffer__ and __release_buffer__ methods.

collections.abc.Buffer

ABC for buffer types.

3.3.12. Special method lookup

For custom classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an object's type, not in the object's instance dictionary. That behaviour is the reason why the following code raises an exception:

>>> class C:
...     pass
...
>>> c = C()
>>> c.__len__ = lambda: 5
>>> len(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'C' has no len()

The rationale behind this behaviour lies with a number of special methods such as __hash__() and __repr__() that are implemented by all objects, including type objects. If the implicit lookup of these methods used the conventional lookup process, they would fail when invoked on the type object itself:

>>> 1 .__hash__() == hash(1)
True
>>> int.__hash__() == hash(int)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: descriptor '__hash__' of 'int' object needs an argument

Incorrectly attempting to invoke an unbound method of a class in this way is sometimes referred to as 'metaclass confusion', and is avoided by bypassing the instance when looking up special methods:

>>> type(1).__hash__(1) == hash(1)
True
>>> type(int).__hash__(int) == hash(int)
True

In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the __getattribute__() method even of the object's metaclass:

>>> class Meta(type):
...     def __getattribute__(*args):
...         print("Metaclass getattribute invoked")
...         return type.__getattribute__(*args)
...
>>> class C(object, metaclass=Meta):
...     def __len__(self):
...         return 10
...     def __getattribute__(*args):
...         print("Class getattribute invoked")
...         return object.__getattribute__(*args)
...
>>> c = C()
>>> c.__len__()                 # Explicit lookup via instance
Class getattribute invoked
10
>>> type(c).__len__(c)          # Explicit lookup via type
Metaclass getattribute invoked
10
>>> len(c)                      # Implicit lookup
10

Bypassing the __getattribute__() machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method must be set on the class object itself in order to be consistently invoked by the interpreter).

3.4. 協程

3.4.1. Awaitable Objects

An awaitable object generally implements an __await__() method. Coroutine objects returned from async def functions are awaitable.

備註

The generator iterator objects returned from generators decorated with types.coroutine() are also awaitable, but they do not implement __await__().

object.__await__(self)

Must return an iterator. Should be used to implement awaitable objects. For instance, asyncio.Future implements this method to be compatible with the await expression. The object class itself is not awaitable and does not provide this method.

備註

The language doesn't place any restriction on the type or value of the objects yielded by the iterator returned by __await__, as this is specific to the implementation of the asynchronous execution framework (e.g. asyncio) that will be managing the awaitable object.

在 3.5 版被加入.

也參考

PEP 492 for additional information about awaitable objects.

3.4.2. 協程物件

Coroutine objects are awaitable objects. A coroutine's execution can be controlled by calling __await__() and iterating over the result. When the coroutine has finished executing and returns, the iterator raises StopIteration, and the exception's value attribute holds the return value. If the coroutine raises an exception, it is propagated by the iterator. Coroutines should not directly raise unhandled StopIteration exceptions.

Coroutines also have the methods listed below, which are analogous to those of generators (see Generator-iterator methods). However, unlike generators, coroutines do not directly support iteration.

在 3.5.2 版的變更: It is a RuntimeError to await on a coroutine more than once.

coroutine.send(value)

Starts or resumes execution of the coroutine. If value is None, this is equivalent to advancing the iterator returned by __await__(). If value is not None, this method delegates to the send() method of the iterator that caused the coroutine to suspend. The result (return value, StopIteration, or other exception) is the same as when iterating over the __await__() return value, described above.

coroutine.throw(value)

coroutine.throw(type[, value[, traceback]])

Raises the specified exception in the coroutine. This method delegates to the throw() method of the iterator that caused the coroutine to suspend, if it has such a method. Otherwise, the exception is raised at the suspension point. The result (return value, StopIteration, or other exception) is the same as when iterating over the __await__() return value, described above. If the exception is not caught in the coroutine, it propagates back to the caller.

在 3.12 版的變更: The second signature (type[, value[, traceback]]) is deprecated and may be removed in a future version of Python.

coroutine.close()

Causes the coroutine to clean itself up and exit. If the coroutine is suspended, this method first delegates to the close() method of the iterator that caused the coroutine to suspend, if it has such a method. Then it raises GeneratorExit at the suspension point, causing the coroutine to immediately clean itself up. Finally, the coroutine is marked as having finished executing, even if it was never started.

Coroutine objects are automatically closed using the above process when they are about to be destroyed.

3.4.3. Asynchronous Iterators

An asynchronous iterator can call asynchronous code in its __anext__ method.

Asynchronous iterators can be used in an async for statement.

The object class itself does not provide these methods.

object.__aiter__(self)

Must return an asynchronous iterator object.

object.__anext__(self)

Must return an awaitable resulting in a next value of the iterator. Should raise a StopAsyncIteration error when the iteration is over.

An example of an asynchronous iterable object:

class Reader:
    async def readline(self):
        ...

    def __aiter__(self):
        return self

    async def __anext__(self):
        val = await self.readline()
        if val == b'':
            raise StopAsyncIteration
        return val

在 3.5 版被加入.

在 3.7 版的變更: Prior to Python 3.7, __aiter__() could return an awaitable that would resolve to an asynchronous iterator.

Starting with Python 3.7, __aiter__() must return an asynchronous iterator object. Returning anything else will result in a TypeError error.

3.4.4. Asynchronous Context Managers

An asynchronous context manager is a context manager that is able to suspend execution in its __aenter__ and __aexit__ methods.

Asynchronous context managers can be used in an async with statement.

The object class itself does not provide these methods.

object.__aenter__(self)

Semantically similar to __enter__(), the only difference being that it must return an awaitable.

object.__aexit__(self, exc_type, exc_value, traceback)

Semantically similar to __exit__(), the only difference being that it must return an awaitable.

An example of an asynchronous context manager class:

class AsyncContextManager:
    async def __aenter__(self):
        await log('entering context')

    async def __aexit__(self, exc_type, exc, tb):
        await log('exiting context')

在 3.5 版被加入.

註解

[1]

It is possible in some cases to change an object's type, under certain controlled conditions. It generally isn't a good idea though, since it can lead to some very strange behaviour if it is handled incorrectly.

[2]

The __hash__(), __iter__(), __reversed__(), __contains__(), __class_getitem__() and __fspath__() methods have special handling for this. Others will still raise a TypeError, but may do so by relying on the behavior that None is not callable.

[3]

"Does not support" here means that the class has no such method, or the method returns NotImplemented. Do not set the method to None if you want to force fallback to the right operand's reflected method—that will instead have the opposite effect of explicitly blocking such fallback.

[4]

For operands of the same type, it is assumed that if the non-reflected method -- such as __add__() -- fails then the overall operation is not supported, which is why the reflected method is not called.