# 4. 深入了解流程控制

除了剛才介紹的 `while`，Python 擁有在其他程式語言中常用的流程控制語法，並有ㄧ些不一樣的改變。

## 4.1. `if` 语句

或許最常見的陳述式種類就是 `if` 了。舉例來說：

    >>> x = int(input("Please enter an integer: "))
    Please enter an integer: 42
    >>> if x < 0:
    ...     x = 0
    ...     print('Negative changed to zero')
    ... elif x == 0:
    ...     print('Zero')
    ... elif x == 1:
    ...     print('Single')
    ... else:
    ...     print('More')
    ...
    More

可以有零个或多个 `elif` 部分，以及一个可选的 `else` 部分。 关键字 '`elif`' 是 'else if' 的缩写，适合用于避免过多的缩进。 一个 `if` ... `elif` ... `elif` ... 序列可以看作是其他语言中的 `switch` 或 `case` 语句的替代。

## 4.2. `for` 语句

Python 中的 `for` 语句与你在 C 或 Pascal 中可能用到的有所不同。 Python 中的 `for` 语句并不总是对算术递增的数值进行迭代（如同 Pascal），或是给予用户定义迭代步骤和暂停条件的能力（如同 C），而是对任意序列进行迭代（例如列表或字符串），条目的迭代顺序与它们在序列中出现的顺序一致。 例如（此处英文为双关语）:

    >>> # Measure some strings:
    ... words = ['cat', 'window', 'defenestrate']
    >>> for w in words:
    ...     print(w, len(w))
    ...
    cat 3
    window 6
    defenestrate 12

如果你在迴圈中需要修改一個你正在疊代的序列（例如重複一些選擇的元素），那麼會建議你先建立一個序列的拷貝。疊代序列並不暗示建立新的拷貝。此時 slice 語法就讓這件事十分容易完成：

    >>> for w in words[:]:  # Loop over a slice copy of the entire list.
    ...     if len(w) > 6:
    ...         words.insert(0, w)
    ...
    >>> words
    ['defenestrate', 'cat', 'window', 'defenestrate']

在 `for w in words:` 的情況，這個例子會試著重覆不斷地插入 `defenestrate`，產生一個無限長的 list。

## 4.3. `range()` 函式

如果你需要疊代一個數列的話，使用內建 `range()` 函式就很方便。它可以生成一等差級數：

    >>> for i in range(5):
    ...     print(i)
    ...
    0
    1
    2
    3
    4

給定的結束值永遠不會出現在生成的序列中；`range(10)` 生成的 10 個數值，即對應存取一個長度為 10 的序列內每一個元素的索引值。也可以讓 range 從其他數值計數，或者給定不同的級距（甚至為負；有時稱之為 step）：

    range(5, 10)
       5, 6, 7, 8, 9

    range(0, 10, 3)
       0, 3, 6, 9

    range(-10, -100, -30)
      -10, -40, -70

欲疊代一個序列的索引值，你可以搭配使用 `range()` 和 `len()` 如下：

    >>> a = ['Mary', 'had', 'a', 'little', 'lamb']
    >>> for i in range(len(a)):
    ...     print(i, a[i])
    ...
    0 Mary
    1 had
    2 a
    3 little
    4 lamb

然而，在多數的情況，使用 `enumerate()` 函式將更為方便，詳見迴圈技巧。

如果直接印出一個 range 則會出現奇怪的輸出：

    >>> print(range(10))
    range(0, 10)

在很多情況下，由 `range()` 回傳的物件的行為如同一個 list，但實際上它並不是。它是一個物件在你疊代時會回傳想要的序列的連續元素，並不會真正建出這個序列的 list，以節省空間。

We say such an object is iterable, that is, suitable as a target for functions and constructs that expect something from which they can obtain successive items until the supply is exhausted. We have seen that the `for` statement is such a construct, while an example of function that takes an iterable is `sum()`:

    >>> sum(range(4))  # 0 + 1 + 2 + 3
    6

Later we will see more functions that return iterables and take iterables as arguments. Lastly, maybe you are curious about how to get a list from a range. Here is the solution:

    >>> list(range(4))
    [0, 1, 2, 3]

In chapter 資料結構, we will discuss in more detail about `list()`.

## 4.4. `break` 和 `continue` 语句，以及循环中的 `else` 子句

`break` 陳述，如同 C 語言，終止包含其最內部的 `for` 或 `while` 迴圈。

Loop statements may have an `else` clause; it is executed when the loop terminates through exhaustion of the iterable (with `for`) or when the condition becomes false (with `while`), but not when the loop is terminated by a `break` statement. This is exemplified by the following loop, which searches for prime numbers:

    >>> for n in range(2, 10):
    ...     for x in range(2, n):
    ...         if n % x == 0:
    ...             print(n, 'equals', x, '*', n//x)
    ...             break
    ...     else:
    ...         # loop fell through without finding a factor
    ...         print(n, 'is a prime number')
    ...
    2 is a prime number
    3 is a prime number
    4 equals 2 * 2
    5 is a prime number
    6 equals 2 * 3
    7 is a prime number
    8 equals 2 * 4
    9 equals 3 * 3

（沒錯，這是正確的程式碼。請看仔細：`else` 段落屬於 `for` 迴圈，並非 `if` 陳述。）

When used with a loop, the `else` clause has more in common with the `else` clause of a `try` statement than it does with that of `if` statements: a `try` statement's `else` clause runs when no exception occurs, and a loop's `else` clause runs when no `break` occurs. For more on the `try` statement and exceptions, see 處理例外.

`continue` 陳述，亦承襲於 C 語言，讓所屬的迴圈繼續執行下個疊代：

    >>> for num in range(2, 10):
    ...     if num % 2 == 0:
    ...         print("Found an even number", num)
    ...         continue
    ...     print("Found a number", num)
    Found an even number 2
    Found a number 3
    Found an even number 4
    Found a number 5
    Found an even number 6
    Found a number 7
    Found an even number 8
    Found a number 9

## 4.5. `pass` 语句

`pass` 陳述不執行任何動作。它用在語法上需要一個陳述但不需要執行任何動作的時候。例如：

    >>> while True:
    ...     pass  # Busy-wait for keyboard interrupt (Ctrl+C)
    ...

這經常用於定義一個最簡單的類別：

    >>> class MyEmptyClass:
    ...     pass
    ...

`pass` 的另一个可以使用的场合是在你编写新的代码时作为一个函数或条件子句体的占位符，允许你保持在更抽象的层次上进行思考。 `pass` 会被静默地忽略:

    >>> def initlog(*args):
    ...     pass   # Remember to implement this!
    ...

## 4.6. 定義函式 (function)

我們可以建立一個函式來產生費式數列到任何一個上界：

    >>> def fib(n):    # write Fibonacci series up to n
    ...     """Print a Fibonacci series up to n."""
    ...     a, b = 0, 1
    ...     while a < n:
    ...         print(a, end=' ')
    ...         a, b = b, a+b
    ...     print()
    ...
    >>> # Now call the function we just defined:
    ... fib(2000)
    0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597

關鍵字 `def` 帶入一個函式的定義。它之後必須連著該函式的名稱和置於括號之中的參數。自下一行起，所有縮排的陳述成為該函式的主體。

一個函式的第一個陳述可以是一個字串值；此情況該字串值被視為該函式的說明文件字串，即 docstring。（關於 docstring 的細節請參見說明文件字串段落。）有些工具可以使用 docstring 來自動產生線上或可列印的文件，或讓使用者能自由地自原始碼中瀏覽文件。在原始碼中加入 docstring 是個好慣例，應該養成這樣的習慣。

The execution of a function introduces a new symbol table used for the local variables of the function. More precisely, all variable assignments in a function store the value in the local symbol table; whereas variable references first look in the local symbol table, then in the local symbol tables of enclosing functions, then in the global symbol table, and finally in the table of built-in names. Thus, global variables and variables of enclosing functions cannot be directly assigned a value within a function (unless, for global variables, named in a `global` statement, or, for variables of enclosing functions, named in a `nonlocal` statement), although they may be referenced.

在一個函式被呼叫的時候，實際傳入的參數（引數）會被加入至該函數的區域符號表。因此，引數傳入的方式為傳值呼叫 (call by value)（這裡傳遞的「值」永遠是一個物件的參照（reference），而不是該物件的值）。1 當一個函式呼叫別的函式時，在被呼叫的函式中會建立一個新的區域符號表。

一個函式定義會把該函式名稱加入至當前的符號表。該函式名稱的值帶有一個型別，並被直譯器辨識為使用者自定函式（user-defined function）。該值可以被賦予給別的變數名，而該變數也可以被當作函式使用。這即是常見的重新命名方式：

    >>> fib
    <function fib at 10042ed0>
    >>> f = fib
    >>> f(100)
    0 1 1 2 3 5 8 13 21 34 55 89

如果你是來自別的語言，你可能不同意 `fib` 是個函式，而是個程序 (procedure)，因為它並沒有回傳值。實際上，即使一個函式缺少一個 `return` 陳述，它亦有一個固定的回傳值。這個值為 `None`（它是一個內建名稱）。在直譯器中單獨使用 `None` 時，通常不會被顯示。你可以使用 `print()` 來看到它：

    >>> fib(0)
    >>> print(fib(0))
    None

如果要寫一個函式回傳費式數列的 list 而不是直接印出它，這也很容易：

    >>> def fib2(n):  # return Fibonacci series up to n
    ...     """Return a list containing the Fibonacci series up to n."""
    ...     result = []
    ...     a, b = 0, 1
    ...     while a < n:
    ...         result.append(a)    # see below
    ...         a, b = b, a+b
    ...     return result
    ...
    >>> f100 = fib2(100)    # call it
    >>> f100                # write the result
    [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

這個例子一樣示範了一些新的 Python 特性：

- `return` 语句会从函数内部返回一个值。 不带表达式参数的 `return` 会返回 `None`。 函数执行完毕退出也会返回 `None`。

- `result.append(a)` 陳述呼叫了一個 list 物件的 `result` method（方法）。method 為「屬於」一個物件的函式，命名規則為 `obj.methodname`，其中 `obj` 為某個物件（亦可為一表達式），而 `methodname` 為該 method 的名稱，並由該物件的型別所定義。不同的型別代表不同的 method。不同型別的 method 可以擁有一樣的名稱而不會讓 Python 混淆。（你可以使用 class 定義自己的物件型別和 method，見 Classes）這裡 `append()` method 定義在 list 物件中；它會加入一個新的元素在該 list 的末端。這個例子等同於 `result = result + [a]`，但更有效率。

## 4.7. More on Defining Functions

It is also possible to define functions with a variable number of arguments. There are three forms, which can be combined.

### 4.7.1. Default Argument Values

The most useful form is to specify a default value for one or more arguments. This creates a function that can be called with fewer arguments than it is defined to allow. For example:

    def ask_ok(prompt, retries=4, reminder='Please try again!'):
        while True:
            ok = input(prompt)
            if ok in ('y', 'ye', 'yes'):
                return True
            if ok in ('n', 'no', 'nop', 'nope'):
                return False
            retries = retries - 1
            if retries < 0:
                raise ValueError('invalid user response')
            print(reminder)

This function can be called in several ways:

- giving only the mandatory argument: `ask_ok('Do you really want to quit?')`

- giving one of the optional arguments: `ask_ok('OK to overwrite the file?', 2)`

- or even giving all arguments: `ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')`

This example also introduces the `in` keyword. This tests whether or not a sequence contains a certain value.

The default values are evaluated at the point of function definition in the defining scope, so that

    i = 5

    def f(arg=i):
        print(arg)

    i = 6
    f()

will print `5`.

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:

    def f(a, L=[]):
        L.append(a)
        return L

    print(f(1))
    print(f(2))
    print(f(3))

This will print

    [1]
    [1, 2]
    [1, 2, 3]

If you don't want the default to be shared between subsequent calls, you can write the function like this instead:

    def f(a, L=None):
        if L is None:
            L = []
        L.append(a)
        return L

### 4.7.2. Keyword Arguments

Functions can also be called using keyword arguments of the form `kwarg=value`. For instance, the following function:

    def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'):
        print("-- This parrot wouldn't", action, end=' ')
        print("if you put", voltage, "volts through it.")
        print("-- Lovely plumage, the", type)
        print("-- It's", state, "!")

accepts one required argument (`voltage`) and three optional arguments (`state`, `action`, and `type`). This function can be called in any of the following ways:

    parrot(1000)                                          # 1 positional argument
    parrot(voltage=1000)                                  # 1 keyword argument
    parrot(voltage=1000000, action='VOOOOOM')             # 2 keyword arguments
    parrot(action='VOOOOOM', voltage=1000000)             # 2 keyword arguments
    parrot('a million', 'bereft of life', 'jump')         # 3 positional arguments
    parrot('a thousand', state='pushing up the daisies')  # 1 positional, 1 keyword

but all the following calls would be invalid:

    parrot()                     # required argument missing
    parrot(voltage=5.0, 'dead')  # non-keyword argument after a keyword argument
    parrot(110, voltage=220)     # duplicate value for the same argument
    parrot(actor='John Cleese')  # unknown keyword argument

In a function call, keyword arguments must follow positional arguments. All the keyword arguments passed must match one of the arguments accepted by the function (e.g. `actor` is not a valid argument for the `parrot` function), and their order is not important. This also includes non-optional arguments (e.g. `parrot(voltage=1000)` is valid too). No argument may receive a value more than once. Here's an example that fails due to this restriction:

    >>> def function(a):
    ...     pass
    ...
    >>> function(0, a=0)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: function() got multiple values for keyword argument 'a'

When a final formal parameter of the form `**name` is present, it receives a dictionary (see Mapping Types --- dict) containing all keyword arguments except for those corresponding to a formal parameter. This may be combined with a formal parameter of the form `*name` (described in the next subsection) which receives a tuple containing the positional arguments beyond the formal parameter list. (`*name` must occur before `**name`.) For example, if we define a function like this:

    def cheeseshop(kind, *arguments, **keywords):
        print("-- Do you have any", kind, "?")
        print("-- I'm sorry, we're all out of", kind)
        for arg in arguments:
            print(arg)
        print("-" * 40)
        for kw in keywords:
            print(kw, ":", keywords[kw])

It could be called like this:

    cheeseshop("Limburger", "It's very runny, sir.",
               "It's really very, VERY runny, sir.",
               shopkeeper="Michael Palin",
               client="John Cleese",
               sketch="Cheese Shop Sketch")

and of course it would print:

    -- Do you have any Limburger ?
    -- I'm sorry, we're all out of Limburger
    It's very runny, sir.
    It's really very, VERY runny, sir.
    ----------------------------------------
    shopkeeper : Michael Palin
    client : John Cleese
    sketch : Cheese Shop Sketch

Note that the order in which the keyword arguments are printed is guaranteed to match the order in which they were provided in the function call.

### 4.7.3. Special parameters

By default, arguments may be passed to a Python function either by position or explicitly by keyword. For readability and performance, it makes sense to restrict the way arguments can be passed so that a developer need only look at the function definition to determine if items are passed by position, by position or keyword, or by keyword.

A function definition may look like:

    def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2):
          -----------    ----------     ----------
            |             |                  |
            |        Positional or keyword   |
            |                                - Keyword only
             -- Positional only

where `/` and `*` are optional. If used, these symbols indicate the kind of parameter by how the arguments may be passed to the function: positional-only, positional-or-keyword, and keyword-only. Keyword parameters are also referred to as named parameters.

#### 4.7.3.1. Positional-or-Keyword Arguments

If `/` and `*` are not present in the function definition, arguments may be passed to a function by position or by keyword.

#### 4.7.3.2. Positional-Only Parameters

Looking at this in a bit more detail, it is possible to mark certain parameters as positional-only. If positional-only, the parameters' order matters, and the parameters cannot be passed by keyword. Positional-only parameters are placed before a `/` (forward-slash). The `/` is used to logically separate the positional-only parameters from the rest of the parameters. If there is no `/` in the function definition, there are no positional-only parameters.

Parameters following the `/` may be positional-or-keyword or keyword-only.

#### 4.7.3.3. Keyword-Only Arguments

To mark parameters as keyword-only, indicating the parameters must be passed by keyword argument, place an `*` in the arguments list just before the first keyword-only parameter.

#### 4.7.3.4. Function Examples

Consider the following example function definitions paying close attention to the markers `/` and `*`:

    >>> def standard_arg(arg):
    ...     print(arg)
    ...
    >>> def pos_only_arg(arg, /):
    ...     print(arg)
    ...
    >>> def kwd_only_arg(*, arg):
    ...     print(arg)
    ...
    >>> def combined_example(pos_only, /, standard, *, kwd_only):
    ...     print(pos_only, standard, kwd_only)

The first function definition, `standard_arg`, the most familiar form, places no restrictions on the calling convention and arguments may be passed by position or keyword:

    >>> standard_arg(2)
    2

    >>> standard_arg(arg=2)
    2

The second function `pos_only_arg` is restricted to only use positional parameters as there is a `/` in the function definition:

    >>> pos_only_arg(1)
    1

    >>> pos_only_arg(arg=1)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: pos_only_arg() got an unexpected keyword argument 'arg'

The third function `kwd_only_args` only allows keyword arguments as indicated by a `*` in the function definition:

    >>> kwd_only_arg(3)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: kwd_only_arg() takes 0 positional arguments but 1 was given

    >>> kwd_only_arg(arg=3)
    3

And the last uses all three calling conventions in the same function definition:

    >>> combined_example(1, 2, 3)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: combined_example() takes 2 positional arguments but 3 were given

    >>> combined_example(1, 2, kwd_only=3)
    1 2 3

    >>> combined_example(1, standard=2, kwd_only=3)
    1 2 3

    >>> combined_example(pos_only=1, standard=2, kwd_only=3)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: combined_example() got an unexpected keyword argument 'pos_only'

Finally, consider this function definition which has a potential collision between the positional argument `name` and `**kwds` which has `name` as a key:

    def foo(name, **kwds):
        return 'name' in kwds

There is no possible call that will make it return `True` as the keyword `'name'` will always to bind to the first parameter. For example:

    >>> foo(1, **{'name': 2})
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: foo() got multiple values for argument 'name'
    >>>

But using `/` (positional only arguments), it is possible since it allows `name` as a positional argument and `'name'` as a key in the keyword arguments:

    def foo(name, /, **kwds):
        return 'name' in kwds
    >>> foo(1, **{'name': 2})
    True

In other words, the names of positional-only parameters can be used in `**kwds` without ambiguity.

#### 4.7.3.5. Recap

The use case will determine which parameters to use in the function definition:

    def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2):

As guidance:

- Use positional-only if you want the name of the parameters to not be available to the user. This is useful when parameter names have no real meaning, if you want to enforce the order of the arguments when the function is called or if you need to take some positional parameters and arbitrary keywords.

- Use keyword-only when names have meaning and the function definition is more understandable by being explicit with names or you want to prevent users relying on the position of the argument being passed.

- For an API, use positional-only to prevent prevent breaking API changes if the parameter's name is modified in the future.

### 4.7.4. Arbitrary Argument Lists

Finally, the least frequently used option is to specify that a function can be called with an arbitrary number of arguments. These arguments will be wrapped up in a tuple (see Tuples 和序列 (Sequences)). Before the variable number of arguments, zero or more normal arguments may occur.

    def write_multiple_items(file, separator, *args):
        file.write(separator.join(args))

Normally, these `variadic` arguments will be last in the list of formal parameters, because they scoop up all remaining input arguments that are passed to the function. Any formal parameters which occur after the `*args` parameter are 'keyword-only' arguments, meaning that they can only be used as keywords rather than positional arguments.

    >>> def concat(*args, sep="/"):
    ...     return sep.join(args)
    ...
    >>> concat("earth", "mars", "venus")
    'earth/mars/venus'
    >>> concat("earth", "mars", "venus", sep=".")
    'earth.mars.venus'

### 4.7.5. Unpacking Argument Lists

The reverse situation occurs when the arguments are already in a list or tuple but need to be unpacked for a function call requiring separate positional arguments. For instance, the built-in `range()` function expects separate start and stop arguments. If they are not available separately, write the function call with the `*`-operator to unpack the arguments out of a list or tuple:

    >>> list(range(3, 6))            # normal call with separate arguments
    [3, 4, 5]
    >>> args = [3, 6]
    >>> list(range(*args))            # call with arguments unpacked from a list
    [3, 4, 5]

In the same fashion, dictionaries can deliver keyword arguments with the `**`-operator:

    >>> def parrot(voltage, state='a stiff', action='voom'):
    ...     print("-- This parrot wouldn't", action, end=' ')
    ...     print("if you put", voltage, "volts through it.", end=' ')
    ...     print("E's", state, "!")
    ...
    >>> d = {"voltage": "four million", "state": "bleedin' demised", "action": "VOOM"}
    >>> parrot(**d)
    -- This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !

### 4.7.6. Lambda Expressions

Small anonymous functions can be created with the `lambda` keyword. This function returns the sum of its two arguments: `lambda a, b: a+b`. Lambda functions can be used wherever function objects are required. They are syntactically restricted to a single expression. Semantically, they are just syntactic sugar for a normal function definition. Like nested function definitions, lambda functions can reference variables from the containing scope:

    >>> def make_incrementor(n):
    ...     return lambda x: x + n
    ...
    >>> f = make_incrementor(42)
    >>> f(0)
    42
    >>> f(1)
    43

The above example uses a lambda expression to return a function. Another use is to pass a small function as an argument:

    >>> pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
    >>> pairs.sort(key=lambda pair: pair[1])
    >>> pairs
    [(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]

### 4.7.7. 說明文件字串

Here are some conventions about the content and formatting of documentation strings.

The first line should always be a short, concise summary of the object's purpose. For brevity, it should not explicitly state the object's name or type, since these are available by other means (except if the name happens to be a verb describing a function's operation). This line should begin with a capital letter and end with a period.

If there are more lines in the documentation string, the second line should be blank, visually separating the summary from the rest of the description. The following lines should be one or more paragraphs describing the object's calling conventions, its side effects, etc.

The Python parser does not strip indentation from multi-line string literals in Python, so tools that process documentation have to strip indentation if desired. This is done using the following convention. The first non-blank line after the first line of the string determines the amount of indentation for the entire documentation string. (We can't use the first line since it is generally adjacent to the string's opening quotes so its indentation is not apparent in the string literal.) Whitespace "equivalent" to this indentation is then stripped from the start of all lines of the string. Lines that are indented less should not occur, but if they occur all their leading whitespace should be stripped. Equivalence of whitespace should be tested after expansion of tabs (to 8 spaces, normally).

Here is an example of a multi-line docstring:

    >>> def my_function():
    ...     """Do nothing, but document it.
    ...
    ...     No, really, it doesn't do anything.
    ...     """
    ...     pass
    ...
    >>> print(my_function.__doc__)
    Do nothing, but document it.

        No, really, it doesn't do anything.

### 4.7.8. Function Annotations

Function annotations are completely optional metadata information about the types used by user-defined functions (see PEP 3107 and PEP 484 for more information).

函数标注 以字典的形式存放在函数的 `__annotations__` 属性中，并且不会影响函数的任何其他部分。 形参标注的定义方式是在形参名称后加上冒号，后面跟一个表达式，该表达式会被求值为标注的值。 返回值标注的定义方式是加上一个组合符号 `->`，后面跟一个表达式，该标注位于形参列表和表示 `def` 语句结束的冒号之间。 下面的示例有一个位置参数，一个关键字参数以及返回值带有相应标注:

    >>> def f(ham: str, eggs: str = 'eggs') -> str:
    ...     print("Annotations:", f.__annotations__)
    ...     print("Arguments:", ham, eggs)
    ...     return ham + ' and ' + eggs
    ...
    >>> f('spam')
    Annotations: {'ham': <class 'str'>, 'return': <class 'str'>, 'eggs': <class 'str'>}
    Arguments: spam eggs
    'spam and eggs'

## 4.8. Intermezzo: Coding Style

Now that you are about to write longer, more complex pieces of Python, it is a good time to talk about coding style. Most languages can be written (or more concise, formatted) in different styles; some are more readable than others. Making it easy for others to read your code is always a good idea, and adopting a nice coding style helps tremendously for that.

For Python, PEP 8 has emerged as the style guide that most projects adhere to; it promotes a very readable and eye-pleasing coding style. Every Python developer should read it at some point; here are the most important points extracted for you:

- Use 4-space indentation, and no tabs.

  4 spaces are a good compromise between small indentation (allows greater nesting depth) and large indentation (easier to read). Tabs introduce confusion, and are best left out.

- Wrap lines so that they don't exceed 79 characters.

  This helps users with small displays and makes it possible to have several code files side-by-side on larger displays.

- Use blank lines to separate functions and classes, and larger blocks of code inside functions.

- When possible, put comments on a line of their own.

- Use docstrings.

- Use spaces around operators and after commas, but not directly inside bracketing constructs: `a = f(1, 2) + g(3, 4)`.

- Name your classes and functions consistently; the convention is to use `CamelCase` for classes and `lower_case_with_underscores` for functions and methods. Always use `self` as the name for the first method argument (see A First Look at Classes for more on classes and methods).

- Don't use fancy encodings if your code is meant to be used in international environments. Python's default, UTF-8, or even plain ASCII work best in any case.

- Likewise, don't use non-ASCII characters in identifiers if there is only the slightest chance people speaking a different language will read or maintain the code.

註解

1

  Actually, call by object reference would be a better description, since if a mutable object is passed, the caller will see any changes the callee makes to it (items inserted into a list).