# `importlib` --- `import` 的实现

3.1 版新加入.

Source code: Lib/importlib/__init__.py

## 簡介

`importlib` 包的目的有两个。 第一个目的是在 Python 源代码中提供 `import` 语句的实现（并且因此而扩展 `__import__()` 函数）。 这提供了一个可移植到任何 Python 解释器的 `import` 实现。 相比使用 Python 以外的编程语言实现方式，这一实现更加易于理解。

Two, the components to implement `import` are exposed in this package, making it easier for users to create their own custom objects (known generically as an importer) to participate in the import process.

也參考

import 语句

  The language reference for the `import` statement.

Packages specification

  Original specification of packages. Some semantics have changed since the writing of this document (e.g. redirecting based on `None` in `sys.modules`).

The `__import__()` function

  The `import` statement is syntactic sugar for this function.

PEP 235

  Import on Case-Insensitive Platforms

PEP 263

  Defining Python Source Code Encodings

PEP 302

  New Import Hooks

PEP 328

  Imports: Multi-Line and Absolute/Relative

PEP 366

  Main module explicit relative imports

PEP 420

  Implicit namespace packages

PEP 451

  A ModuleSpec Type for the Import System

PEP 488

  Elimination of PYO files

PEP 489

  Multi-phase extension module initialization

PEP 552

  Deterministic pycs

PEP 3120

  Using UTF-8 as the Default Source Encoding

PEP 3147

  PYC Repository Directories

## Functions

`importlib.``__import__`(name, globals=None, locals=None, fromlist=(), level=0)

  An implementation of the built-in `__import__()` function.

  備註

  Programmatic importing of modules should use `import_module()` instead of this function.

`importlib.``import_module`(name, package=None)

  Import a module. The name argument specifies what module to import in absolute or relative terms (e.g. either `pkg.mod` or `..mod`). If the name is specified in relative terms, then the package argument must be set to the name of the package which is to act as the anchor for resolving the package name (e.g. `import_module('..mod', 'pkg.subpkg')` will import `pkg.mod`).

  The `import_module()` function acts as a simplifying wrapper around `importlib.__import__()`. This means all semantics of the function are derived from `importlib.__import__()`. The most important difference between these two functions is that `import_module()` returns the specified package or module (e.g. `pkg.mod`), while `__import__()` returns the top-level package or module (e.g. `pkg`).

  If you are dynamically importing a module that was created since the interpreter began execution (e.g., created a Python source file), you may need to call `invalidate_caches()` in order for the new module to be noticed by the import system.

  3.3 版更變: Parent packages are automatically imported.

`importlib.``find_loader`(name, path=None)

  Find the loader for a module, optionally within the specified path. If the module is in `sys.modules`, then `sys.modules[name].__loader__` is returned (unless the loader would be `None` or is not set, in which case `ValueError` is raised). Otherwise a search using `sys.meta_path` is done. `None` is returned if no loader is found.

  A dotted name does not have its parents implicitly imported as that requires loading them and that may not be desired. To properly import a submodule you will need to import all parent packages of the submodule and use the correct argument to path.

  3.3 版新加入.

  3.4 版更變: If `__loader__` is not set, raise `ValueError`, just like when the attribute is set to `None`.

  3.4 版後已棄用: Use `importlib.util.find_spec()` instead.

`importlib.``invalidate_caches`()

  Invalidate the internal caches of finders stored at `sys.meta_path`. If a finder implements `invalidate_caches()` then it will be called to perform the invalidation. This function should be called if any modules are created/installed while your program is running to guarantee all finders will notice the new module's existence.

  3.3 版新加入.

`importlib.``reload`(module)

  Reload a previously imported module. The argument must be a module object, so it must have been successfully imported before. This is useful if you have edited the module source file using an external editor and want to try out the new version without leaving the Python interpreter. The return value is the module object (which can be different if re-importing causes a different object to be placed in `sys.modules`).

  When `reload()` is executed:

  - Python module's code is recompiled and the module-level code re-executed, defining a new set of objects which are bound to names in the module's dictionary by reusing the loader which originally loaded the module. The `init` function of extension modules is not called a second time.

  - As with all other objects in Python the old objects are only reclaimed after their reference counts drop to zero.

  - The names in the module namespace are updated to point to any new or changed objects.

  - Other references to the old objects (such as names external to the module) are not rebound to refer to the new objects and must be updated in each namespace where they occur if that is desired.

  There are a number of other caveats:

  When a module is reloaded, its dictionary (containing the module's global variables) is retained. Redefinitions of names will override the old definitions, so this is generally not a problem. If the new version of a module does not define a name that was defined by the old version, the old definition remains. This feature can be used to the module's advantage if it maintains a global table or cache of objects --- with a `try` statement it can test for the table's presence and skip its initialization if desired:

      try:
          cache
      except NameError:
          cache = {}

  It is generally not very useful to reload built-in or dynamically loaded modules. Reloading `sys`, `__main__`, `builtins` and other key modules is not recommended. In many cases extension modules are not designed to be initialized more than once, and may fail in arbitrary ways when reloaded.

  如果一个模块使用 `from` ... `import` ... 导入的对象来自另外一个模块，给其它模块调用 `reload()` 不会重新定义来自这个模块的对象 —— 解决这个问题的一种方式是重新执行 `from` 语句，另一种方式是使用 `import` 和限定名称(module.name)来代替。

  If a module instantiates instances of a class, reloading the module that defines the class does not affect the method definitions of the instances --- they continue to use the old class definition. The same is true for derived classes.

  3.4 版新加入.

  3.7 版更變: `ModuleNotFoundError` is raised when the module being reloaded lacks a `ModuleSpec`.

## `importlib.abc` -- Abstract base classes related to import

Source code: Lib/importlib/abc.py

The `importlib.abc` module contains all of the core abstract base classes used by `import`. Some subclasses of the core abstract base classes are also provided to help in implementing the core ABCs.

ABC hierarchy:

    object
     +-- Finder (deprecated)
     |    +-- MetaPathFinder
     |    +-- PathEntryFinder
     +-- Loader
          +-- ResourceLoader --------+
          +-- InspectLoader          |
               +-- ExecutionLoader --+
                                     +-- FileLoader
                                     +-- SourceLoader

class `importlib.abc.``Finder`

  An abstract base class representing a finder.

  3.3 版後已棄用: Use `MetaPathFinder` or `PathEntryFinder` instead.

  abstractmethod `find_module`(fullname, path=None)

    An abstract method for finding a loader for the specified module. Originally specified in PEP 302, this method was meant for use in `sys.meta_path` and in the path-based import subsystem.

    3.4 版更變: Returns `None` when called instead of raising `NotImplementedError`.

class `importlib.abc.``MetaPathFinder`

  An abstract base class representing a meta path finder. For compatibility, this is a subclass of `Finder`.

  3.3 版新加入.

  `find_spec`(fullname, path, target=None)

    An abstract method for finding a spec for the specified module. If this is a top-level import, path will be `None`. Otherwise, this is a search for a subpackage or module and path will be the value of `__path__` from the parent package. If a spec cannot be found, `None` is returned. When passed in, `target` is a module object that the finder may use to make a more educated guess about what spec to return.

    3.4 版新加入.

  `find_module`(fullname, path)

    A legacy method for finding a loader for the specified module. If this is a top-level import, path will be `None`. Otherwise, this is a search for a subpackage or module and path will be the value of `__path__` from the parent package. If a loader cannot be found, `None` is returned.

    If `find_spec()` is defined, backwards-compatible functionality is provided.

    3.4 版更變: Returns `None` when called instead of raising `NotImplementedError`. Can use `find_spec()` to provide functionality.

    3.4 版後已棄用: Use `find_spec()` instead.

  `invalidate_caches`()

    An optional method which, when called, should invalidate any internal cache used by the finder. Used by `importlib.invalidate_caches()` when invalidating the caches of all finders on `sys.meta_path`.

    3.4 版更變: Returns `None` when called instead of `NotImplemented`.

class `importlib.abc.``PathEntryFinder`

  An abstract base class representing a path entry finder. Though it bears some similarities to `MetaPathFinder`, `PathEntryFinder` is meant for use only within the path-based import subsystem provided by `PathFinder`. This ABC is a subclass of `Finder` for compatibility reasons only.

  3.3 版新加入.

  `find_spec`(fullname, target=None)

    An abstract method for finding a spec for the specified module. The finder will search for the module only within the path entry to which it is assigned. If a spec cannot be found, `None` is returned. When passed in, `target` is a module object that the finder may use to make a more educated guess about what spec to return.

    3.4 版新加入.

  `find_loader`(fullname)

    A legacy method for finding a loader for the specified module. Returns a 2-tuple of `(loader, portion)` where `portion` is a sequence of file system locations contributing to part of a namespace package. The loader may be `None` while specifying `portion` to signify the contribution of the file system locations to a namespace package. An empty list can be used for `portion` to signify the loader is not part of a namespace package. If `loader` is `None` and `portion` is the empty list then no loader or location for a namespace package were found (i.e. failure to find anything for the module).

    If `find_spec()` is defined then backwards-compatible functionality is provided.

    3.4 版更變: Returns `(None, [])` instead of raising `NotImplementedError`. Uses `find_spec()` when available to provide functionality.

    3.4 版後已棄用: Use `find_spec()` instead.

  `find_module`(fullname)

    A concrete implementation of `Finder.find_module()` which is equivalent to `self.find_loader(fullname)[0]`.

    3.4 版後已棄用: Use `find_spec()` instead.

  `invalidate_caches`()

    An optional method which, when called, should invalidate any internal cache used by the finder. Used by `PathFinder.invalidate_caches()` when invalidating the caches of all cached finders.

class `importlib.abc.``Loader`

  An abstract base class for a loader. See PEP 302 for the exact definition for a loader.

  Loaders that wish to support resource reading should implement a `get_resource_reader(fullname)` method as specified by `importlib.abc.ResourceReader`.

  3.7 版更變: Introduced the optional `get_resource_reader()` method.

  `create_module`(spec)

    A method that returns the module object to use when importing a module. This method may return `None`, indicating that default module creation semantics should take place.

    3.4 版新加入.

    3.5 版更變: Starting in Python 3.6, this method will not be optional when `exec_module()` is defined.

  `exec_module`(module)

    An abstract method that executes the module in its own namespace when a module is imported or reloaded. The module should already be initialized when `exec_module()` is called. When this method exists, `create_module()` must be defined.

    3.4 版新加入.

    3.6 版更變: `create_module()` must also be defined.

  `load_module`(fullname)

    A legacy method for loading a module. If the module cannot be loaded, `ImportError` is raised, otherwise the loaded module is returned.

    If the requested module already exists in `sys.modules`, that module should be used and reloaded. Otherwise the loader should create a new module and insert it into `sys.modules` before any loading begins, to prevent recursion from the import. If the loader inserted a module and the load fails, it must be removed by the loader from `sys.modules`; modules already in `sys.modules` before the loader began execution should be left alone (see `importlib.util.module_for_loader()`).

    The loader should set several attributes on the module. (Note that some of these attributes can change when a module is reloaded):

    - `__name__`

        The name of the module.

    - `__file__`

        The path to where the module data is stored (not set for built-in modules).

    - `__cached__`

        The path to where a compiled version of the module is/should be stored (not set when the attribute would be inappropriate).

    - `__path__`

        A list of strings specifying the search path within a package. This attribute is not set on modules.

    - `__package__`

        The parent package for the module/package. If the module is top-level then it has a value of the empty string. The `importlib.util.module_for_loader()` decorator can handle the details for `__package__`.

    - `__loader__`

        The loader used to load the module. The `importlib.util.module_for_loader()` decorator can handle the details for `__package__`.

    When `exec_module()` is available then backwards-compatible functionality is provided.

    3.4 版更變: Raise `ImportError` when called instead of `NotImplementedError`. Functionality provided when `exec_module()` is available.

    3.4 版後已棄用: The recommended API for loading a module is `exec_module()` (and `create_module()`). Loaders should implement it instead of load_module(). The import machinery takes care of all the other responsibilities of load_module() when exec_module() is implemented.

  `module_repr`(module)

    A legacy method which when implemented calculates and returns the given module's repr, as a string. The module type's default repr() will use the result of this method as appropriate.

    3.3 版新加入.

    3.4 版更變: Made optional instead of an abstractmethod.

    3.4 版後已棄用: The import machinery now takes care of this automatically.

class `importlib.abc.``ResourceReader`

  An abstract base class to provide the ability to read resources.

  From the perspective of this ABC, a resource is a binary artifact that is shipped within a package. Typically this is something like a data file that lives next to the `__init__.py` file of the package. The purpose of this class is to help abstract out the accessing of such data files so that it does not matter if the package and its data file(s) are stored in a e.g. zip file versus on the file system.

  For any of methods of this class, a resource argument is expected to be a path-like object which represents conceptually just a file name. This means that no subdirectory paths should be included in the resource argument. This is because the location of the package the reader is for, acts as the "directory". Hence the metaphor for directories and file names is packages and resources, respectively. This is also why instances of this class are expected to directly correlate to a specific package (instead of potentially representing multiple packages or a module).

  想支持资源读取的加载器需要提供一个返回实现了此 ABC 的接口的 `get_resource_reader(fullname)` 方法。如果通过全名指定的模块不是一个包，这个方法应该返回 `None`。 当指定的模块是一个包时，应该只返回一个与这个抽象类ABC兼容的对象。

  3.7 版新加入.

  abstractmethod `open_resource`(resource)

    Returns an opened, file-like object for binary reading of the resource.

    If the resource cannot be found, `FileNotFoundError` is raised.

  abstractmethod `resource_path`(resource)

    Returns the file system path to the resource.

    If the resource does not concretely exist on the file system, raise `FileNotFoundError`.

  abstractmethod `is_resource`(name)

    Returns `True` if the named name is considered a resource. `FileNotFoundError` is raised if name does not exist.

  abstractmethod `contents`()

    Returns an iterable of strings over the contents of the package. Do note that it is not required that all names returned by the iterator be actual resources, e.g. it is acceptable to return names for which `is_resource()` would be false.

    Allowing non-resource names to be returned is to allow for situations where how a package and its resources are stored are known a priori and the non-resource names would be useful. For instance, returning subdirectory names is allowed so that when it is known that the package and resources are stored on the file system then those subdirectory names can be used directly.

    The abstract method returns an iterable of no items.

class `importlib.abc.``ResourceLoader`

  An abstract base class for a loader which implements the optional PEP 302 protocol for loading arbitrary resources from the storage back-end.

  3.7 版後已棄用: This ABC is deprecated in favour of supporting resource loading through `importlib.abc.ResourceReader`.

  abstractmethod `get_data`(path)

    An abstract method to return the bytes for the data located at path. Loaders that have a file-like storage back-end that allows storing arbitrary data can implement this abstract method to give direct access to the data stored. `OSError` is to be raised if the path cannot be found. The path is expected to be constructed using a module's `__file__` attribute or an item from a package's `__path__`.

    3.4 版更變: Raises `OSError` instead of `NotImplementedError`.

class `importlib.abc.``InspectLoader`

  An abstract base class for a loader which implements the optional PEP 302 protocol for loaders that inspect modules.

  `get_code`(fullname)

    Return the code object for a module, or `None` if the module does not have a code object (as would be the case, for example, for a built-in module). Raise an `ImportError` if loader cannot find the requested module.

    備註

    While the method has a default implementation, it is suggested that it be overridden if possible for performance.

    3.4 版更變: No longer abstract and a concrete implementation is provided.

  abstractmethod `get_source`(fullname)

    An abstract method to return the source of a module. It is returned as a text string using universal newlines, translating all recognized line separators into `'\n'` characters. Returns `None` if no source is available (e.g. a built-in module). Raises `ImportError` if the loader cannot find the module specified.

    3.4 版更變: Raises `ImportError` instead of `NotImplementedError`.

  `is_package`(fullname)

    An abstract method to return a true value if the module is a package, a false value otherwise. `ImportError` is raised if the loader cannot find the module.

    3.4 版更變: Raises `ImportError` instead of `NotImplementedError`.

  static `source_to_code`(data, path='<string>')

    Create a code object from Python source.

    The data argument can be whatever the `compile()` function supports (i.e. string or bytes). The path argument should be the "path" to where the source code originated from, which can be an abstract concept (e.g. location in a zip file).

    With the subsequent code object one can execute it in a module by running `exec(code, module.__dict__)`.

    3.4 版新加入.

    3.5 版更變: Made the method static.

  `exec_module`(module)

    Implementation of `Loader.exec_module()`.

    3.4 版新加入.

  `load_module`(fullname)

    Implementation of `Loader.load_module()`.

    3.4 版後已棄用: use `exec_module()` instead.

class `importlib.abc.``ExecutionLoader`

  An abstract base class which inherits from `InspectLoader` that, when implemented, helps a module to be executed as a script. The ABC represents an optional PEP 302 protocol.

  abstractmethod `get_filename`(fullname)

    An abstract method that is to return the value of `__file__` for the specified module. If no path is available, `ImportError` is raised.

    If source code is available, then the method should return the path to the source file, regardless of whether a bytecode was used to load the module.

    3.4 版更變: Raises `ImportError` instead of `NotImplementedError`.

class `importlib.abc.``FileLoader`(fullname, path)

  An abstract base class which inherits from `ResourceLoader` and `ExecutionLoader`, providing concrete implementations of `ResourceLoader.get_data()` and `ExecutionLoader.get_filename()`.

  The fullname argument is a fully resolved name of the module the loader is to handle. The path argument is the path to the file for the module.

  3.3 版新加入.

  `name`

    The name of the module the loader can handle.

  `path`

    Path to the file of the module.

  `load_module`(fullname)

    Calls super's `load_module()`.

    3.4 版後已棄用: Use `Loader.exec_module()` instead.

  abstractmethod `get_filename`(fullname)

    Returns `path`.

  abstractmethod `get_data`(path)

    Reads path as a binary file and returns the bytes from it.

class `importlib.abc.``SourceLoader`

  An abstract base class for implementing source (and optionally bytecode) file loading. The class inherits from both `ResourceLoader` and `ExecutionLoader`, requiring the implementation of:

  - `ResourceLoader.get_data()`

  - `ExecutionLoader.get_filename()`

      Should only return the path to the source file; sourceless loading is not supported.

  The abstract methods defined by this class are to add optional bytecode file support. Not implementing these optional methods (or causing them to raise `NotImplementedError`) causes the loader to only work with source code. Implementing the methods allows the loader to work with source and bytecode files; it does not allow for sourceless loading where only bytecode is provided. Bytecode files are an optimization to speed up loading by removing the parsing step of Python's compiler, and so no bytecode-specific API is exposed.

  `path_stats`(path)

    Optional abstract method which returns a `dict` containing metadata about the specified path. Supported dictionary keys are:

    - `'mtime'` (mandatory): an integer or floating-point number representing the modification time of the source code;

    - `'size'` (optional): the size in bytes of the source code.

    Any other keys in the dictionary are ignored, to allow for future extensions. If the path cannot be handled, `OSError` is raised.

    3.3 版新加入.

    3.4 版更變: Raise `OSError` instead of `NotImplementedError`.

  `path_mtime`(path)

    Optional abstract method which returns the modification time for the specified path.

    3.3 版後已棄用: This method is deprecated in favour of `path_stats()`. You don't have to implement it, but it is still available for compatibility purposes. Raise `OSError` if the path cannot be handled.

    3.4 版更變: Raise `OSError` instead of `NotImplementedError`.

  `set_data`(path, data)

    Optional abstract method which writes the specified bytes to a file path. Any intermediate directories which do not exist are to be created automatically.

    When writing to the path fails because the path is read-only (`errno.EACCES`/`PermissionError`), do not propagate the exception.

    3.4 版更變: No longer raises `NotImplementedError` when called.

  `get_code`(fullname)

    Concrete implementation of `InspectLoader.get_code()`.

  `exec_module`(module)

    Concrete implementation of `Loader.exec_module()`.

    3.4 版新加入.

  `load_module`(fullname)

    Concrete implementation of `Loader.load_module()`.

    3.4 版後已棄用: Use `exec_module()` instead.

  `get_source`(fullname)

    Concrete implementation of `InspectLoader.get_source()`.

  `is_package`(fullname)

    Concrete implementation of `InspectLoader.is_package()`. A module is determined to be a package if its file path (as provided by `ExecutionLoader.get_filename()`) is a file named `__init__` when the file extension is removed and the module name itself does not end in `__init__`.

## `importlib.resources` -- Resources

Source code: Lib/importlib/resources.py

3.7 版新加入.

This module leverages Python's import system to provide access to resources within packages. If you can import a package, you can access resources within that package. Resources can be opened or read, in either binary or text mode.

Resources are roughly akin to files inside directories, though it's important to keep in mind that this is just a metaphor. Resources and packages do not have to exist as physical files and directories on the file system.

備註

This module provides functionality similar to pkg_resources Basic Resource Access without the performance overhead of that package. This makes reading resources included in packages easier, with more stable and consistent semantics.

The standalone backport of this module provides more information on using importlib.resources and migrating from pkg_resources to importlib.resources.

Loaders that wish to support resource reading should implement a `get_resource_reader(fullname)` method as specified by `importlib.abc.ResourceReader`.

The following types are defined.

`importlib.resources.``Package`

  The `Package` type is defined as `Union[str, ModuleType]`. This means that where the function describes accepting a `Package`, you can pass in either a string or a module. Module objects must have a resolvable `__spec__.submodule_search_locations` that is not `None`.

`importlib.resources.``Resource`

  This type describes the resource names passed into the various functions in this package. This is defined as `Union[str, os.PathLike]`.

The following functions are available.

`importlib.resources.``open_binary`(package, resource)

  Open for binary reading the resource within package.

  package is either a name or a module object which conforms to the `Package` requirements. resource is the name of the resource to open within package; it may not contain path separators and it may not have sub-resources (i.e. it cannot be a directory). This function returns a `typing.BinaryIO` instance, a binary I/O stream open for reading.

`importlib.resources.``open_text`(package, resource, encoding='utf-8', errors='strict')

  Open for text reading the resource within package. By default, the resource is opened for reading as UTF-8.

  package is either a name or a module object which conforms to the `Package` requirements. resource is the name of the resource to open within package; it may not contain path separators and it may not have sub-resources (i.e. it cannot be a directory). encoding and errors have the same meaning as with built-in `open()`.

  This function returns a `typing.TextIO` instance, a text I/O stream open for reading.

`importlib.resources.``read_binary`(package, resource)

  Read and return the contents of the resource within package as `bytes`.

  package is either a name or a module object which conforms to the `Package` requirements. resource is the name of the resource to open within package; it may not contain path separators and it may not have sub-resources (i.e. it cannot be a directory). This function returns the contents of the resource as `bytes`.

`importlib.resources.``read_text`(package, resource, encoding='utf-8', errors='strict')

  Read and return the contents of resource within package as a `str`. By default, the contents are read as strict UTF-8.

  package is either a name or a module object which conforms to the `Package` requirements. resource is the name of the resource to open within package; it may not contain path separators and it may not have sub-resources (i.e. it cannot be a directory). encoding and errors have the same meaning as with built-in `open()`. This function returns the contents of the resource as `str`.

`importlib.resources.``path`(package, resource)

  Return the path to the resource as an actual file system path. This function returns a context manager for use in a `with` statement. The context manager provides a `pathlib.Path` object.

  Exiting the context manager cleans up any temporary file created when the resource needs to be extracted from e.g. a zip file.

  package is either a name or a module object which conforms to the `Package` requirements. resource is the name of the resource to open within package; it may not contain path separators and it may not have sub-resources (i.e. it cannot be a directory).

`importlib.resources.``is_resource`(package, name)

  Return `True` if there is a resource named name in the package, otherwise `False`. Remember that directories are not resources! package is either a name or a module object which conforms to the `Package` requirements.

`importlib.resources.``contents`(package)

  Return an iterable over the named items within the package. The iterable returns `str` resources (e.g. files) and non-resources (e.g. directories). The iterable does not recurse into subdirectories.

  package is either a name or a module object which conforms to the `Package` requirements.

## `importlib.machinery` -- Importers and path hooks

Source code: Lib/importlib/machinery.py

This module contains the various objects that help `import` find and load modules.

`importlib.machinery.``SOURCE_SUFFIXES`

  A list of strings representing the recognized file suffixes for source modules.

  3.3 版新加入.

`importlib.machinery.``DEBUG_BYTECODE_SUFFIXES`

  A list of strings representing the file suffixes for non-optimized bytecode modules.

  3.3 版新加入.

  3.5 版後已棄用: Use `BYTECODE_SUFFIXES` instead.

`importlib.machinery.``OPTIMIZED_BYTECODE_SUFFIXES`

  A list of strings representing the file suffixes for optimized bytecode modules.

  3.3 版新加入.

  3.5 版後已棄用: Use `BYTECODE_SUFFIXES` instead.

`importlib.machinery.``BYTECODE_SUFFIXES`

  A list of strings representing the recognized file suffixes for bytecode modules (including the leading dot).

  3.3 版新加入.

  3.5 版更變: The value is no longer dependent on `__debug__`.

`importlib.machinery.``EXTENSION_SUFFIXES`

  A list of strings representing the recognized file suffixes for extension modules.

  3.3 版新加入.

`importlib.machinery.``all_suffixes`()

  Returns a combined list of strings representing all file suffixes for modules recognized by the standard import machinery. This is a helper for code which simply needs to know if a filesystem path potentially refers to a module without needing any details on the kind of module (for example, `inspect.getmodulename()`).

  3.3 版新加入.

class `importlib.machinery.``BuiltinImporter`

  An importer for built-in modules. All known built-in modules are listed in `sys.builtin_module_names`. This class implements the `importlib.abc.MetaPathFinder` and `importlib.abc.InspectLoader` ABCs.

  Only class methods are defined by this class to alleviate the need for instantiation.

  3.5 版更變: As part of PEP 489, the builtin importer now implements `Loader.create_module()` and `Loader.exec_module()`

class `importlib.machinery.``FrozenImporter`

  An importer for frozen modules. This class implements the `importlib.abc.MetaPathFinder` and `importlib.abc.InspectLoader` ABCs.

  Only class methods are defined by this class to alleviate the need for instantiation.

class `importlib.machinery.``WindowsRegistryFinder`

  Finder for modules declared in the Windows registry. This class implements the `importlib.abc.MetaPathFinder` ABC.

  Only class methods are defined by this class to alleviate the need for instantiation.

  3.3 版新加入.

  3.6 版後已棄用: Use `site` configuration instead. Future versions of Python may not enable this finder by default.

class `importlib.machinery.``PathFinder`

  A Finder for `sys.path` and package `__path__` attributes. This class implements the `importlib.abc.MetaPathFinder` ABC.

  Only class methods are defined by this class to alleviate the need for instantiation.

  classmethod `find_spec`(fullname, path=None, target=None)

    Class method that attempts to find a spec for the module specified by fullname on `sys.path` or, if defined, on path. For each path entry that is searched, `sys.path_importer_cache` is checked. If a non-false object is found then it is used as the path entry finder to look for the module being searched for. If no entry is found in `sys.path_importer_cache`, then `sys.path_hooks` is searched for a finder for the path entry and, if found, is stored in `sys.path_importer_cache` along with being queried about the module. If no finder is ever found then `None` is both stored in the cache and returned.

    3.4 版新加入.

    3.5 版更變: If the current working directory -- represented by an empty string -- is no longer valid then `None` is returned but no value is cached in `sys.path_importer_cache`.

  classmethod `find_module`(fullname, path=None)

    A legacy wrapper around `find_spec()`.

    3.4 版後已棄用: Use `find_spec()` instead.

  classmethod `invalidate_caches`()

    Calls `importlib.abc.PathEntryFinder.invalidate_caches()` on all finders stored in `sys.path_importer_cache` that define the method. Otherwise entries in `sys.path_importer_cache` set to `None` are deleted.

    3.7 版更變: Entries of `None` in `sys.path_importer_cache` are deleted.

  3.4 版更變: Calls objects in `sys.path_hooks` with the current working directory for `''` (i.e. the empty string).

class `importlib.machinery.``FileFinder`(path, *loader_details)

  A concrete implementation of `importlib.abc.PathEntryFinder` which caches results from the file system.

  The path argument is the directory for which the finder is in charge of searching.

  The loader_details argument is a variable number of 2-item tuples each containing a loader and a sequence of file suffixes the loader recognizes. The loaders are expected to be callables which accept two arguments of the module's name and the path to the file found.

  The finder will cache the directory contents as necessary, making stat calls for each module search to verify the cache is not outdated. Because cache staleness relies upon the granularity of the operating system's state information of the file system, there is a potential race condition of searching for a module, creating a new file, and then searching for the module the new file represents. If the operations happen fast enough to fit within the granularity of stat calls, then the module search will fail. To prevent this from happening, when you create a module dynamically, make sure to call `importlib.invalidate_caches()`.

  3.3 版新加入.

  `path`

    The path the finder will search in.

  `find_spec`(fullname, target=None)

    Attempt to find the spec to handle fullname within `path`.

    3.4 版新加入.

  `find_loader`(fullname)

    Attempt to find the loader to handle fullname within `path`.

  `invalidate_caches`()

    Clear out the internal cache.

  classmethod `path_hook`(*loader_details)

    A class method which returns a closure for use on `sys.path_hooks`. An instance of `FileFinder` is returned by the closure using the path argument given to the closure directly and loader_details indirectly.

    If the argument to the closure is not an existing directory, `ImportError` is raised.

class `importlib.machinery.``SourceFileLoader`(fullname, path)

  A concrete implementation of `importlib.abc.SourceLoader` by subclassing `importlib.abc.FileLoader` and providing some concrete implementations of other methods.

  3.3 版新加入.

  `name`

    The name of the module that this loader will handle.

  `path`

    The path to the source file.

  `is_package`(fullname)

    Return true if `path` appears to be for a package.

  `path_stats`(path)

    Concrete implementation of `importlib.abc.SourceLoader.path_stats()`.

  `set_data`(path, data)

    Concrete implementation of `importlib.abc.SourceLoader.set_data()`.

  `load_module`(name=None)

    Concrete implementation of `importlib.abc.Loader.load_module()` where specifying the name of the module to load is optional.

    3.6 版後已棄用: Use `importlib.abc.Loader.exec_module()` instead.

class `importlib.machinery.``SourcelessFileLoader`(fullname, path)

  A concrete implementation of `importlib.abc.FileLoader` which can import bytecode files (i.e. no source code files exist).

  Please note that direct use of bytecode files (and thus not source code files) inhibits your modules from being usable by all Python implementations or new versions of Python which change the bytecode format.

  3.3 版新加入.

  `name`

    The name of the module the loader will handle.

  `path`

    The path to the bytecode file.

  `is_package`(fullname)

    Determines if the module is a package based on `path`.

  `get_code`(fullname)

    Returns the code object for `name` created from `path`.

  `get_source`(fullname)

    Returns `None` as bytecode files have no source when this loader is used.

  `load_module`(name=None)

  Concrete implementation of `importlib.abc.Loader.load_module()` where specifying the name of the module to load is optional.

  3.6 版後已棄用: Use `importlib.abc.Loader.exec_module()` instead.

class `importlib.machinery.``ExtensionFileLoader`(fullname, path)

  A concrete implementation of `importlib.abc.ExecutionLoader` for extension modules.

  The fullname argument specifies the name of the module the loader is to support. The path argument is the path to the extension module's file.

  3.3 版新加入.

  `name`

    Name of the module the loader supports.

  `path`

    Path to the extension module.

  `create_module`(spec)

    Creates the module object from the given specification in accordance with PEP 489.

    3.5 版新加入.

  `exec_module`(module)

    Initializes the given module object in accordance with PEP 489.

    3.5 版新加入.

  `is_package`(fullname)

    Returns `True` if the file path points to a package's `__init__` module based on `EXTENSION_SUFFIXES`.

  `get_code`(fullname)

    Returns `None` as extension modules lack a code object.

  `get_source`(fullname)

    Returns `None` as extension modules do not have source code.

  `get_filename`(fullname)

    Returns `path`.

    3.4 版新加入.

class `importlib.machinery.``ModuleSpec`(name, loader, *, origin=None, loader_state=None, is_package=None)

  A specification for a module's import-system-related state. This is typically exposed as the module's `__spec__` attribute. In the descriptions below, the names in parentheses give the corresponding attribute available directly on the module object. E.g. `module.__spec__.origin == module.__file__`. Note however that while the values are usually equivalent, they can differ since there is no synchronization between the two objects. Thus it is possible to update the module's `__path__` at runtime, and this will not be automatically reflected in `__spec__.submodule_search_locations`.

  3.4 版新加入.

  `name`

  (`__name__`)

  A string for the fully-qualified name of the module.

  `loader`

  (`__loader__`)

  The loader to use for loading. For namespace packages this should be set to `None`.

  `origin`

  (`__file__`)

  Name of the place from which the module is loaded, e.g. "builtin" for built-in modules and the filename for modules loaded from source. Normally "origin" should be set, but it may be `None` (the default) which indicates it is unspecified (e.g. for namespace packages).

  `submodule_search_locations`

  (`__path__`)

  List of strings for where to find submodules, if a package (`None` otherwise).

  `loader_state`

  Container of extra module-specific data for use during loading (or `None`).

  `cached`

  (`__cached__`)

  String for where the compiled module should be stored (or `None`).

  `parent`

  (`__package__`)

  (Read-only) Fully-qualified name of the package to which the module belongs as a submodule (or `None`).

  `has_location`

  Boolean indicating whether or not the module's "origin" attribute refers to a loadable location.

## `importlib.util` -- Utility code for importers

Source code: Lib/importlib/util.py

This module contains the various objects that help in the construction of an importer.

`importlib.util.``MAGIC_NUMBER`

  The bytes which represent the bytecode version number. If you need help with loading/writing bytecode then consider `importlib.abc.SourceLoader`.

  3.4 版新加入.

`importlib.util.``cache_from_source`(path, debug_override=None, *, optimization=None)

  Return the PEP 3147/PEP 488 path to the byte-compiled file associated with the source path. For example, if path is `/foo/bar/baz.py` the return value would be `/foo/bar/__pycache__/baz.cpython-32.pyc` for Python 3.2. The `cpython-32` string comes from the current magic tag (see `get_tag()`; if `sys.implementation.cache_tag` is not defined then `NotImplementedError` will be raised).

  The optimization parameter is used to specify the optimization level of the bytecode file. An empty string represents no optimization, so `/foo/bar/baz.py` with an optimization of `''` will result in a bytecode path of `/foo/bar/__pycache__/baz.cpython-32.pyc`. `None` causes the interpter's optimization level to be used. Any other value's string representation being used, so `/foo/bar/baz.py` with an optimization of `2` will lead to the bytecode path of `/foo/bar/__pycache__/baz.cpython-32.opt-2.pyc`. The string representation of optimization can only be alphanumeric, else `ValueError` is raised.

  The debug_override parameter is deprecated and can be used to override the system's value for `__debug__`. A `True` value is the equivalent of setting optimization to the empty string. A `False` value is the same as setting optimization to `1`. If both debug_override an optimization are not `None` then `TypeError` is raised.

  3.4 版新加入.

  3.5 版更變: The optimization parameter was added and the debug_override parameter was deprecated.

  3.6 版更變: Accepts a path-like object.

`importlib.util.``source_from_cache`(path)

  Given the path to a PEP 3147 file name, return the associated source code file path. For example, if path is `/foo/bar/__pycache__/baz.cpython-32.pyc` the returned path would be `/foo/bar/baz.py`. path need not exist, however if it does not conform to PEP 3147 or PEP 488 format, a `ValueError` is raised. If `sys.implementation.cache_tag` is not defined, `NotImplementedError` is raised.

  3.4 版新加入.

  3.6 版更變: Accepts a path-like object.

`importlib.util.``decode_source`(source_bytes)

  Decode the given bytes representing source code and return it as a string with universal newlines (as required by `importlib.abc.InspectLoader.get_source()`).

  3.4 版新加入.

`importlib.util.``resolve_name`(name, package)

  Resolve a relative module name to an absolute one.

  If name has no leading dots, then name is simply returned. This allows for usage such as `importlib.util.resolve_name('sys', __package__)` without doing a check to see if the package argument is needed.

  `ValueError` is raised if name is a relative module name but package is a false value (e.g. `None` or the empty string). `ValueError` is also raised a relative name would escape its containing package (e.g. requesting `..bacon` from within the `spam` package).

  3.3 版新加入.

`importlib.util.``find_spec`(name, package=None)

  Find the spec for a module, optionally relative to the specified package name. If the module is in `sys.modules`, then `sys.modules[name].__spec__` is returned (unless the spec would be `None` or is not set, in which case `ValueError` is raised). Otherwise a search using `sys.meta_path` is done. `None` is returned if no spec is found.

  If name is for a submodule (contains a dot), the parent module is automatically imported.

  name and package work the same as for `import_module()`.

  3.4 版新加入.

  3.7 版更變: Raises `ModuleNotFoundError` instead of `AttributeError` if package is in fact not a package (i.e. lacks a `__path__` attribute).

`importlib.util.``module_from_spec`(spec)

  Create a new module based on spec and `spec.loader.create_module`.

  If `spec.loader.create_module` does not return `None`, then any pre-existing attributes will not be reset. Also, no `AttributeError` will be raised if triggered while accessing spec or setting an attribute on the module.

  This function is preferred over using `types.ModuleType` to create a new module as spec is used to set as many import-controlled attributes on the module as possible.

  3.5 版新加入.

`@``importlib.util.``module_for_loader`

  A decorator for `importlib.abc.Loader.load_module()` to handle selecting the proper module object to load with. The decorated method is expected to have a call signature taking two positional arguments (e.g. `load_module(self, module)`) for which the second argument will be the module object to be used by the loader. Note that the decorator will not work on static methods because of the assumption of two arguments.

  The decorated method will take in the name of the module to be loaded as expected for a loader. If the module is not found in `sys.modules` then a new one is constructed. Regardless of where the module came from, `__loader__` set to self and `__package__` is set based on what `importlib.abc.InspectLoader.is_package()` returns (if available). These attributes are set unconditionally to support reloading.

  If an exception is raised by the decorated method and a module was added to `sys.modules`, then the module will be removed to prevent a partially initialized module from being in left in `sys.modules`. If the module was already in `sys.modules` then it is left alone.

  3.3 版更變: `__loader__` and `__package__` are automatically set (when possible).

  3.4 版更變: Set `__name__`, `__loader__` `__package__` unconditionally to support reloading.

  3.4 版後已棄用: The import machinery now directly performs all the functionality provided by this function.

`@``importlib.util.``set_loader`

  A decorator for `importlib.abc.Loader.load_module()` to set the `__loader__` attribute on the returned module. If the attribute is already set the decorator does nothing. It is assumed that the first positional argument to the wrapped method (i.e. `self`) is what `__loader__` should be set to.

  3.4 版更變: Set `__loader__` if set to `None`, as if the attribute does not exist.

  3.4 版後已棄用: The import machinery takes care of this automatically.

`@``importlib.util.``set_package`

  A decorator for `importlib.abc.Loader.load_module()` to set the `__package__` attribute on the returned module. If `__package__` is set and has a value other than `None` it will not be changed.

  3.4 版後已棄用: The import machinery takes care of this automatically.

`importlib.util.``spec_from_loader`(name, loader, *, origin=None, is_package=None)

  A factory function for creating a `ModuleSpec` instance based on a loader. The parameters have the same meaning as they do for ModuleSpec. The function uses available loader APIs, such as `InspectLoader.is_package()`, to fill in any missing information on the spec.

  3.4 版新加入.

`importlib.util.``spec_from_file_location`(name, location, *, loader=None, submodule_search_locations=None)

  A factory function for creating a `ModuleSpec` instance based on the path to a file. Missing information will be filled in on the spec by making use of loader APIs and by the implication that the module will be file-based.

  3.4 版新加入.

  3.6 版更變: Accepts a path-like object.

`importlib.util.``source_hash`(source_bytes)

  Return the hash of source_bytes as bytes. A hash-based `.pyc` file embeds the `source_hash()` of the corresponding source file's contents in its header.

  3.7 版新加入.

class `importlib.util.``LazyLoader`(loader)

  A class which postpones the execution of the loader of a module until the module has an attribute accessed.

  This class only works with loaders that define `exec_module()` as control over what module type is used for the module is required. For those same reasons, the loader's `create_module()` method must return `None` or a type for which its `__class__` attribute can be mutated along with not using slots. Finally, modules which substitute the object placed into `sys.modules` will not work as there is no way to properly replace the module references throughout the interpreter safely; `ValueError` is raised if such a substitution is detected.

  備註

  For projects where startup time is critical, this class allows for potentially minimizing the cost of loading a module if it is never used. For projects where startup time is not essential then use of this class is heavily discouraged due to error messages created during loading being postponed and thus occurring out of context.

  3.5 版新加入.

  3.6 版更變: Began calling `create_module()`, removing the compatibility warning for `importlib.machinery.BuiltinImporter` and `importlib.machinery.ExtensionFileLoader`.

  classmethod `factory`(loader)

    A static method which returns a callable that creates a lazy loader. This is meant to be used in situations where the loader is passed by class instead of by instance.

        suffixes = importlib.machinery.SOURCE_SUFFIXES
        loader = importlib.machinery.SourceFileLoader
        lazy_loader = importlib.util.LazyLoader.factory(loader)
        finder = importlib.machinery.FileFinder(path, (lazy_loader, suffixes))

## Examples

### Importing programmatically

To programmatically import a module, use `importlib.import_module()`.

    import importlib

    itertools = importlib.import_module('itertools')

### Checking if a module can be imported

If you need to find out if a module can be imported without actually doing the import, then you should use `importlib.util.find_spec()`.

    import importlib.util
    import sys

    # For illustrative purposes.
    name = 'itertools'

    if name in sys.modules:
        print(f"{name!r} already in sys.modules")
    elif (spec := importlib.util.find_spec(name)) is not None:
        # If you chose to perform the actual import ...
        module = importlib.util.module_from_spec(spec)
        sys.modules[name] = module
        spec.loader.exec_module(module)
        print(f"{name!r} has been imported")
    else:
        print(f"can't find the {name!r} module")

### Importing a source file directly

To import a Python source file directly, use the following recipe (Python 3.5 and newer only):

    import importlib.util
    import sys

    # For illustrative purposes.
    import tokenize
    file_path = tokenize.__file__
    module_name = tokenize.__name__

    spec = importlib.util.spec_from_file_location(module_name, file_path)
    module = importlib.util.module_from_spec(spec)
    sys.modules[module_name] = module
    spec.loader.exec_module(module)

### Setting up an importer

For deep customizations of import, you typically want to implement an importer. This means managing both the finder and loader side of things. For finders there are two flavours to choose from depending on your needs: a meta path finder or a path entry finder. The former is what you would put on `sys.meta_path` while the latter is what you create using a path entry hook on `sys.path_hooks` which works with `sys.path` entries to potentially create a finder. This example will show you how to register your own importers so that import will use them (for creating an importer for yourself, read the documentation for the appropriate classes defined within this package):

    import importlib.machinery
    import sys

    # For illustrative purposes only.
    SpamMetaPathFinder = importlib.machinery.PathFinder
    SpamPathEntryFinder = importlib.machinery.FileFinder
    loader_details = (importlib.machinery.SourceFileLoader,
                      importlib.machinery.SOURCE_SUFFIXES)

    # Setting up a meta path finder.
    # Make sure to put the finder in the proper location in the list in terms of
    # priority.
    sys.meta_path.append(SpamMetaPathFinder)

    # Setting up a path entry finder.
    # Make sure to put the path hook in the proper location in the list in terms
    # of priority.
    sys.path_hooks.append(SpamPathEntryFinder.path_hook(loader_details))

### Approximating `importlib.import_module()`

Import itself is implemented in Python code, making it possible to expose most of the import machinery through importlib. The following helps illustrate the various APIs that importlib exposes by providing an approximate implementation of `importlib.import_module()` (Python 3.4 and newer for the importlib usage, Python 3.6 and newer for other parts of the code).

    import importlib.util
    import sys

    def import_module(name, package=None):
        """An approximate implementation of import."""
        absolute_name = importlib.util.resolve_name(name, package)
        try:
            return sys.modules[absolute_name]
        except KeyError:
            pass

        path = None
        if '.' in absolute_name:
            parent_name, _, child_name = absolute_name.rpartition('.')
            parent_module = import_module(parent_name)
            path = parent_module.__spec__.submodule_search_locations
        for finder in sys.meta_path:
            spec = finder.find_spec(absolute_name, path)
            if spec is not None:
                break
        else:
            msg = f'No module named {absolute_name!r}'
            raise ModuleNotFoundError(msg, name=absolute_name)
        module = importlib.util.module_from_spec(spec)
        sys.modules[absolute_name] = module
        spec.loader.exec_module(module)
        if path is not None:
            setattr(parent_module, child_name, module)
        return module