5. 导入系统 ¶

5.2.1. Regular packages ¶

Python defines two types of packages, regular packages and namespace packages . Regular packages are traditional packages as they existed in Python 3.2 and earlier. A regular package is typically implemented as a directory containing an __init__.py file. When a regular package is imported, this __init__.py file is implicitly executed, and the objects it defines are bound to names in the package’s namespace. The __init__.py file can contain the same Python code that any other module can contain, and Python will add some additional attributes to the module when it is imported.

For example, the following file system layout defines a top level parent package with three subpackages:

parent/
    __init__.py
    one/
        __init__.py
    two/
        __init__.py
    three/
        __init__.py
									

导入 parent.one will implicitly execute parent/__init__.py and parent/one/__init__.py . Subsequent imports of parent.two or parent.three will execute parent/two/__init__.py and parent/three/__init__.py 分别。

5.2.2. Namespace packages ¶

A namespace package is a composite of various portions , where each portion contributes a subpackage to the parent package. Portions may reside in different locations on the file system. Portions may also be found in zip files, on the network, or anywhere else that Python searches during import. Namespace packages may or may not correspond directly to objects on the file system; they may be virtual modules that have no concrete representation.

Namespace packages do not use an ordinary list for their __path__ attribute. They instead use a custom iterable type which will automatically perform a new search for package portions on the next import attempt within that package if the path of their parent package (or sys.path for a top level package) changes.

With namespace packages, there is no parent/__init__.py file. In fact, there may be multiple parent directories found during import search, where each one is provided by a different portion. Thus parent/one may not be physically located next to parent/two . In this case, Python will create a namespace package for the top-level parent package whenever it or one of its subpackages is imported.

另请参阅 PEP 420 for the namespace package specification.

5.3.1. The module cache ¶

The first place checked during import search is sys.modules . This mapping serves as a cache of all modules that have been previously imported, including the intermediate paths. So if foo.bar.baz was previously imported, sys.modules will contain entries for foo , foo.bar ，和 foo.bar.baz . Each key will have as its value the corresponding module object.

During import, the module name is looked up in sys.modules and if present, the associated value is the module satisfying the import, and the process completes. However, if the value is None , then a ModuleNotFoundError is raised. If the module name is missing, Python will continue searching for the module.

sys.modules is writable. Deleting a key may not destroy the associated module (as other modules may hold references to it), but it will invalidate the cache entry for the named module, causing Python to search anew for the named module upon its next import. The key can also be assigned to None , forcing the next import of the module to result in a ModuleNotFoundError .

Beware though, as if you keep a reference to the module object, invalidate its cache entry in sys.modules , and then re-import the named module, the two module objects will not be the same. By contrast, importlib.reload() will reuse the same module object, and simply reinitialise the module contents by rerunning the module’s code.

5.3.2. Finders and loaders ¶

If the named module is not found in sys.modules , then Python’s import protocol is invoked to find and load the module. This protocol consists of two conceptual objects, finders and loaders . A finder’s job is to determine whether it can find the named module using whatever strategy it knows about. Objects that implement both of these interfaces are referred to as importers - they return themselves when they find that they can load the requested module.

Python includes a number of default finders and importers. The first one knows how to locate built-in modules, and the second knows how to locate frozen modules. A third default finder searches an 导入路径 for modules. The 导入路径 is a list of locations that may name file system paths or zip files. It can also be extended to search for any locatable resource, such as those identified by URLs.

The import machinery is extensible, so new finders can be added to extend the range and scope of module searching.

Finders do not actually load modules. If they can find the named module, they return a module spec , an encapsulation of the module’s import-related information, which the import machinery then uses when loading the module.

The following sections describe the protocol for finders and loaders in more detail, including how you can create and register new ones to extend the import machinery.

5.3.3. Import hooks ¶

The import machinery is designed to be extensible; the primary mechanism for this are the import hooks . There are two types of import hooks: meta hooks and import path hooks .

Meta hooks are called at the start of import processing, before any other import processing has occurred, other than sys.modules cache look up. This allows meta hooks to override sys.path processing, frozen modules, or even built-in modules. Meta hooks are registered by adding new finder objects to sys.meta_path , as described below.

Import path hooks are called as part of sys.path (或 package.__path__ ) processing, at the point where their associated path item is encountered. Import path hooks are registered by adding new callables to sys.path_hooks as described below.

5.3.4. The meta path ¶

When the named module is not found in sys.modules , Python next searches sys.meta_path , which contains a list of meta path finder objects. These finders are queried in order to see if they know how to handle the named module. Meta path finders must implement a method called find_spec() which takes three arguments: a name, an import path, and (optionally) a target module. The meta path finder can use any strategy it wants to determine whether it can handle the named module or not.

If the meta path finder knows how to handle the named module, it returns a spec object. If it cannot handle the named module, it returns None 。若 sys.meta_path processing reaches the end of its list without returning a spec, then a ModuleNotFoundError is raised. Any other exceptions raised are simply propagated up, aborting the import process.

find_spec() method of meta path finders is called with two or three arguments. The first is the fully qualified name of the module being imported, for example foo.bar.baz . The second argument is the path entries to use for the module search. For top-level modules, the second argument is None , but for submodules or subpackages, the second argument is the value of the parent package’s __path__ attribute. If the appropriate __path__ attribute cannot be accessed, a ModuleNotFoundError is raised. The third argument is an existing module object that will be the target of loading later. The import system passes in a target module only during reload.

The meta path may be traversed multiple times for a single import request. For example, assuming none of the modules involved has already been cached, importing foo.bar.baz will first perform a top level import, calling mpf.find_spec("foo", None, None) on each meta path finder ( mpf ). After foo has been imported, foo.bar will be imported by traversing the meta path a second time, calling mpf.find_spec("foo.bar", foo.__path__, None) . Once foo.bar has been imported, the final traversal will call mpf.find_spec("foo.bar.baz", foo.bar.__path__, None) .

Some meta path finders only support top level imports. These importers will always return None when anything other than None is passed as the second argument.

Python’s default sys.meta_path has three meta path finders, one that knows how to import built-in modules, one that knows how to import frozen modules, and one that knows how to import modules from an 导入路径 (i.e. the 基于路径的查找器 ).

5.4.1. Loaders ¶

Module loaders provide the critical function of loading: module execution. The import machinery calls the importlib.abc.Loader.exec_module() method with a single argument, the module object to execute. Any value returned from exec_module() 被忽略。

Loaders must satisfy the following requirements:

• If the module is a Python module (as opposed to a built-in module or a dynamically loaded extension), the loader should execute the module’s code in the module’s global name space ( module.__dict__ ).
• If the loader cannot execute the module, it should raise an ImportError , although any other exception raised during exec_module() will be propagated.

In many cases, the finder and loader can be the same object; in such cases the find_spec() method would just return a spec with the loader set to self .

Module loaders may opt in to creating the module object during loading by implementing a create_module() method. It takes one argument, the module spec, and returns the new module object to use during loading. create_module() does not need to set any attributes on the module object. If the method returns None , the import machinery will create the new module itself.

5.4.2. Submodules ¶

When a submodule is loaded using any mechanism (e.g. importlib APIs, the import or import-from statements, or built-in __import__() ) a binding is placed in the parent module’s namespace to the submodule object. For example, if package spam has a submodule foo , after importing spam.foo , spam will have an attribute foo which is bound to the submodule. Let’s say you have the following directory structure:

spam/
    __init__.py
    foo.py
    bar.py
									

and spam/__init__.py has the following lines in it:

from .foo import Foo
from .bar import Bar
									

then executing the following puts a name binding to foo and bar 在 spam 模块：

>>> import spam
>>> spam.foo
<module 'spam.foo' from '/tmp/imports/spam/foo.py'>
>>> spam.bar
<module 'spam.bar' from '/tmp/imports/spam/bar.py'>
									

Given Python’s familiar name binding rules this might seem surprising, but it’s actually a fundamental feature of the import system. The invariant holding is that if you have sys.modules['spam'] and sys.modules['spam.foo'] (as you would after the above import), the latter must appear as the foo attribute of the former.

5.4.3. Module spec ¶

The import machinery uses a variety of information about each module during import, especially before loading. Most of the information is common to all modules. The purpose of a module’s spec is to encapsulate this import-related information on a per-module basis.

Using a spec during import allows state to be transferred between import system components, e.g. between the finder that creates the module spec and the loader that executes it. Most importantly, it allows the import machinery to perform the boilerplate operations of loading, whereas without a module spec the loader had that responsibility.

The module’s spec is exposed as the __spec__ attribute on a module object. See ModuleSpec for details on the contents of the module spec.

5.4.4. Import-related module attributes ¶

The import machinery fills in these attributes on each module object during loading, based on the module’s spec, before the loader executes the module.

__name__ ¶

__name__ attribute must be set to the fully-qualified name of the module. This name is used to uniquely identify the module in the import system.

__loader__ ¶

__loader__ attribute must be set to the loader object that the import machinery used when loading the module. This is mostly for introspection, but can be used for additional loader-specific functionality, for example getting data associated with a loader.

__package__ ¶

模块的 __package__ attribute must be set. Its value must be a string, but it can be the same value as its __name__ . When the module is a package, its __package__ value should be set to its __name__ . When the module is not a package, __package__ should be set to the empty string for top-level modules, or for submodules, to the parent package’s name. See PEP 366 进一步了解细节。

This attribute is used instead of __name__ to calculate explicit relative imports for main modules, as defined in PEP 366 . It is expected to have the same value as __spec__.parent .

3.6 版改变： 值 __package__ is expected to be the same as __spec__.parent .

__spec__ ¶

__spec__ attribute must be set to the module spec that was used when importing the module. Setting __spec__ appropriately applies equally to modules initialized during interpreter startup . The one exception is __main__ ，其中 __spec__ is set to None in some cases .

当 __package__ is not defined, __spec__.parent is used as a fallback.

3.4 版新增。

3.6 版改变： __spec__.parent is used as a fallback when __package__ is not defined.

__path__ ¶

If the module is a package (either regular or namespace), the module object’s __path__ attribute must be set. The value must be iterable, but may be empty if __path__ has no further significance. If __path__ is not empty, it must produce strings when iterated over. More details on the semantics of __path__ are given below .

Non-package modules should not have a __path__ 属性。

__file__ ¶

__cached__ ¶

__file__ is optional. If set, this attribute’s value must be a string. The import system may opt to leave __file__ unset if it has no semantic meaning (e.g. a module loaded from a database).

若 __file__ is set, it may also be appropriate to set the __cached__ attribute which is the path to any compiled version of the code (e.g. 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 ).

It is also appropriate to set __cached__ when __file__ is not set. However, that scenario is quite atypical. Ultimately, the loader is what makes use of __file__ and/or __cached__ . So if a loader can load from a cached module but otherwise does not load from a file, that atypical scenario may be appropriate.

5.4.5. module.__path__ ¶

By definition, if a module has a __path__ attribute, it is a package.

A package’s __path__ attribute is used during imports of its subpackages. Within the import machinery, it functions much the same as sys.path , i.e. providing a list of locations to search for modules during import. However, __path__ is typically much more constrained than sys.path .

__path__ must be an iterable of strings, but it may be empty. The same rules used for sys.path also apply to a package’s __path__ ，和 sys.path_hooks (described below) are consulted when traversing a package’s __path__ .

A package’s __init__.py file may set or alter the package’s __path__ attribute, and this was typically the way namespace packages were implemented prior to PEP 420 . With the adoption of PEP 420 , namespace packages no longer need to supply __init__.py files containing only __path__ manipulation code; the import machinery automatically sets __path__ correctly for the namespace package.

5.4.6. Module reprs ¶

By default, all modules have a usable repr, however depending on the attributes set above, and in the module’s spec, you can more explicitly control the repr of module objects.

If the module has a spec ( __spec__ ), the import machinery will try to generate a repr from it. If that fails or there is no spec, the import system will craft a default repr using whatever information is available on the module. It will try to use the module.__name__ , module.__file__ ，和 module.__loader__ as input into the repr, with defaults for whatever information is missing.

Here are the exact rules used:

• If the module has a __spec__ attribute, the information in the spec is used to generate the repr. The “name”, “loader”, “origin”, and “has_location” attributes are consulted.
• If the module has a __file__ attribute, this is used as part of the module’s repr.
• If the module has no __file__ but does have a __loader__ that is not None , then the loader’s repr is used as part of the module’s repr.
• Otherwise, just use the module’s __name__ in the repr.

5.5.1. 路径条目查找器 ¶

基于路径的查找器 is responsible for finding and loading Python modules and packages whose location is specified with a string 路径条目 . Most path entries name locations in the file system, but they need not be limited to this.

As a meta path finder, the 基于路径的查找器 实现 find_spec() protocol previously described, however it exposes additional hooks that can be used to customize how modules are found and loaded from the 导入路径 .

Three variables are used by the 基于路径的查找器 , sys.path , sys.path_hooks and sys.path_importer_cache 。 __path__ attributes on package objects are also used. These provide additional ways that the import machinery can be customized.

sys.path contains a list of strings providing search locations for modules and packages. It is initialized from the PYTHONPATH environment variable and various other installation- and implementation-specific defaults. Entries in sys.path can name directories on the file system, zip files, and potentially other “locations” (see the site module) that should be searched for modules, such as URLs, or database queries. Only strings and bytes should be present on sys.path ; all other data types are ignored. The encoding of bytes entries is determined by the individual 路径条目查找器 .

基于路径的查找器 是 meta path finder , so the import machinery begins the 导入路径 search by calling the path based finder’s find_spec() method as described previously. When the path 自变量为 find_spec() is given, it will be a list of string paths to traverse - typically a package’s __path__ attribute for an import within that package. If the path 自变量为 None , this indicates a top level import and sys.path 被使用。

The path based finder iterates over every entry in the search path, and for each of these, looks for an appropriate path entry finder ( PathEntryFinder ) for the path entry. Because this can be an expensive operation (e.g. there may be stat() call overheads for this search), the path based finder maintains a cache mapping path entries to path entry finders. This cache is maintained in sys.path_importer_cache (despite the name, this cache actually stores finder objects rather than being limited to importer objects). In this way, the expensive search for a particular 路径条目 location’s path entry finder need only be done once. User code is free to remove cache entries from sys.path_importer_cache forcing the path based finder to perform the path entry search again [3] .

If the path entry is not present in the cache, the path based finder iterates over every callable in sys.path_hooks . Each of the path entry hooks in this list is called with a single argument, the path entry to be searched. This callable may either return a path entry finder that can handle the path entry, or it may raise ImportError . An ImportError is used by the path based finder to signal that the hook cannot find a path entry finder for that 路径条目 . The exception is ignored and 导入路径 iteration continues. The hook should expect either a string or bytes object; the encoding of bytes objects is up to the hook (e.g. it may be a file system encoding, UTF-8, or something else), and if the hook cannot decode the argument, it should raise ImportError .

若 sys.path_hooks iteration ends with no path entry finder being returned, then the path based finder’s find_spec() method will store None in sys.path_importer_cache (to indicate that there is no finder for this path entry) and return None , indicating that this meta path finder could not find the module.

若 path entry finder is returned by one of the path entry hook callables on sys.path_hooks , then the following protocol is used to ask the finder for a module spec, which is then used when loading the module.

The current working directory – denoted by an empty string – is handled slightly differently from other entries on sys.path . First, if the current working directory is found to not exist, no value is stored in sys.path_importer_cache . Second, the value for the current working directory is looked up fresh for each module lookup. Third, the path used for sys.path_importer_cache and returned by importlib.machinery.PathFinder.find_spec() will be the actual current working directory and not the empty string.

5.5.2. Path entry finder protocol ¶

In order to support imports of modules and initialized packages and also to contribute portions to namespace packages, path entry finders must implement the find_spec() 方法。

find_spec() takes two argument, the fully qualified name of the module being imported, and the (optional) target module. find_spec() returns a fully populated spec for the module. This spec will always have “loader” set (with one exception).

To indicate to the import machinery that the spec represents a namespace portion . the path entry finder sets “loader” on the spec to None and “submodule_search_locations” to a list containing the portion.

5.7.1. __main__.__spec__ ¶

Depending on how __main__ is initialized, __main__.__spec__ gets set appropriately or to None .

When Python is started with the -m option, __spec__ is set to the module spec of the corresponding module or package. __spec__ is also populated when the __main__ module is loaded as part of executing a directory, zipfile or other sys.path entry.

在 the remaining cases __main__.__spec__ 被设为 None , as the code used to populate the __main__ does not correspond directly with an importable module:

• interactive prompt
• -c option
• running from stdin
• running directly from a source or bytecode file

注意， __main__.__spec__ is always None in the last case, even if the file could technically be imported directly as a module instead. Use the -m switch if valid module metadata is desired in __main__ .

Note also that even when __main__ corresponds with an importable module and __main__.__spec__ is set accordingly, they’re still considered distinct modules. This is due to the fact that blocks guarded by if __name__ == "__main__": checks only execute when the module is used to populate the __main__ namespace, and not during normal import.