Python 代码在一
module
gains access to the code in another module by the process of
importing
it. The
import
statement is the most common way of invoking the import machinery, but it is not the only way. Functions such as
importlib.import_module()
和内置
__import__()
can also be used to invoke the import machinery.
import
statement combines two operations; it searches for the named module, then it binds the results of that search to a name in the local scope. The search operation of the
import
statement is defined as a call to the
__import__()
function, with the appropriate arguments. The return value of
__import__()
is used to perform the name binding operation of the
import
statement. See the
import
statement for the exact details of that name binding operation.
直接调用
__import__()
performs only the module search and, if found, the module creation operation. While certain side-effects may occur, such as the importing of parent packages, and the updating of various caches (including
sys.modules
), only the
import
statement performs a name binding operation.
当
import
statement is executed, the standard builtin
__import__()
function is called. Other mechanisms for invoking the import system (such as
importlib.import_module()
) may choose to bypass
__import__()
and use their own solutions to implement import semantics.
When a module is first imported, Python searches for the module and if found, it creates a module object
1
, initializing it. If the named module cannot be found, a
ModuleNotFoundError
is raised. Python implements various strategies to search for the named module when the import machinery is invoked. These strategies can be modified and extended by using various hooks described in the sections below.
3.3 版改变：
The import system has been updated to fully implement the second phase of
PEP 302
. There is no longer any implicit import machinery - the full import system is exposed through
sys.meta_path
. In addition, native namespace package support has been implemented (see
PEP 420
).
importlib
¶
importlib
module provides a rich API for interacting with the import system. For example
importlib.import_module()
provides a recommended, simpler API than built-in
__import__()
for invoking the import machinery. Refer to the
importlib
library documentation for additional detail.
Python has only one type of module object, and all modules are of this type, regardless of whether the module is implemented in Python, C, or something else. To help organize modules and provide a naming hierarchy, Python has a concept of packages .
You can think of packages as the directories on a file system and modules as files within directories, but don’t take this analogy too literally since packages and modules need not originate from the file system. For the purposes of this documentation, we’ll use this convenient analogy of directories and files. Like file system directories, packages are organized hierarchically, and packages may themselves contain subpackages, as well as regular modules.
It’s important to keep in mind that all packages are modules, but not all modules are packages. Or put another way, packages are just a special kind of module. Specifically, any module that contains a
__path__
attribute is considered a package.
All modules have a name. Subpackage names are separated from their parent package name by a dot, akin to Python’s standard attribute access syntax. Thus you might have a package called
email
, which in turn has a subpackage called
email.mime
and a module within that subpackage called
email.mime.text
.
Python defines two types of packages,
常规包
and
名称空间包
. 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
将隐式执行
parent/__init__.py
and
parent/one/__init__.py
. Subsequent imports of
parent.two
or
parent.three
将执行
parent/two/__init__.py
and
parent/three/__init__.py
分别。
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.
To begin the search, Python needs the
fully qualified
name of the module (or package, but for the purposes of this discussion, the difference is immaterial) being imported. This name may come from various arguments to the
import
statement, or from the parameters to the
importlib.import_module()
or
__import__()
函数。
This name will be used in various phases of the import search, and it may be the dotted path to a submodule, e.g.
foo.bar.baz
. In this case, Python first tries to import
foo
，那么
foo.bar
, and finally
foo.bar.baz
. If any of the intermediate imports fail, a
ModuleNotFoundError
被引发。
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.
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 模块特定 , 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.
3.4 版改变： In previous versions of Python, finders returned loaders directly, whereas now they return module specs which contain loaders. Loaders are still used during import but have fewer responsibilities.
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.
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 的默认
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
).
3.4 版改变：
find_spec()
method of meta path finders replaced
find_module()
, which is now deprecated. While it will continue to work without change, the import machinery will try it only if the finder does not implement
find_spec()
.
3.10 版改变：
使用
find_module()
by the import system now raises
ImportWarning
.
If and when a module spec is found, the import machinery will use it (and the loader it contains) when loading the module. Here is an approximation of what happens during the loading portion of import:
module = None if spec.loader is not None and hasattr(spec.loader, 'create_module'): # It is assumed 'exec_module' will also be defined on the loader. module = spec.loader.create_module(spec) if module is None: module = ModuleType(spec.name) # The import-related module attributes get set here: _init_module_attrs(spec, module) if spec.loader is None: # unsupported raise ImportError if spec.origin is None and spec.submodule_search_locations is not None: # namespace package sys.modules[spec.name] = module elif not hasattr(spec.loader, 'exec_module'): module = spec.loader.load_module(spec.name) # Set __loader__ and __package__ if missing. else: sys.modules[spec.name] = module try: spec.loader.exec_module(module) except BaseException: try: del sys.modules[spec.name] except KeyError: pass raise return sys.modules[spec.name]
Note the following details:
If there is an existing module object with the given name in
sys.modules
, import will have already returned it.
The module will exist in
sys.modules
before the loader executes the module code. This is crucial because the module code may (directly or indirectly) import itself; adding it to
sys.modules
beforehand prevents unbounded recursion in the worst case and multiple loading in the best.
If loading fails, the failing module – and only the failing module – gets removed from
sys.modules
. Any module already in the
sys.modules
cache, and any module that was successfully loaded as a side-effect, must remain in the cache. This contrasts with reloading where even the failing module is left in
sys.modules
.
After the module is created but before execution, the import machinery sets the import-related module attributes (“_init_module_attrs” in the pseudo-code example above), as summarized in a later section .
Module execution is the key moment of loading in which the module’s namespace gets populated. Execution is entirely delegated to the loader, which gets to decide what gets populated and how.
The module created during loading and passed to exec_module() may not be the one returned at the end of import 2 .
3.4 版改变：
The import system has taken over the boilerplate responsibilities of loaders. These were previously performed by the
importlib.abc.Loader.load_module()
方法。
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.
3.4 版新增：
create_module()
method of loaders.
3.4 版改变：
load_module()
method was replaced by
exec_module()
and the import machinery assumed all the boilerplate responsibilities of loading.
For compatibility with existing loaders, the import machinery will use the
load_module()
method of loaders if it exists and the loader does not also implement
exec_module()
. However,
load_module()
has been deprecated and loaders should implement
exec_module()
代替。
load_module()
method must implement all the boilerplate loading functionality described above in addition to executing the module. All the same constraints apply, with some additional clarification:
If there is an existing module object with the given name in
sys.modules
, the loader must use that existing module. (Otherwise,
importlib.reload()
will not work correctly.) If the named module does not exist in
sys.modules
, the loader must create a new module object and add it to
sys.modules
.
模块
must
exist in
sys.modules
before the loader executes the module code, to prevent unbounded recursion or multiple loading.
If loading fails, the loader must remove any modules it has inserted into
sys.modules
, but it must remove
only
the failing module(s), and only if the loader itself has loaded the module(s) explicitly.
3.5 版改变：
A
DeprecationWarning
被引发当
exec_module()
is defined but
create_module()
is not.
3.6 版改变：
An
ImportError
被引发当
exec_module()
is defined but
create_module()
is not.
3.10 版改变：
使用
load_module()
会引发
ImportWarning
.
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
and
spam/__init__.py
has the following line in it:
from .foo import Foo
then executing the following puts name bindings for
foo
and
Foo
在
spam
模块：
>>> import spam >>> spam.foo <module 'spam.foo' from '/tmp/imports/spam/foo.py'> >>> spam.Foo <class 'spam.foo.Foo'>
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.
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.
模块的 spec 被暴露为
__spec__
attribute on a module object. See
ModuleSpec
for details on the contents of the module spec.
3.4 版新增。
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__
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__
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.
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__
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__
未定义，
__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__
未定义。
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__
is optional (if set, value must be a string). It indicates the pathname of the file from which the module was loaded (if loaded from a file), or the pathname of the shared library file for extension modules loaded dynamically from a shared library. It might be missing for certain types of modules, such as C modules that are statically linked into the interpreter, and the import system may opt to leave it unset if it has no semantic meaning (e.g. a module loaded from a database).
若
__file__
is set then the
__cached__
attribute might also be set, 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
).
注意，
__cached__
may be set even if
__file__
is not set. However, that scenario is quite atypical. Ultimately, the loader is what makes use of the module spec provided by the finder (from which
__file__
and
__cached__
are derived). So if a loader can load from a cached module but otherwise does not load from a file, that atypical scenario may be appropriate.
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.
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__
不是
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.
3.4 版改变：
使用
loader.module_repr()
has been deprecated and the module spec is now used by the import machinery to generate a module repr.
For backward compatibility with Python 3.3, the module repr will be generated by calling the loader’s
module_repr()
method, if defined, before trying either approach described above. However, the method is deprecated.
3.10 版改变：
调用
module_repr()
now occurs after trying to use a module’s
__spec__
attribute but before falling back on
__file__
. Use of
module_repr()
is slated to stop in Python 3.12.
Before Python loads cached bytecode from a
.pyc
file, it checks whether the cache is up-to-date with the source
.py
file. By default, Python does this by storing the source’s last-modified timestamp and size in the cache file when writing it. At runtime, the import system then validates the cache file by checking the stored metadata in the cache file against the source’s metadata.
Python also supports “hash-based” cache files, which store a hash of the source file’s contents rather than its metadata. There are two variants of hash-based
.pyc
files: checked and unchecked. For checked hash-based
.pyc
files, Python validates the cache file by hashing the source file and comparing the resulting hash with the hash in the cache file. If a checked hash-based cache file is found to be invalid, Python regenerates it and writes a new checked hash-based cache file. For unchecked hash-based
.pyc
files, Python simply assumes the cache file is valid if it exists. Hash-based
.pyc
files validation behavior may be overridden with the
--check-hash-based-pycs
标志。
3.7 版改变：
Added hash-based
.pyc
files. Previously, Python only supported timestamp-based invalidation of bytecode caches.
As mentioned previously, Python comes with several default meta path finders. One of these, called the
基于路径的查找器
(
PathFinder
)，搜索
导入路径
, which contains a list of
路径条目
. Each path entry names a location to search for modules.
The path based finder itself doesn’t know how to import anything. Instead, it traverses the individual path entries, associating each of them with a path entry finder that knows how to handle that particular kind of path.
The default set of path entry finders implement all the semantics for finding modules on the file system, handling special file types such as Python source code (
.py
files), Python byte code (
.pyc
files) and shared libraries (e.g.
.so
files). When supported by the
zipimport
module in the standard library, the default path entry finders also handle loading all of these file types (other than shared libraries) from zipfiles.
Path entries need not be limited to file system locations. They can refer to URLs, database queries, or any other location that can be specified as a string.
The path based finder provides additional hooks and protocols so that you can extend and customize the types of searchable path entries. For example, if you wanted to support path entries as network URLs, you could write a hook that implements HTTP semantics to find modules on the web. This hook (a callable) would return a 路径条目查找器 supporting the protocol described below, which was then used to get a loader for the module from the web.
A word of warning: this section and the previous both use the term
finder
, distinguishing between them by using the terms
元路径查找器
and
路径条目查找器
. These two types of finders are very similar, support similar protocols, and function in similar ways during the import process, but it’s important to keep in mind that they are subtly different. In particular, meta path finders operate at the beginning of the import process, as keyed off the
sys.meta_path
traversal.
By contrast, path entry finders are in a sense an implementation detail of the path based finder, and in fact, if the path based finder were to be removed from
sys.meta_path
, none of the path entry finder semantics would be invoked.
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 should be present on
sys.path
; all other data types are ignored.
基于路径的查找器
是
元路径查找器
, 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
自变量对于
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
路径条目查找器
(
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
路径条目查找器
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
路径条目挂钩
in this list is called with a single argument, the path entry to be searched. This callable may either return a
路径条目查找器
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
路径条目查找器
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
路径条目查找器
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
元路径查找器
could not find the module.
若
路径条目查找器
is
returned by one of the
路径条目挂钩
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.
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 arguments: 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 “submodule_search_locations” to a list containing the portion.
3.4 版改变：
find_spec()
替换
find_loader()
and
find_module()
, both of which are now deprecated, but will be used if
find_spec()
未定义。
Older path entry finders may implement one of these two deprecated methods instead of
find_spec()
. The methods are still respected for the sake of backward compatibility. However, if
find_spec()
is implemented on the path entry finder, the legacy methods are ignored.
find_loader()
takes one argument, the fully qualified name of the module being imported.
find_loader()
returns a 2-tuple where the first item is the loader and the second item is a namespace
portion
.
For backwards compatibility with other implementations of the import protocol, many path entry finders also support the same, traditional
find_module()
method that meta path finders support. However path entry finder
find_module()
methods are never called with a
path
argument (they are expected to record the appropriate path information from the initial call to the path hook).
find_module()
method on path entry finders is deprecated, as it does not allow the path entry finder to contribute portions to namespace packages. If both
find_loader()
and
find_module()
exist on a path entry finder, the import system will always call
find_loader()
in preference to
find_module()
.
3.10 版改变：
调用
find_module()
and
find_loader()
by the import system will raise
ImportWarning
.
The most reliable mechanism for replacing the entire import system is to delete the default contents of
sys.meta_path
, replacing them entirely with a custom meta path hook.
If it is acceptable to only alter the behaviour of import statements without affecting other APIs that access the import system, then replacing the builtin
__import__()
function may be sufficient. This technique may also be employed at the module level to only alter the behaviour of import statements within that module.
To selectively prevent the import of some modules from a hook early on the meta path (rather than disabling the standard import system entirely), it is sufficient to raise
ModuleNotFoundError
directly from
find_spec()
instead of returning
None
. The latter indicates that the meta path search should continue, while raising an exception terminates it immediately.
Relative imports use leading dots. A single leading dot indicates a relative import, starting with the current package. Two or more leading dots indicate a relative import to the parent(s) of the current package, one level per dot after the first. For example, given the following package layout:
package/ __init__.py subpackage1/ __init__.py moduleX.py moduleY.py subpackage2/ __init__.py moduleZ.py moduleA.py
In either
subpackage1/moduleX.py
or
subpackage1/__init__.py
, the following are valid relative imports:
from .moduleY import spam from .moduleY import spam as ham from . import moduleY from ..subpackage1 import moduleY from ..subpackage2.moduleZ import eggs from ..moduleA import foo
Absolute imports may use either the
import <>
or
from <> import <>
syntax, but relative imports may only use the second form; the reason for this is that:
import XXX.YYY.ZZZ
should expose
XXX.YYY.ZZZ
as a usable expression, but .moduleY is not a valid expression.
__main__
module is a special case relative to Python’s import system. As noted
elsewhere
，
__main__
module is directly initialized at interpreter startup, much like
sys
and
builtins
. However, unlike those two, it doesn’t strictly qualify as a built-in module. This is because the manner in which
__main__
is initialized depends on the flags and other options with which the interpreter is invoked.
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
其余情况
__main__.__spec__
被设为
None
, as the code used to populate the
__main__
does not correspond directly with an importable module:
交互提示
-c
option
从 stdin 运行
直接从源 (或字节码) 文件运行
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.
The import machinery has evolved considerably since Python’s early days. The original 包规范 is still available to read, although some details have changed since the writing of that document.
The original specification for
sys.meta_path
was
PEP 302
, with subsequent extension in
PEP 420
.
PEP 420
introduced
名称空间包
for Python 3.3.
PEP 420
also introduced the
find_loader()
protocol as an alternative to
find_module()
.
PEP 366
describes the addition of the
__package__
attribute for explicit relative imports in main modules.
PEP 328
introduced absolute and explicit relative imports and initially proposed
__name__
for semantics
PEP 366
would eventually specify for
__package__
.
PEP 338 defines executing modules as scripts.
PEP 451 adds the encapsulation of per-module import state in spec objects. It also off-loads most of the boilerplate responsibilities of loaders back onto the import machinery. These changes allow the deprecation of several APIs in the import system and also addition of new methods to finders and loaders.
脚注
见
types.ModuleType
.
The importlib implementation avoids using the return value directly. Instead, it gets the module object by looking the module name up in
sys.modules
. The indirect effect of this is that an imported module may replace itself in
sys.modules
. This is implementation-specific behavior that is not guaranteed to work in other Python implementations.
In legacy code, it is possible to find instances of
imp.NullImporter
在
sys.path_importer_cache
. It is recommended that code be changed to use
None
代替。见
移植 Python 代码
了解更多细节。