31.5. "importlib" --- The implementation of "import"
****************************************************

3.1 新版功能.

**Source code:** Lib/importlib/__init__.py

======================================================================


31.5.1. 概述
============

The purpose of the "importlib" package is two-fold. One is to provide
the implementation of the "import" statement (and thus, by extension,
the "__import__()" function) in Python source code. This provides an
implementation of "import" which is portable to any Python
interpreter. This also provides an implementation which is easier to
comprehend than one implemented in a programming language other than
Python.

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.

参见:

  The import statement
     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 3120**
     Using UTF-8 as the Default Source Encoding

  **PEP 3147**
     PYC Repository Directories


31.5.2. Functions
=================

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

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

   注解: 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.

   在 3.3 版更改: 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 新版功能.

   在 3.4 版更改: If "__loader__" is not set, raise "ValueError", just
   like when the attribute is set to "None".

   3.4 版后已移除: 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 新版功能.

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.

   If a module imports objects from another module using "from" ...
   "import" ..., calling "reload()" for the other module does not
   redefine the objects imported from it --- one way around this is to
   re-execute the "from" statement, another is to use "import" and
   qualified names (*module.name*) instead.

   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 新版功能.


31.5.3. "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 版后已移除: 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.

      在 3.4 版更改: 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 新版功能.

   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 新版功能.

   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.

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

      3.4 版后已移除: 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".

      在 3.4 版更改: 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 新版功能.

   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 新版功能.

   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.

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

      3.4 版后已移除: 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 版后已移除: 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.

   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 新版功能.

      在 3.5 版更改: 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 新版功能.

      在 3.6 版更改: "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.

      在 3.4 版更改: Raise "ImportError" when called instead of
      "NotImplementedError". Functionality provided when
      "exec_module()" is available.

      3.4 版后已移除: 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 新版功能.

      在 3.4 版更改: Made optional instead of an abstractmethod.

      3.4 版后已移除: The import machinery now takes care of this
      automatically.

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.

   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__".

      在 3.4 版更改: 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.

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

      在 3.4 版更改: 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.

      在 3.4 版更改: 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.

      在 3.4 版更改: 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 新版功能.

      在 3.5 版更改: Made the method static.

   exec_module(module)

      Implementation of "Loader.exec_module()".

      3.4 新版功能.

   load_module(fullname)

      Implementation of "Loader.load_module()".

      3.4 版后已移除: 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.

      在 3.4 版更改: 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 新版功能.

   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 版后已移除: 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 新版功能.

      在 3.4 版更改: Raise "OSError" instead of "NotImplementedError".

   path_mtime(path)

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

      3.3 版后已移除: 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.

      在 3.4 版更改: 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.

      在 3.4 版更改: 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 新版功能.

   load_module(fullname)

      Concrete implementation of "Loader.load_module()".

      3.4 版后已移除: 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__".


31.5.4. "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 新版功能.

importlib.machinery.DEBUG_BYTECODE_SUFFIXES

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

   3.3 新版功能.

   3.5 版后已移除: Use "BYTECODE_SUFFIXES" instead.

importlib.machinery.OPTIMIZED_BYTECODE_SUFFIXES

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

   3.3 新版功能.

   3.5 版后已移除: 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 新版功能.

   在 3.5 版更改: 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 新版功能.

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 新版功能.

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.

   在 3.5 版更改: 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 新版功能.

   3.6 版后已移除: 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 新版功能.

      在 3.5 版更改: 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 版后已移除: Use "find_spec()" instead.

   classmethod invalidate_caches()

      Calls "importlib.abc.PathEntryFinder.invalidate_caches()" on all
      finders stored in "sys.path_importer_cache".

   在 3.4 版更改: 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 新版功能.

   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 新版功能.

   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 新版功能.

   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 版后已移除: 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 新版功能.

   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 版后已移除: 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 新版功能.

   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 新版功能.

   exec_module(module)

      Initializes the given module object in accordance with **PEP
      489**.

      3.5 新版功能.

   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 新版功能.

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 新版功能.

   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.

   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.


31.5.5. "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 新版功能.

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 新版功能.

   在 3.5 版更改: The *optimization* parameter was added and the
   *debug_override* parameter was deprecated.

   在 3.6 版更改: 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 新版功能.

   在 3.6 版更改: 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 新版功能.

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 新版功能.

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 新版功能.

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 新版功能.

@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.

   在 3.3 版更改: "__loader__" and "__package__" are automatically set
   (when possible).

   在 3.4 版更改: Set "__name__", "__loader__" "__package__"
   unconditionally to support reloading.

   3.4 版后已移除: 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.

   在 3.4 版更改: Set "__loader__" if set to "None", as if the
   attribute does not exist.

   3.4 版后已移除: 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 版后已移除: 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 新版功能.

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 新版功能.

   在 3.6 版更改: Accepts a *path-like object*.

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.

   注解: 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 新版功能.

   在 3.6 版更改: 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))


31.5.6. 示例
============


31.5.6.1. Importing programmatically
------------------------------------

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

   import importlib

   itertools = importlib.import_module('itertools')


31.5.6.2. 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'

   spec = importlib.util.find_spec(name)
   if spec is None:
       print("can't find the itertools module")
   else:
       # If you chose to perform the actual import ...
       module = importlib.util.module_from_spec(spec)
       spec.loader.exec_module(module)
       # Adding the module to sys.modules is optional.
       sys.modules[name] = module


31.5.6.3. Importing a source file directly
------------------------------------------

To import a Python source file directly, use the following recipe
(Python 3.4 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)
   spec.loader.exec_module(module)
   # Optional; only necessary if you want to be able to import the module
   # by name later.
   sys.modules[module_name] = module


31.5.6.4. 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))


31.5.6.5. 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:
           raise ImportError(f'No module named {absolute_name!r}')
       module = importlib.util.module_from_spec(spec)
       spec.loader.exec_module(module)
       sys.modules[absolute_name] = module
       if path is not None:
           setattr(parent_module, child_name, module)
       return module
