"imp" —— 由代码内部访问 import 。
*********************************

**源代码：** Lib/imp.py

3.4 版後已棄用: "imp" 模块已被废弃，请改用 "importlib"。

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

本模块提供了一个接口，用于实现 "import" 语句的机制。这里定义了以下常量
和函数：

imp.get_magic()

   Return the magic string value used to recognize byte-compiled code
   files (".pyc" files).  (This value may be different for each Python
   version.)

   3.4 版後已棄用: Use "importlib.util.MAGIC_NUMBER" instead.

imp.get_suffixes()

   Return a list of 3-element tuples, each describing a particular
   type of module. Each triple has the form "(suffix, mode, type)",
   where *suffix* is a string to be appended to the module name to
   form the filename to search for, *mode* is the mode string to pass
   to the built-in "open()" function to open the file (this can be
   "'r'" for text files or "'rb'" for binary files), and *type* is the
   file type, which has one of the values "PY_SOURCE", "PY_COMPILED",
   or "C_EXTENSION", described below.

   3.3 版後已棄用: Use the constants defined on "importlib.machinery"
   instead.

imp.find_module(name[, path])

   Try to find the module *name*.  If *path* is omitted or "None", the
   list of directory names given by "sys.path" is searched, but first
   a few special places are searched: the function tries to find a
   built-in module with the given name ("C_BUILTIN"), then a frozen
   module ("PY_FROZEN"), and on some systems some other places are
   looked in as well (on Windows, it looks in the registry which may
   point to a specific file).

   Otherwise, *path* must be a list of directory names; each directory
   is searched for files with any of the suffixes returned by
   "get_suffixes()" above.  Invalid names in the list are silently
   ignored (but all list items must be strings).

   If search is successful, the return value is a 3-element tuple
   "(file, pathname, description)":

   *file* is an open *file object* positioned at the beginning,
   *pathname* is the pathname of the file found, and *description* is
   a 3-element tuple as contained in the list returned by
   "get_suffixes()" describing the kind of module found.

   If the module is built-in or frozen then *file* and *pathname* are
   both "None" and the *description* tuple contains empty strings for
   its suffix and mode; the module type is indicated as given in
   parentheses above.  If the search is unsuccessful, "ImportError" is
   raised.  Other exceptions indicate problems with the arguments or
   environment.

   If the module is a package, *file* is "None", *pathname* is the
   package path and the last item in the *description* tuple is
   "PKG_DIRECTORY".

   This function does not handle hierarchical module names (names
   containing dots).  In order to find *P.M*, that is, submodule *M*
   of package *P*, use "find_module()" and "load_module()" to find and
   load package *P*, and then use "find_module()" with the *path*
   argument set to "P.__path__". When *P* itself has a dotted name,
   apply this recipe recursively.

   3.3 版後已棄用: Use "importlib.util.find_spec()" instead unless
   Python 3.3 compatibility is required, in which case use
   "importlib.find_loader()". For example usage of the former case,
   see the 例子 section of the "importlib" documentation.

imp.load_module(name, file, pathname, description)

   Load a module that was previously found by "find_module()" (or by
   an otherwise conducted search yielding compatible results).  This
   function does more than importing the module: if the module was
   already imported, it will reload the module!  The *name* argument
   indicates the full module name (including the package name, if this
   is a submodule of a package).  The *file* argument is an open file,
   and *pathname* is the corresponding file name; these can be "None"
   and "''", respectively, when the module is a package or not being
   loaded from a file.  The *description* argument is a tuple, as
   would be returned by "get_suffixes()", describing what kind of
   module must be loaded.

   If the load is successful, the return value is the module object;
   otherwise, an exception (usually "ImportError") is raised.

   **Important:** the caller is responsible for closing the *file*
   argument, if it was not "None", even when an exception is raised.
   This is best done using a "try" ... "finally" statement.

   3.3 版後已棄用: If previously used in conjunction with
   "imp.find_module()" then consider using
   "importlib.import_module()", otherwise use the loader returned by
   the replacement you chose for "imp.find_module()". If you called
   "imp.load_module()" and related functions directly with file path
   arguments then use a combination of
   "importlib.util.spec_from_file_location()" and
   "importlib.util.module_from_spec()". See the 例子 section of the
   "importlib" documentation for details of the various approaches.

imp.new_module(name)

   Return a new empty module object called *name*.  This object is
   *not* inserted in "sys.modules".

   3.4 版後已棄用: Use "importlib.util.module_from_spec()" instead.

imp.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 (the
   same as the *module* argument).

   When "reload(module)" is executed:

   * Python modules' code is recompiled and the module-level code
     reexecuted, defining a new set of objects which are bound to
     names in the module's dictionary.  The "init" function of
     extension modules is not called a second time.

   * 与Python中的所有的其它对象一样，旧的对象只有在它们的引用计数为0之
     后才会被回收。

   * 模块命名空间中的名称重新指向任何新的或更改后的对象。

   * 其他旧对象的引用（例如那个模块的外部名称）不会被重新绑定到引用的
     新对象的，并且如果有需要，必须在出现的每个命名空间中进行更新。

   有一些其他注意事项：

   当一个模块被重新加载的时候，它的字典（包含了那个模块的全区变量）会
   被保留。名称的重新定义会覆盖旧的定义，所以通常来说这不是问题。如果
   一个新模块没有定义在旧版本模块中定义的名称，则将保留旧版本中的定义
   。这一特性可用于作为那个模块的优点，如果它维护一个全局表或者对象的
   缓存 —— 使用 "try" 语句，就可以测试表的存在并且跳过它的初始化，如果
   有需要的话:

      try:
          cache
      except NameError:
          cache = {}

   It is legal though generally not very useful to reload built-in or
   dynamically loaded modules, except for "sys", "__main__" and
   "builtins". In many cases, however, 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.

   如果一个模块创建一个类的实例，重新加载定义那个类的模块不影响那些实
   例的方法定义———它们继续使用旧类中的定义。对于子类来说同样是正确的。

   3.3 版更變: Relies on both "__name__" and "__loader__" being
   defined on the module being reloaded instead of just "__name__".

   3.4 版後已棄用: Use "importlib.reload()" instead.

The following functions are conveniences for handling **PEP 3147**
byte-compiled file paths.

3.2 版新加入.

imp.cache_from_source(path, debug_override=None)

   Return the **PEP 3147** 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). By passing in "True" or
   "False" for *debug_override* you can override the system's value
   for "__debug__", leading to optimized bytecode.

   *path* need not exist.

   3.3 版更變: If "sys.implementation.cache_tag" is "None", then
   "NotImplementedError" is raised.

   3.4 版後已棄用: Use "importlib.util.cache_from_source()" instead.

   3.5 版更變: The *debug_override* parameter no longer creates a
   ".pyo" file.

imp.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** format, a "ValueError" is raised. If
   "sys.implementation.cache_tag" is not defined,
   "NotImplementedError" is raised.

   3.3 版更變: Raise "NotImplementedError" when
   "sys.implementation.cache_tag" is not defined.

   3.4 版後已棄用: Use "importlib.util.source_from_cache()" instead.

imp.get_tag()

   Return the **PEP 3147** magic tag string matching this version of
   Python's magic number, as returned by "get_magic()".

   3.4 版後已棄用: Use "sys.implementation.cache_tag" directly
   starting in Python 3.3.

The following functions help interact with the import system's
internal locking mechanism.  Locking semantics of imports are an
implementation detail which may vary from release to release.
However, Python ensures that circular imports work without any
deadlocks.

imp.lock_held()

   Return "True" if the global import lock is currently held, else
   "False". On platforms without threads, always return "False".

   On platforms with threads, a thread executing an import first holds
   a global import lock, then sets up a per-module lock for the rest
   of the import.  This blocks other threads from importing the same
   module until the original import completes, preventing other
   threads from seeing incomplete module objects constructed by the
   original thread.  An exception is made for circular imports, which
   by construction have to expose an incomplete module object at some
   point.

   3.3 版更變: The locking scheme has changed to per-module locks for
   the most part.  A global import lock is kept for some critical
   tasks, such as initializing the per-module locks.

   3.4 版後已棄用.

imp.acquire_lock()

   Acquire the interpreter's global import lock for the current
   thread. This lock should be used by import hooks to ensure thread-
   safety when importing modules.

   Once a thread has acquired the import lock, the same thread may
   acquire it again without blocking; the thread must release it once
   for each time it has acquired it.

   On platforms without threads, this function does nothing.

   3.3 版更變: The locking scheme has changed to per-module locks for
   the most part.  A global import lock is kept for some critical
   tasks, such as initializing the per-module locks.

   3.4 版後已棄用.

imp.release_lock()

   Release the interpreter's global import lock. On platforms without
   threads, this function does nothing.

   3.3 版更變: The locking scheme has changed to per-module locks for
   the most part.  A global import lock is kept for some critical
   tasks, such as initializing the per-module locks.

   3.4 版後已棄用.

The following constants with integer values, defined in this module,
are used to indicate the search result of "find_module()".

imp.PY_SOURCE

   The module was found as a source file.

   3.3 版後已棄用.

imp.PY_COMPILED

   The module was found as a compiled code object file.

   3.3 版後已棄用.

imp.C_EXTENSION

   The module was found as dynamically loadable shared library.

   3.3 版後已棄用.

imp.PKG_DIRECTORY

   The module was found as a package directory.

   3.3 版後已棄用.

imp.C_BUILTIN

   The module was found as a built-in module.

   3.3 版後已棄用.

imp.PY_FROZEN

   The module was found as a frozen module.

   3.3 版後已棄用.

class imp.NullImporter(path_string)

   The "NullImporter" type is a **PEP 302** import hook that handles
   non-directory path strings by failing to find any modules.  Calling
   this type with an existing directory or empty string raises
   "ImportError". Otherwise, a "NullImporter" instance is returned.

   Instances have only one method:

   find_module(fullname[, path])

      This method always returns "None", indicating that the requested
      module could not be found.

   3.3 版更變: "None" is inserted into "sys.path_importer_cache"
   instead of an instance of "NullImporter".

   3.4 版後已棄用: Insert "None" into "sys.path_importer_cache"
   instead.


例子
====

The following function emulates what was the standard import statement
up to Python 1.4 (no hierarchical module names).  (This
*implementation* wouldn't work in that version, since "find_module()"
has been extended and "load_module()" has been added in 1.4.)

   import imp
   import sys

   def __import__(name, globals=None, locals=None, fromlist=None):
       # Fast path: see if the module has already been imported.
       try:
           return sys.modules[name]
       except KeyError:
           pass

       # If any of the following calls raises an exception,
       # there's a problem we can't handle -- let the caller handle it.

       fp, pathname, description = imp.find_module(name)

       try:
           return imp.load_module(name, fp, pathname, description)
       finally:
           # Since we may exit via an exception, close fp explicitly.
           if fp:
               fp.close()
