31.1. "imp" — Access the "import" internals
*******************************************

This module provides an interface to the mechanisms used to implement
the "import" statement.  It defines the following constants and
functions:

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.)

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.

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 does not live in a file, the returned *file* is
   "None", *pathname* is the empty string, 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.

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 is equivalent to a "reload()"!  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.

imp.new_module(name)

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

imp.lock_held()

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

   On platforms with threads, a thread executing an import holds an
   internal lock until the import is complete. This lock blocks other
   threads from doing an import until the original import completes,
   which in turn prevents other threads from seeing incomplete module
   objects constructed by the original thread while in the process of
   completing its import (and the imports, if any, triggered by that).

imp.acquire_lock()

   Acquire the interpreter’s 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.

   2.3 新版功能.

imp.release_lock()

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

   2.3 新版功能.

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.

imp.PY_COMPILED

   The module was found as a compiled code object file.

imp.C_EXTENSION

   The module was found as dynamically loadable shared library.

imp.PKG_DIRECTORY

   The module was found as a package directory.

imp.C_BUILTIN

   The module was found as a built-in module.

imp.PY_FROZEN

   The module was found as a frozen module (see "init_frozen()").

The following constant and functions are obsolete; their functionality
is available through "find_module()" or "load_module()". They are kept
around for backward compatibility:

imp.SEARCH_ERROR

   Unused.

imp.init_builtin(name)

   Initialize the built-in module called *name* and return its module
   object along with storing it in "sys.modules".  If the module was
   already initialized, it will be initialized *again*.  Re-
   initialization involves the copying of the built-in module’s
   "__dict__" from the cached module over the module’s entry in
   "sys.modules".  If there is no built-in module called *name*,
   "None" is returned.

imp.init_frozen(name)

   Initialize the frozen module called *name* and return its module
   object.  If the module was already initialized, it will be
   initialized *again*.  If there is no frozen module called *name*,
   "None" is returned.  (Frozen modules are modules written in Python
   whose compiled byte-code object is incorporated into a custom-built
   Python interpreter by Python’s **freeze** utility. See
   "Tools/freeze/" for now.)

imp.is_builtin(name)

   Return "1" if there is a built-in module called *name* which can be
   initialized again.  Return "-1" if there is a built-in module
   called *name* which cannot be initialized again (see
   "init_builtin()").  Return "0" if there is no built-in module
   called *name*.

imp.is_frozen(name)

   Return "True" if there is a frozen module (see "init_frozen()")
   called *name*, or "False" if there is no such module.

imp.load_compiled(name, pathname[, file])

   Load and initialize a module implemented as a byte-compiled code
   file and return its module object.  If the module was already
   initialized, it will be initialized *again*.  The *name* argument
   is used to create or access a module object.  The *pathname*
   argument points to the byte-compiled code file.  The *file*
   argument is the byte-compiled code file, open for reading in binary
   mode, from the beginning. It must currently be a real file object,
   not a user-defined class emulating a file.

imp.load_dynamic(name, pathname[, file])

   Load and initialize a module implemented as a dynamically loadable
   shared library and return its module object.  If the module was
   already initialized, it will be initialized *again*. Re-
   initialization involves copying the "__dict__" attribute of the
   cached instance of the module over the value used in the module
   cached in "sys.modules".  The *pathname* argument must point to the
   shared library.  The *name* argument is used to construct the name
   of the initialization function: an external C function called
   "initname()" in the shared library is called.  The optional *file*
   argument is ignored.  (Note: using shared libraries is highly
   system dependent, and not all systems support it.)

   **CPython implementation detail:** The import internals identify
   extension modules by filename, so doing "foo = load_dynamic("foo",
   "mod.so")" and "bar = load_dynamic("bar", "mod.so")" will result in
   both foo and bar referring to the same module, regardless of
   whether or not "mod.so" exports an "initbar" function. On systems
   which support them, symlinks can be used to import multiple modules
   from the same shared library, as each reference to the module will
   use a different file name.

imp.load_source(name, pathname[, file])

   Load and initialize a module implemented as a Python source file
   and return its module object.  If the module was already
   initialized, it will be initialized *again*.  The *name* argument
   is used to create or access a module object.  The *pathname*
   argument points to the source file.  The *file* argument is the
   source file, open for reading as text, from the beginning. It must
   currently be a real file object, not a user-defined class emulating
   a file.  Note that if a properly matching byte-compiled file (with
   suffix ".pyc" or ".pyo") exists, it will be used instead of parsing
   the given source file.

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.

   Python adds instances of this type to "sys.path_importer_cache" for
   any path entries that are not directories and are not handled by
   any other path hooks on "sys.path_hooks".  Instances have only one
   method:

   find_module(fullname[, path])

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

   2.5 新版功能.


31.1.1. 示例
============

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()

A more complete example that implements hierarchical module names and
includes a "reload()" function can be found in the module "knee".  The
"knee" module can be found in "Demo/imputil/" in the Python source
distribution.
