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# 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. 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()

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.