# 36.2. `imp` — Access the import internals

Source code: Lib/imp.py

3.4 版后已移除: The `imp` package is pending deprecation in favor of `importlib`.

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.)

  3.4 版后已移除: 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 版后已移除: 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 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.

  3.3 版后已移除: 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 版后已移除: 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 版后已移除: 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。

  在 3.3 版更改: Relies on both `__name__` and `__loader__` being defined on the module being reloaded instead of just `__name__`.

  3.4 版后已移除: Use `importlib.reload()` instead.

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

3.2 新版功能.

`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.

  在 3.3 版更改: If `sys.implementation.cache_tag` is `None`, then `NotImplementedError` is raised.

  3.4 版后已移除: Use `importlib.util.cache_from_source()` instead.

  在 3.5 版更改: 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.

  在 3.3 版更改: Raise `NotImplementedError` when `sys.implementation.cache_tag` is not defined.

  3.4 版后已移除: 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 版后已移除: 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.

  在 3.3 版更改: 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 版后已移除.

`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.

  在 3.3 版更改: 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 版后已移除.

`imp.``release_lock`()

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

  在 3.3 版更改: 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 版后已移除.

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 版后已移除.

`imp.``PY_COMPILED`

  The module was found as a compiled code object file.

  3.3 版后已移除.

`imp.``C_EXTENSION`

  The module was found as dynamically loadable shared library.

  3.3 版后已移除.

`imp.``PKG_DIRECTORY`

  The module was found as a package directory.

  3.3 版后已移除.

`imp.``C_BUILTIN`

  The module was found as a built-in module.

  3.3 版后已移除.

`imp.``PY_FROZEN`

  The module was found as a frozen module.

  3.3 版后已移除.

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.

  在 3.3 版更改: `None` is inserted into `sys.path_importer_cache` instead of an instance of `NullImporter`.

  3.4 版后已移除: Insert `None` into `sys.path_importer_cache` instead.

## 36.2.1. 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()