# 3.21 `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:

`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.)

`get_suffixes`( | )

  Return a list of triples, 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.

`find_module`( | name[, path])

  Try to find the module name on the search path path. If path is 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 path is omitted or `None`, the list of directory names given by `sys.path` is searched, but first it searches a few special places: it 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 the Mac, it looks for a resource (`PY_RESOURCE`); on Windows, it looks in the registry which may point to a specific file).

  If search is successful, the return value is a triple `(file, pathname, description)` where file is an open file object positioned at the beginning, pathname is the pathname of the file found, and description is a triple 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`, filename is the empty string, and the description tuple contains empty strings for its suffix and mode; the module type is as indicate in parentheses above. If the search is unsuccessful, `ImportError` is raised. Other exceptions indicate problems with the arguments or environment.

  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.

`load_module`( | name, file, filename, 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 filename is the corresponding file name; these can be `None` and `''`, respectively, when the module is 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.

`new_module`( | name)

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

`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).

`acquire_lock`( | )

  Acquires the interpreter's import lock for the current thread. This lock should be used by import hooks to ensure thread-safety when importing modules. On platforms without threads, this function does nothing. New in version 2.3.

`release_lock`( | )

  Release the interpreter's import lock. On platforms without threads, this function does nothing. New in version 2.3.

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

`PY_SOURCE`
  The module was found as a source file.

`PY_COMPILED`
  The module was found as a compiled code object file.

`C_EXTENSION`
  The module was found as dynamically loadable shared library.

`PY_RESOURCE`
  The module was found as a Macintosh resource. This value can only be returned on a Macintosh.

`PKG_DIRECTORY`
  The module was found as a package directory.

`C_BUILTIN`
  The module was found as a built-in module.

`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:

`SEARCH_ERROR`
  Unused.

`init_builtin`( | name)

  Initialize the built-in module called name and return its module object. If the module was already initialized, it will be initialized again. A few modules cannot be initialized twice -- attempting to initialize these again will raise an `ImportError` exception. If there is no built-in module called name, `None` is returned.

`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.)

`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.

`is_frozen`( | name)

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

`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.

`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. Some modules don't like that and may raise an exception. 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.)

`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.