29.1. 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 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.

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. On platforms without threads, this function does nothing.

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.

imp.release_lock()

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

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.
• As with all other objects in Python the old objects are only reclaimed after their reference counts drop to zero.
• The names in the module namespace are updated to point to any new or changed objects.
• Other references to the old objects (such as names external to the module) are not rebound to refer to the new objects and must be updated in each namespace where they occur if that is desired.

There are a number of other caveats:

If a module is syntactically correct but its initialization fails, the first import statement for it does not bind its name locally, but does store a (partially initialized) module object in sys.modules. To reload the module you must first import it again (this will bind the name to the partially initialized module object) before you can reload() it.

When a module is reloaded, its dictionary (containing the module’s global variables) is retained. Redefinitions of names will override the old definitions, so this is generally not a problem. If the new version of a module does not define a name that was defined by the old version, the old definition remains. This feature can be used to the module’s advantage if it maintains a global table or cache of objects — with a try statement it can test for the table’s presence and skip its initialization if desired:

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 __builtin__. 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.

If a module instantiates instances of a class, reloading the module that defines the class does not affect the method definitions of the instances — they continue to use the old class definition. The same is true for derived classes.

imp.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.

imp.release_lock()

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

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.)

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.

29.1.1. Examples

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.