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378 lines
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ReStructuredText
378 lines
15 KiB
ReStructuredText
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:mod:`imp` --- Access the :keyword:`import` internals
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=====================================================
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.. module:: imp
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:synopsis: Access the implementation of the import statement.
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.. index:: statement: import
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This module provides an interface to the mechanisms used to implement the
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:keyword:`import` statement. It defines the following constants and functions:
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.. function:: get_magic()
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.. index:: pair: file; byte-code
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Return the magic string value used to recognize byte-compiled code files
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(:file:`.pyc` files). (This value may be different for each Python version.)
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.. function:: get_suffixes()
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Return a list of 3-element tuples, each describing a particular type of
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module. Each triple has the form ``(suffix, mode, type)``, where *suffix* is
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a string to be appended to the module name to form the filename to search
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for, *mode* is the mode string to pass to the built-in :func:`open` function
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to open the file (this can be ``'r'`` for text files or ``'rb'`` for binary
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files), and *type* is the file type, which has one of the values
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:const:`PY_SOURCE`, :const:`PY_COMPILED`, or :const:`C_EXTENSION`, described
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below.
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.. function:: find_module(name[, path])
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Try to find the module *name* on the search path *path*. If *path* is a list
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of directory names, each directory is searched for files with any of the
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suffixes returned by :func:`get_suffixes` above. Invalid names in the list
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are silently ignored (but all list items must be strings). If *path* is
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omitted or ``None``, the list of directory names given by ``sys.path`` is
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searched, but first it searches a few special places: it tries to find a
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built-in module with the given name (:const:`C_BUILTIN`), then a frozen
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module (:const:`PY_FROZEN`), and on some systems some other places are looked
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in as well (on the Mac, it looks for a resource (:const:`PY_RESOURCE`); on
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Windows, it looks in the registry which may point to a specific file).
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If search is successful, the return value is a 3-element tuple ``(file,
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pathname, description)``:
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*file* is an open file object positioned at the beginning, *pathname* is the
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pathname of the file found, and *description* is a 3-element tuple as
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contained in the list returned by :func:`get_suffixes` describing the kind of
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module found.
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If the module does not live in a file, the returned *file* is ``None``,
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*pathname* is the empty string, and the *description* tuple contains empty
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strings for its suffix and mode; the module type is indicated as given in
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parentheses above. If the search is unsuccessful, :exc:`ImportError` is
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raised. Other exceptions indicate problems with the arguments or
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environment.
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If the module is a package, *file* is ``None``, *pathname* is the package
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path and the last item in the *description* tuple is :const:`PKG_DIRECTORY`.
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This function does not handle hierarchical module names (names containing
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dots). In order to find *P*.*M*, that is, submodule *M* of package *P*, use
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:func:`find_module` and :func:`load_module` to find and load package *P*, and
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then use :func:`find_module` with the *path* argument set to ``P.__path__``.
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When *P* itself has a dotted name, apply this recipe recursively.
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.. function:: load_module(name, file, pathname, description)
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Load a module that was previously found by :func:`find_module` (or by an
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otherwise conducted search yielding compatible results). This function does
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more than importing the module: if the module was already imported, it will
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reload the module! The *name* argument indicates the full
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module name (including the package name, if this is a submodule of a
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package). The *file* argument is an open file, and *pathname* is the
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corresponding file name; these can be ``None`` and ``''``, respectively, when
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the module is a package or not being loaded from a file. The *description*
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argument is a tuple, as would be returned by :func:`get_suffixes`, describing
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what kind of module must be loaded.
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If the load is successful, the return value is the module object; otherwise,
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an exception (usually :exc:`ImportError`) is raised.
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**Important:** the caller is responsible for closing the *file* argument, if
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it was not ``None``, even when an exception is raised. This is best done
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using a :keyword:`try` ... :keyword:`finally` statement.
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.. function:: new_module(name)
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Return a new empty module object called *name*. This object is *not* inserted
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in ``sys.modules``.
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.. function:: lock_held()
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Return ``True`` if the import lock is currently held, else ``False``. On
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platforms without threads, always return ``False``.
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On platforms with threads, a thread executing an import holds an internal lock
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until the import is complete. This lock blocks other threads from doing an
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import until the original import completes, which in turn prevents other threads
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from seeing incomplete module objects constructed by the original thread while
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in the process of completing its import (and the imports, if any, triggered by
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that).
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.. function:: acquire_lock()
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Acquires the interpreter's import lock for the current thread. This lock should
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be used by import hooks to ensure thread-safety when importing modules. On
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platforms without threads, this function does nothing.
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.. function:: release_lock()
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Release the interpreter's import lock. On platforms without threads, this
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function does nothing.
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.. function:: reload(module)
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Reload a previously imported *module*. The argument must be a module object, so
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it must have been successfully imported before. This is useful if you have
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edited the module source file using an external editor and want to try out the
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new version without leaving the Python interpreter. The return value is the
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module object (the same as the *module* argument).
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When ``reload(module)`` is executed:
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* Python modules' code is recompiled and the module-level code reexecuted,
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defining a new set of objects which are bound to names in the module's
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dictionary. The ``init`` function of extension modules is not called a second
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time.
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* As with all other objects in Python the old objects are only reclaimed after
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their reference counts drop to zero.
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* The names in the module namespace are updated to point to any new or changed
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objects.
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* Other references to the old objects (such as names external to the module) are
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not rebound to refer to the new objects and must be updated in each namespace
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where they occur if that is desired.
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There are a number of other caveats:
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If a module is syntactically correct but its initialization fails, the first
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:keyword:`import` statement for it does not bind its name locally, but does
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store a (partially initialized) module object in ``sys.modules``. To reload the
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module you must first :keyword:`import` it again (this will bind the name to the
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partially initialized module object) before you can :func:`reload` it.
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When a module is reloaded, its dictionary (containing the module's global
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variables) is retained. Redefinitions of names will override the old
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definitions, so this is generally not a problem. If the new version of a module
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does not define a name that was defined by the old version, the old definition
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remains. This feature can be used to the module's advantage if it maintains a
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global table or cache of objects --- with a :keyword:`try` statement it can test
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for the table's presence and skip its initialization if desired::
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try:
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cache
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except NameError:
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cache = {}
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It is legal though generally not very useful to reload built-in or dynamically
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loaded modules, except for :mod:`sys`, :mod:`__main__` and :mod:`__builtin__`.
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In many cases, however, extension modules are not designed to be initialized
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more than once, and may fail in arbitrary ways when reloaded.
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If a module imports objects from another module using :keyword:`from` ...
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:keyword:`import` ..., calling :func:`reload` for the other module does not
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redefine the objects imported from it --- one way around this is to re-execute
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the :keyword:`from` statement, another is to use :keyword:`import` and qualified
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names (*module*.*name*) instead.
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If a module instantiates instances of a class, reloading the module that defines
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the class does not affect the method definitions of the instances --- they
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continue to use the old class definition. The same is true for derived classes.
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.. function:: acquire_lock()
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Acquires the interpreter's import lock for the current thread. This lock should
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be used by import hooks to ensure thread-safety when importing modules. On
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platforms without threads, this function does nothing.
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.. function:: release_lock()
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Release the interpreter's import lock. On platforms without threads, this
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function does nothing.
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The following constants with integer values, defined in this module, are used to
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indicate the search result of :func:`find_module`.
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.. data:: PY_SOURCE
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The module was found as a source file.
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.. data:: PY_COMPILED
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The module was found as a compiled code object file.
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.. data:: C_EXTENSION
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The module was found as dynamically loadable shared library.
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.. data:: PY_RESOURCE
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The module was found as a Mac OS 9 resource. This value can only be returned on
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a Mac OS 9 or earlier Macintosh.
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.. data:: PKG_DIRECTORY
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The module was found as a package directory.
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.. data:: C_BUILTIN
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The module was found as a built-in module.
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.. data:: PY_FROZEN
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The module was found as a frozen module (see :func:`init_frozen`).
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The following constant and functions are obsolete; their functionality is
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available through :func:`find_module` or :func:`load_module`. They are kept
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around for backward compatibility:
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.. data:: SEARCH_ERROR
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Unused.
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.. function:: init_builtin(name)
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Initialize the built-in module called *name* and return its module object along
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with storing it in ``sys.modules``. If the module was already initialized, it
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will be initialized *again*. Re-initialization involves the copying of the
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built-in module's ``__dict__`` from the cached module over the module's entry in
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``sys.modules``. If there is no built-in module called *name*, ``None`` is
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returned.
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.. function:: init_frozen(name)
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Initialize the frozen module called *name* and return its module object. If
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the module was already initialized, it will be initialized *again*. If there
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is no frozen module called *name*, ``None`` is returned. (Frozen modules are
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modules written in Python whose compiled byte-code object is incorporated
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into a custom-built Python interpreter by Python's :program:`freeze`
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utility. See :file:`Tools/freeze/` for now.)
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.. function:: is_builtin(name)
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Return ``1`` if there is a built-in module called *name* which can be
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initialized again. Return ``-1`` if there is a built-in module called *name*
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which cannot be initialized again (see :func:`init_builtin`). Return ``0`` if
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there is no built-in module called *name*.
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.. function:: is_frozen(name)
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Return ``True`` if there is a frozen module (see :func:`init_frozen`) called
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*name*, or ``False`` if there is no such module.
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.. function:: load_compiled(name, pathname, [file])
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.. index:: pair: file; byte-code
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Load and initialize a module implemented as a byte-compiled code file and return
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its module object. If the module was already initialized, it will be
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initialized *again*. The *name* argument is used to create or access a module
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object. The *pathname* argument points to the byte-compiled code file. The
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*file* argument is the byte-compiled code file, open for reading in binary mode,
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from the beginning. It must currently be a real file object, not a user-defined
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class emulating a file.
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.. function:: load_dynamic(name, pathname[, file])
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Load and initialize a module implemented as a dynamically loadable shared
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library and return its module object. If the module was already initialized, it
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will be initialized *again*. Re-initialization involves copying the ``__dict__``
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attribute of the cached instance of the module over the value used in the module
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cached in ``sys.modules``. The *pathname* argument must point to the shared
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library. The *name* argument is used to construct the name of the
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initialization function: an external C function called ``initname()`` in the
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shared library is called. The optional *file* argument is ignored. (Note:
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using shared libraries is highly system dependent, and not all systems support
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it.)
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.. function:: load_source(name, pathname[, file])
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Load and initialize a module implemented as a Python source file and return its
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module object. If the module was already initialized, it will be initialized
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*again*. The *name* argument is used to create or access a module object. The
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*pathname* argument points to the source file. The *file* argument is the
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source file, open for reading as text, from the beginning. It must currently be
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a real file object, not a user-defined class emulating a file. Note that if a
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properly matching byte-compiled file (with suffix :file:`.pyc` or :file:`.pyo`)
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exists, it will be used instead of parsing the given source file.
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.. class:: NullImporter(path_string)
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The :class:`NullImporter` type is a :pep:`302` import hook that handles
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non-directory path strings by failing to find any modules. Calling this type
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with an existing directory or empty string raises :exc:`ImportError`.
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Otherwise, a :class:`NullImporter` instance is returned.
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Python adds instances of this type to ``sys.path_importer_cache`` for any path
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entries that are not directories and are not handled by any other path hooks on
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``sys.path_hooks``. Instances have only one method:
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.. method:: NullImporter.find_module(fullname [, path])
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This method always returns ``None``, indicating that the requested module could
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not be found.
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.. _examples-imp:
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Examples
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--------
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The following function emulates what was the standard import statement up to
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Python 1.4 (no hierarchical module names). (This *implementation* wouldn't work
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in that version, since :func:`find_module` has been extended and
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:func:`load_module` has been added in 1.4.) ::
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import imp
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import sys
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def __import__(name, globals=None, locals=None, fromlist=None):
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# Fast path: see if the module has already been imported.
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try:
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return sys.modules[name]
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except KeyError:
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pass
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# If any of the following calls raises an exception,
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# there's a problem we can't handle -- let the caller handle it.
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fp, pathname, description = imp.find_module(name)
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try:
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return imp.load_module(name, fp, pathname, description)
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finally:
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# Since we may exit via an exception, close fp explicitly.
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if fp:
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fp.close()
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.. index:: module: knee
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A more complete example that implements hierarchical module names and includes a
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:func:`reload` function can be found in the module :mod:`knee`. The :mod:`knee`
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module can be found in :file:`Demo/imputil/` in the Python source distribution.
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