/Doc/library/imp.rst

  1
  2:mod:`imp` --- Access the :keyword:`import` internals
  3=====================================================
  4
  5.. module:: imp
  6   :synopsis: Access the implementation of the import statement.
  7
  8
  9.. index:: statement: import
 10
 11This module provides an interface to the mechanisms used to implement the
 12:keyword:`import` statement.  It defines the following constants and functions:
 13
 14
 15.. function:: get_magic()
 16
 17   .. index:: pair: file; byte-code
 18
 19   Return the magic string value used to recognize byte-compiled code files
 20   (:file:`.pyc` files).  (This value may be different for each Python version.)
 21
 22
 23.. function:: get_suffixes()
 24
 25   Return a list of 3-element tuples, each describing a particular type of
 26   module. Each triple has the form ``(suffix, mode, type)``, where *suffix* is
 27   a string to be appended to the module name to form the filename to search
 28   for, *mode* is the mode string to pass to the built-in :func:`open` function
 29   to open the file (this can be ``'r'`` for text files or ``'rb'`` for binary
 30   files), and *type* is the file type, which has one of the values
 31   :const:`PY_SOURCE`, :const:`PY_COMPILED`, or :const:`C_EXTENSION`, described
 32   below.
 33
 34
 35.. function:: find_module(name[, path])
 36
 37   Try to find the module *name* on the search path *path*.  If *path* is a list
 38   of directory names, each directory is searched for files with any of the
 39   suffixes returned by :func:`get_suffixes` above.  Invalid names in the list
 40   are silently ignored (but all list items must be strings).  If *path* is
 41   omitted or ``None``, the list of directory names given by ``sys.path`` is
 42   searched, but first it searches a few special places: it tries to find a
 43   built-in module with the given name (:const:`C_BUILTIN`), then a frozen
 44   module (:const:`PY_FROZEN`), and on some systems some other places are looked
 45   in as well (on Windows, it looks in the registry which may point to a
 46   specific file).
 47
 48   If search is successful, the return value is a 3-element tuple ``(file,
 49   pathname, description)``:
 50
 51   *file* is an open file object positioned at the beginning, *pathname* is the
 52   pathname of the file found, and *description* is a 3-element tuple as
 53   contained in the list returned by :func:`get_suffixes` describing the kind of
 54   module found.
 55
 56   If the module does not live in a file, the returned *file* is ``None``,
 57   *pathname* is the empty string, and the *description* tuple contains empty
 58   strings for its suffix and mode; the module type is indicated as given in
 59   parentheses above.  If the search is unsuccessful, :exc:`ImportError` is
 60   raised.  Other exceptions indicate problems with the arguments or
 61   environment.
 62
 63   If the module is a package, *file* is ``None``, *pathname* is the package
 64   path and the last item in the *description* tuple is :const:`PKG_DIRECTORY`.
 65
 66   This function does not handle hierarchical module names (names containing
 67   dots).  In order to find *P*.*M*, that is, submodule *M* of package *P*, use
 68   :func:`find_module` and :func:`load_module` to find and load package *P*, and
 69   then use :func:`find_module` with the *path* argument set to ``P.__path__``.
 70   When *P* itself has a dotted name, apply this recipe recursively.
 71
 72
 73.. function:: load_module(name, file, pathname, description)
 74
 75   .. index:: builtin: reload
 76
 77   Load a module that was previously found by :func:`find_module` (or by an
 78   otherwise conducted search yielding compatible results).  This function does
 79   more than importing the module: if the module was already imported, it is
 80   equivalent to a :func:`reload`!  The *name* argument indicates the full
 81   module name (including the package name, if this is a submodule of a
 82   package).  The *file* argument is an open file, and *pathname* is the
 83   corresponding file name; these can be ``None`` and ``''``, respectively, when
 84   the module is a package or not being loaded from a file.  The *description*
 85   argument is a tuple, as would be returned by :func:`get_suffixes`, describing
 86   what kind of module must be loaded.
 87
 88   If the load is successful, the return value is the module object; otherwise,
 89   an exception (usually :exc:`ImportError`) is raised.
 90
 91   **Important:** the caller is responsible for closing the *file* argument, if
 92   it was not ``None``, even when an exception is raised.  This is best done
 93   using a :keyword:`try` ... :keyword:`finally` statement.
 94
 95
 96.. function:: new_module(name)
 97
 98   Return a new empty module object called *name*.  This object is *not* inserted
 99   in ``sys.modules``.
100
101
102.. function:: lock_held()
103
104   Return ``True`` if the import lock is currently held, else ``False``. On
105   platforms without threads, always return ``False``.
106
107   On platforms with threads, a thread executing an import holds an internal lock
108   until the import is complete. This lock blocks other threads from doing an
109   import until the original import completes, which in turn prevents other threads
110   from seeing incomplete module objects constructed by the original thread while
111   in the process of completing its import (and the imports, if any, triggered by
112   that).
113
114
115.. function:: acquire_lock()
116
117   Acquire the interpreter's import lock for the current thread.  This lock should
118   be used by import hooks to ensure thread-safety when importing modules. On
119   platforms without threads, this function does nothing.
120
121   Once a thread has acquired the import lock, the same thread may acquire it
122   again without blocking; the thread must release it once for each time it has
123   acquired it.
124
125   On platforms without threads, this function does nothing.
126
127   .. versionadded:: 2.3
128
129
130.. function:: release_lock()
131
132   Release the interpreter's import lock. On platforms without threads, this
133   function does nothing.
134
135   .. versionadded:: 2.3
136
137The following constants with integer values, defined in this module, are used to
138indicate the search result of :func:`find_module`.
139
140
141.. data:: PY_SOURCE
142
143   The module was found as a source file.
144
145
146.. data:: PY_COMPILED
147
148   The module was found as a compiled code object file.
149
150
151.. data:: C_EXTENSION
152
153   The module was found as dynamically loadable shared library.
154
155
156.. data:: PKG_DIRECTORY
157
158   The module was found as a package directory.
159
160
161.. data:: C_BUILTIN
162
163   The module was found as a built-in module.
164
165
166.. data:: PY_FROZEN
167
168   The module was found as a frozen module (see :func:`init_frozen`).
169
170The following constant and functions are obsolete; their functionality is
171available through :func:`find_module` or :func:`load_module`. They are kept
172around for backward compatibility:
173
174
175.. data:: SEARCH_ERROR
176
177   Unused.
178
179
180.. function:: init_builtin(name)
181
182   Initialize the built-in module called *name* and return its module object along
183   with storing it in ``sys.modules``.  If the module was already initialized, it
184   will be initialized *again*.  Re-initialization involves the copying of the
185   built-in module's ``__dict__`` from the cached module over the module's entry in
186   ``sys.modules``.  If there is no built-in module called *name*, ``None`` is
187   returned.
188
189
190.. function:: init_frozen(name)
191
192   Initialize the frozen module called *name* and return its module object.  If
193   the module was already initialized, it will be initialized *again*.  If there
194   is no frozen module called *name*, ``None`` is returned.  (Frozen modules are
195   modules written in Python whose compiled byte-code object is incorporated
196   into a custom-built Python interpreter by Python's :program:`freeze`
197   utility. See :file:`Tools/freeze/` for now.)
198
199
200.. function:: is_builtin(name)
201
202   Return ``1`` if there is a built-in module called *name* which can be
203   initialized again.  Return ``-1`` if there is a built-in module called *name*
204   which cannot be initialized again (see :func:`init_builtin`).  Return ``0`` if
205   there is no built-in module called *name*.
206
207
208.. function:: is_frozen(name)
209
210   Return ``True`` if there is a frozen module (see :func:`init_frozen`) called
211   *name*, or ``False`` if there is no such module.
212
213
214.. function:: load_compiled(name, pathname, [file])
215
216   .. index:: pair: file; byte-code
217
218   Load and initialize a module implemented as a byte-compiled code file and return
219   its module object.  If the module was already initialized, it will be
220   initialized *again*.  The *name* argument is used to create or access a module
221   object.  The *pathname* argument points to the byte-compiled code file.  The
222   *file* argument is the byte-compiled code file, open for reading in binary mode,
223   from the beginning. It must currently be a real file object, not a user-defined
224   class emulating a file.
225
226
227.. function:: load_dynamic(name, pathname[, file])
228
229   Load and initialize a module implemented as a dynamically loadable shared
230   library and return its module object.  If the module was already initialized, it
231   will be initialized *again*. Re-initialization involves copying the ``__dict__``
232   attribute of the cached instance of the module over the value used in the module
233   cached in ``sys.modules``.  The *pathname* argument must point to the shared
234   library.  The *name* argument is used to construct the name of the
235   initialization function: an external C function called ``initname()`` in the
236   shared library is called.  The optional *file* argument is ignored.  (Note:
237   using shared libraries is highly system dependent, and not all systems support
238   it.)
239
240
241.. function:: load_source(name, pathname[, file])
242
243   Load and initialize a module implemented as a Python source file and return its
244   module object.  If the module was already initialized, it will be initialized
245   *again*.  The *name* argument is used to create or access a module object.  The
246   *pathname* argument points to the source file.  The *file* argument is the
247   source file, open for reading as text, from the beginning. It must currently be
248   a real file object, not a user-defined class emulating a file.  Note that if a
249   properly matching byte-compiled file (with suffix :file:`.pyc` or :file:`.pyo`)
250   exists, it will be used instead of parsing the given source file.
251
252
253.. class:: NullImporter(path_string)
254
255   The :class:`NullImporter` type is a :pep:`302` import hook that handles
256   non-directory path strings by failing to find any modules.  Calling this type
257   with an existing directory or empty string raises :exc:`ImportError`.
258   Otherwise, a :class:`NullImporter` instance is returned.
259
260   Python adds instances of this type to ``sys.path_importer_cache`` for any path
261   entries that are not directories and are not handled by any other path hooks on
262   ``sys.path_hooks``.  Instances have only one method:
263
264
265   .. method:: NullImporter.find_module(fullname [, path])
266
267      This method always returns ``None``, indicating that the requested module could
268      not be found.
269
270   .. versionadded:: 2.5
271
272
273.. _examples-imp:
274
275Examples
276--------
277
278The following function emulates what was the standard import statement up to
279Python 1.4 (no hierarchical module names).  (This *implementation* wouldn't work
280in that version, since :func:`find_module` has been extended and
281:func:`load_module` has been added in 1.4.) ::
282
283   import imp
284   import sys
285
286   def __import__(name, globals=None, locals=None, fromlist=None):
287       # Fast path: see if the module has already been imported.
288       try:
289           return sys.modules[name]
290       except KeyError:
291           pass
292
293       # If any of the following calls raises an exception,
294       # there's a problem we can't handle -- let the caller handle it.
295
296       fp, pathname, description = imp.find_module(name)
297
298       try:
299           return imp.load_module(name, fp, pathname, description)
300       finally:
301           # Since we may exit via an exception, close fp explicitly.
302           if fp:
303               fp.close()
304
305.. index::
306   builtin: reload
307   module: knee
308
309A more complete example that implements hierarchical module names and includes a
310:func:`reload` function can be found in the module :mod:`knee`.  The :mod:`knee`
311module can be found in :file:`Demo/imputil/` in the Python source distribution.
312