/Doc/library/imp.rst

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