2 :mod:`imp` --- Access the :keyword:`import` internals
3 =====================================================
6 :synopsis: Access the implementation of the import statement.
9 .. index:: statement: import
11 This module provides an interface to the mechanisms used to implement the
12 :keyword:`import` statement. It defines the following constants and functions:
15 .. function:: get_magic()
17 .. index:: pair: file; byte-code
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.)
23 .. function:: get_suffixes()
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
35 .. function:: find_module(name[, path])
37 Try to find the module *name*. If *path* is omitted or ``None``, the list of
38 directory names given by ``sys.path`` is searched, but first a few special
39 places are searched: the function tries to find a built-in module with the
40 given name (:const:`C_BUILTIN`), then a frozen module (:const:`PY_FROZEN`),
41 and on some systems some other places are looked in as well (on Windows, it
42 looks in the registry which may point to a specific file).
44 Otherwise, *path* must be a list of directory names; each directory is
45 searched for files with any of the suffixes returned by :func:`get_suffixes`
46 above. Invalid names in the list are silently ignored (but all list items
49 If search is successful, the return value is a 3-element tuple ``(file,
50 pathname, description)``:
52 *file* is an open file object positioned at the beginning, *pathname* is the
53 pathname of the file found, and *description* is a 3-element tuple as
54 contained in the list returned by :func:`get_suffixes` describing the kind of
57 If the module does not live in a file, the returned *file* is ``None``,
58 *pathname* is the empty string, and the *description* tuple contains empty
59 strings for its suffix and mode; the module type is indicated as given in
60 parentheses above. If the search is unsuccessful, :exc:`ImportError` is
61 raised. Other exceptions indicate problems with the arguments or
64 If the module is a package, *file* is ``None``, *pathname* is the package
65 path and the last item in the *description* tuple is :const:`PKG_DIRECTORY`.
67 This function does not handle hierarchical module names (names containing
68 dots). In order to find *P*.*M*, that is, submodule *M* of package *P*, use
69 :func:`find_module` and :func:`load_module` to find and load package *P*, and
70 then use :func:`find_module` with the *path* argument set to ``P.__path__``.
71 When *P* itself has a dotted name, apply this recipe recursively.
74 .. function:: load_module(name, file, pathname, description)
76 .. index:: builtin: reload
78 Load a module that was previously found by :func:`find_module` (or by an
79 otherwise conducted search yielding compatible results). This function does
80 more than importing the module: if the module was already imported, it is
81 equivalent to a :func:`reload`! The *name* argument indicates the full
82 module name (including the package name, if this is a submodule of a
83 package). The *file* argument is an open file, and *pathname* is the
84 corresponding file name; these can be ``None`` and ``''``, respectively, when
85 the module is a package or not being loaded from a file. The *description*
86 argument is a tuple, as would be returned by :func:`get_suffixes`, describing
87 what kind of module must be loaded.
89 If the load is successful, the return value is the module object; otherwise,
90 an exception (usually :exc:`ImportError`) is raised.
92 **Important:** the caller is responsible for closing the *file* argument, if
93 it was not ``None``, even when an exception is raised. This is best done
94 using a :keyword:`try` ... :keyword:`finally` statement.
97 .. function:: new_module(name)
99 Return a new empty module object called *name*. This object is *not* inserted
103 .. function:: lock_held()
105 Return ``True`` if the import lock is currently held, else ``False``. On
106 platforms without threads, always return ``False``.
108 On platforms with threads, a thread executing an import holds an internal lock
109 until the import is complete. This lock blocks other threads from doing an
110 import until the original import completes, which in turn prevents other threads
111 from seeing incomplete module objects constructed by the original thread while
112 in the process of completing its import (and the imports, if any, triggered by
116 .. function:: acquire_lock()
118 Acquire the interpreter's import lock for the current thread. This lock should
119 be used by import hooks to ensure thread-safety when importing modules. On
120 platforms without threads, this function does nothing.
122 Once a thread has acquired the import lock, the same thread may acquire it
123 again without blocking; the thread must release it once for each time it has
126 On platforms without threads, this function does nothing.
128 .. versionadded:: 2.3
131 .. function:: release_lock()
133 Release the interpreter's import lock. On platforms without threads, this
134 function does nothing.
136 .. versionadded:: 2.3
138 The following constants with integer values, defined in this module, are used to
139 indicate the search result of :func:`find_module`.
144 The module was found as a source file.
147 .. data:: PY_COMPILED
149 The module was found as a compiled code object file.
152 .. data:: C_EXTENSION
154 The module was found as dynamically loadable shared library.
157 .. data:: PKG_DIRECTORY
159 The module was found as a package directory.
164 The module was found as a built-in module.
169 The module was found as a frozen module (see :func:`init_frozen`).
171 The following constant and functions are obsolete; their functionality is
172 available through :func:`find_module` or :func:`load_module`. They are kept
173 around for backward compatibility:
176 .. data:: SEARCH_ERROR
181 .. function:: init_builtin(name)
183 Initialize the built-in module called *name* and return its module object along
184 with storing it in ``sys.modules``. If the module was already initialized, it
185 will be initialized *again*. Re-initialization involves the copying of the
186 built-in module's ``__dict__`` from the cached module over the module's entry in
187 ``sys.modules``. If there is no built-in module called *name*, ``None`` is
191 .. function:: init_frozen(name)
193 Initialize the frozen module called *name* and return its module object. If
194 the module was already initialized, it will be initialized *again*. If there
195 is no frozen module called *name*, ``None`` is returned. (Frozen modules are
196 modules written in Python whose compiled byte-code object is incorporated
197 into a custom-built Python interpreter by Python's :program:`freeze`
198 utility. See :file:`Tools/freeze/` for now.)
201 .. function:: is_builtin(name)
203 Return ``1`` if there is a built-in module called *name* which can be
204 initialized again. Return ``-1`` if there is a built-in module called *name*
205 which cannot be initialized again (see :func:`init_builtin`). Return ``0`` if
206 there is no built-in module called *name*.
209 .. function:: is_frozen(name)
211 Return ``True`` if there is a frozen module (see :func:`init_frozen`) called
212 *name*, or ``False`` if there is no such module.
215 .. function:: load_compiled(name, pathname, [file])
217 .. index:: pair: file; byte-code
219 Load and initialize a module implemented as a byte-compiled code file and return
220 its module object. If the module was already initialized, it will be
221 initialized *again*. The *name* argument is used to create or access a module
222 object. The *pathname* argument points to the byte-compiled code file. The
223 *file* argument is the byte-compiled code file, open for reading in binary mode,
224 from the beginning. It must currently be a real file object, not a user-defined
225 class emulating a file.
228 .. function:: load_dynamic(name, pathname[, file])
230 Load and initialize a module implemented as a dynamically loadable shared
231 library and return its module object. If the module was already initialized, it
232 will be initialized *again*. Re-initialization involves copying the ``__dict__``
233 attribute of the cached instance of the module over the value used in the module
234 cached in ``sys.modules``. The *pathname* argument must point to the shared
235 library. The *name* argument is used to construct the name of the
236 initialization function: an external C function called ``initname()`` in the
237 shared library is called. The optional *file* argument is ignored. (Note:
238 using shared libraries is highly system dependent, and not all systems support
242 .. function:: load_source(name, pathname[, file])
244 Load and initialize a module implemented as a Python source file and return its
245 module object. If the module was already initialized, it will be initialized
246 *again*. The *name* argument is used to create or access a module object. The
247 *pathname* argument points to the source file. The *file* argument is the
248 source file, open for reading as text, from the beginning. It must currently be
249 a real file object, not a user-defined class emulating a file. Note that if a
250 properly matching byte-compiled file (with suffix :file:`.pyc` or :file:`.pyo`)
251 exists, it will be used instead of parsing the given source file.
254 .. class:: NullImporter(path_string)
256 The :class:`NullImporter` type is a :pep:`302` import hook that handles
257 non-directory path strings by failing to find any modules. Calling this type
258 with an existing directory or empty string raises :exc:`ImportError`.
259 Otherwise, a :class:`NullImporter` instance is returned.
261 Python adds instances of this type to ``sys.path_importer_cache`` for any path
262 entries that are not directories and are not handled by any other path hooks on
263 ``sys.path_hooks``. Instances have only one method:
266 .. method:: NullImporter.find_module(fullname [, path])
268 This method always returns ``None``, indicating that the requested module could
271 .. versionadded:: 2.5
279 The following function emulates what was the standard import statement up to
280 Python 1.4 (no hierarchical module names). (This *implementation* wouldn't work
281 in that version, since :func:`find_module` has been extended and
282 :func:`load_module` has been added in 1.4.) ::
287 def __import__(name, globals=None, locals=None, fromlist=None):
288 # Fast path: see if the module has already been imported.
290 return sys.modules[name]
294 # If any of the following calls raises an exception,
295 # there's a problem we can't handle -- let the caller handle it.
297 fp, pathname, description = imp.find_module(name)
300 return imp.load_module(name, fp, pathname, description)
302 # Since we may exit via an exception, close fp explicitly.
310 A more complete example that implements hierarchical module names and includes a
311 :func:`reload` function can be found in the module :mod:`knee`. The :mod:`knee`
312 module can be found in :file:`Demo/imputil/` in the Python source distribution.