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* 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
48 If search is successful, the return value is a 3-element tuple ``(file,
49 pathname, description)``:
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
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
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`.
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.
73 .. function:: load_module(name, file, pathname, description)
75 .. index:: builtin: reload
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.
88 If the load is successful, the return value is the module object; otherwise,
89 an exception (usually :exc:`ImportError`) is raised.
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.
96 .. function:: new_module(name)
98 Return a new empty module object called *name*. This object is *not* inserted
102 .. function:: lock_held()
104 Return ``True`` if the import lock is currently held, else ``False``. On
105 platforms without threads, always return ``False``.
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
115 .. function:: acquire_lock()
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.
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
125 On platforms without threads, this function does nothing.
127 .. versionadded:: 2.3
130 .. function:: release_lock()
132 Release the interpreter's import lock. On platforms without threads, this
133 function does nothing.
135 .. versionadded:: 2.3
137 The following constants with integer values, defined in this module, are used to
138 indicate the search result of :func:`find_module`.
143 The module was found as a source file.
146 .. data:: PY_COMPILED
148 The module was found as a compiled code object file.
151 .. data:: C_EXTENSION
153 The module was found as dynamically loadable shared library.
156 .. data:: PKG_DIRECTORY
158 The module was found as a package directory.
163 The module was found as a built-in module.
168 The module was found as a frozen module (see :func:`init_frozen`).
170 The following constant and functions are obsolete; their functionality is
171 available through :func:`find_module` or :func:`load_module`. They are kept
172 around for backward compatibility:
175 .. data:: SEARCH_ERROR
180 .. function:: init_builtin(name)
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
190 .. function:: init_frozen(name)
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.)
200 .. function:: is_builtin(name)
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*.
208 .. function:: is_frozen(name)
210 Return ``True`` if there is a frozen module (see :func:`init_frozen`) called
211 *name*, or ``False`` if there is no such module.
214 .. function:: load_compiled(name, pathname, [file])
216 .. index:: pair: file; byte-code
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.
227 .. function:: load_dynamic(name, pathname[, file])
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
241 .. function:: load_source(name, pathname[, file])
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.
253 .. class:: NullImporter(path_string)
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.
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:
265 .. method:: NullImporter.find_module(fullname [, path])
267 This method always returns ``None``, indicating that the requested module could
270 .. versionadded:: 2.5
278 The following function emulates what was the standard import statement up to
279 Python 1.4 (no hierarchical module names). (This *implementation* wouldn't work
280 in that version, since :func:`find_module` has been extended and
281 :func:`load_module` has been added in 1.4.) ::
286 def __import__(name, globals=None, locals=None, fromlist=None):
287 # Fast path: see if the module has already been imported.
289 return sys.modules[name]
293 # If any of the following calls raises an exception,
294 # there's a problem we can't handle -- let the caller handle it.
296 fp, pathname, description = imp.find_module(name)
299 return imp.load_module(name, fp, pathname, description)
301 # Since we may exit via an exception, close fp explicitly.
309 A 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`
311 module can be found in :file:`Demo/imputil/` in the Python source distribution.