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 

    11 This 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 

   137 The following constants with integer values, defined in this module, are used to

   138 indicate 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 

   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:

   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 

   275 Examples

   276 --------

   277 

   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.) ::

   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 

   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.

   312