| :mod:`imp` --- Access the :keyword:`import` internals |
| ===================================================== |
| |
| .. module:: imp |
| :synopsis: Access the implementation of the import statement. |
| |
| |
| .. index:: statement: import |
| |
| This module provides an interface to the mechanisms used to implement the |
| :keyword:`import` statement. It defines the following constants and functions: |
| |
| |
| .. function:: get_magic() |
| |
| .. index:: pair: file; byte-code |
| |
| Return the magic string value used to recognize byte-compiled code files |
| (:file:`.pyc` files). (This value may be different for each Python version.) |
| |
| |
| .. function:: get_suffixes() |
| |
| Return a list of 3-element tuples, each describing a particular type of |
| module. Each triple has the form ``(suffix, mode, type)``, where *suffix* is |
| a string to be appended to the module name to form the filename to search |
| for, *mode* is the mode string to pass to the built-in :func:`open` function |
| to open the file (this can be ``'r'`` for text files or ``'rb'`` for binary |
| files), and *type* is the file type, which has one of the values |
| :const:`PY_SOURCE`, :const:`PY_COMPILED`, or :const:`C_EXTENSION`, described |
| below. |
| |
| |
| .. function:: find_module(name[, path]) |
| |
| Try to find the module *name* on the search path *path*. If *path* is a list |
| of directory names, each directory is searched for files with any of the |
| suffixes returned by :func:`get_suffixes` above. Invalid names in the list |
| are silently ignored (but all list items must be strings). If *path* is |
| omitted or ``None``, the list of directory names given by ``sys.path`` is |
| searched, but first it searches a few special places: it tries to find a |
| built-in module with the given name (:const:`C_BUILTIN`), then a frozen |
| module (:const:`PY_FROZEN`), and on some systems some other places are looked |
| in as well (on Windows, it looks in the registry which may point to a |
| specific file). |
| |
| If search is successful, the return value is a 3-element tuple ``(file, |
| pathname, description)``: |
| |
| *file* is an open file object positioned at the beginning, *pathname* is the |
| pathname of the file found, and *description* is a 3-element tuple as |
| contained in the list returned by :func:`get_suffixes` describing the kind of |
| module found. |
| |
| If the module does not live in a file, the returned *file* is ``None``, |
| *pathname* is the empty string, and the *description* tuple contains empty |
| strings for its suffix and mode; the module type is indicated as given in |
| parentheses above. If the search is unsuccessful, :exc:`ImportError` is |
| raised. Other exceptions indicate problems with the arguments or |
| environment. |
| |
| If the module is a package, *file* is ``None``, *pathname* is the package |
| path and the last item in the *description* tuple is :const:`PKG_DIRECTORY`. |
| |
| This function does not handle hierarchical module names (names containing |
| dots). In order to find *P*.*M*, that is, submodule *M* of package *P*, use |
| :func:`find_module` and :func:`load_module` to find and load package *P*, and |
| then use :func:`find_module` with the *path* argument set to ``P.__path__``. |
| When *P* itself has a dotted name, apply this recipe recursively. |
| |
| |
| .. function:: load_module(name, file, pathname, description) |
| |
| Load a module that was previously found by :func:`find_module` (or by an |
| otherwise conducted search yielding compatible results). This function does |
| more than importing the module: if the module was already imported, it will |
| reload the module! The *name* argument indicates the full |
| module name (including the package name, if this is a submodule of a |
| package). The *file* argument is an open file, and *pathname* is the |
| corresponding file name; these can be ``None`` and ``''``, respectively, when |
| the module is a package or not being loaded from a file. The *description* |
| argument is a tuple, as would be returned by :func:`get_suffixes`, describing |
| what kind of module must be loaded. |
| |
| If the load is successful, the return value is the module object; otherwise, |
| an exception (usually :exc:`ImportError`) is raised. |
| |
| **Important:** the caller is responsible for closing the *file* argument, if |
| it was not ``None``, even when an exception is raised. This is best done |
| using a :keyword:`try` ... :keyword:`finally` statement. |
| |
| |
| .. function:: new_module(name) |
| |
| Return a new empty module object called *name*. This object is *not* inserted |
| in ``sys.modules``. |
| |
| |
| .. function:: lock_held() |
| |
| Return ``True`` if the import lock is currently held, else ``False``. On |
| platforms without threads, always return ``False``. |
| |
| On platforms with threads, a thread executing an import holds an internal lock |
| until the import is complete. This lock blocks other threads from doing an |
| import until the original import completes, which in turn prevents other threads |
| from seeing incomplete module objects constructed by the original thread while |
| in the process of completing its import (and the imports, if any, triggered by |
| that). |
| |
| |
| .. function:: acquire_lock() |
| |
| Acquire the interpreter's import lock for the current thread. This lock should |
| be used by import hooks to ensure thread-safety when importing modules. On |
| platforms without threads, this function does nothing. |
| |
| Once a thread has acquired the import lock, the same thread may acquire it |
| again without blocking; the thread must release it once for each time it has |
| acquired it. |
| |
| On platforms without threads, this function does nothing. |
| |
| |
| .. function:: release_lock() |
| |
| Release the interpreter's import lock. On platforms without threads, this |
| function does nothing. |
| |
| |
| .. function:: reload(module) |
| |
| Reload a previously imported *module*. The argument must be a module object, so |
| it must have been successfully imported before. This is useful if you have |
| edited the module source file using an external editor and want to try out the |
| new version without leaving the Python interpreter. The return value is the |
| module object (the same as the *module* argument). |
| |
| When ``reload(module)`` is executed: |
| |
| * Python modules' code is recompiled and the module-level code reexecuted, |
| defining a new set of objects which are bound to names in the module's |
| dictionary. The ``init`` function of extension modules is not called a second |
| time. |
| |
| * As with all other objects in Python the old objects are only reclaimed after |
| their reference counts drop to zero. |
| |
| * The names in the module namespace are updated to point to any new or changed |
| objects. |
| |
| * Other references to the old objects (such as names external to the module) are |
| not rebound to refer to the new objects and must be updated in each namespace |
| where they occur if that is desired. |
| |
| There are a number of other caveats: |
| |
| If a module is syntactically correct but its initialization fails, the first |
| :keyword:`import` statement for it does not bind its name locally, but does |
| store a (partially initialized) module object in ``sys.modules``. To reload the |
| module you must first :keyword:`import` it again (this will bind the name to the |
| partially initialized module object) before you can :func:`reload` it. |
| |
| When a module is reloaded, its dictionary (containing the module's global |
| variables) is retained. Redefinitions of names will override the old |
| definitions, so this is generally not a problem. If the new version of a module |
| does not define a name that was defined by the old version, the old definition |
| remains. This feature can be used to the module's advantage if it maintains a |
| global table or cache of objects --- with a :keyword:`try` statement it can test |
| for the table's presence and skip its initialization if desired:: |
| |
| try: |
| cache |
| except NameError: |
| cache = {} |
| |
| It is legal though generally not very useful to reload built-in or dynamically |
| loaded modules, except for :mod:`sys`, :mod:`__main__` and :mod:`__builtin__`. |
| In many cases, however, extension modules are not designed to be initialized |
| more than once, and may fail in arbitrary ways when reloaded. |
| |
| If a module imports objects from another module using :keyword:`from` ... |
| :keyword:`import` ..., calling :func:`reload` for the other module does not |
| redefine the objects imported from it --- one way around this is to re-execute |
| the :keyword:`from` statement, another is to use :keyword:`import` and qualified |
| names (*module*.*name*) instead. |
| |
| If a module instantiates instances of a class, reloading the module that defines |
| the class does not affect the method definitions of the instances --- they |
| continue to use the old class definition. The same is true for derived classes. |
| |
| |
| .. function:: acquire_lock() |
| |
| Acquires the interpreter's import lock for the current thread. This lock should |
| be used by import hooks to ensure thread-safety when importing modules. On |
| platforms without threads, this function does nothing. |
| |
| |
| .. function:: release_lock() |
| |
| Release the interpreter's import lock. On platforms without threads, this |
| function does nothing. |
| |
| |
| The following constants with integer values, defined in this module, are used to |
| indicate the search result of :func:`find_module`. |
| |
| |
| .. data:: PY_SOURCE |
| |
| The module was found as a source file. |
| |
| |
| .. data:: PY_COMPILED |
| |
| The module was found as a compiled code object file. |
| |
| |
| .. data:: C_EXTENSION |
| |
| The module was found as dynamically loadable shared library. |
| |
| |
| .. data:: PKG_DIRECTORY |
| |
| The module was found as a package directory. |
| |
| |
| .. data:: C_BUILTIN |
| |
| The module was found as a built-in module. |
| |
| |
| .. data:: PY_FROZEN |
| |
| The module was found as a frozen module (see :func:`init_frozen`). |
| |
| The following constant and functions are obsolete; their functionality is |
| available through :func:`find_module` or :func:`load_module`. They are kept |
| around for backward compatibility: |
| |
| |
| .. data:: SEARCH_ERROR |
| |
| Unused. |
| |
| |
| .. function:: init_builtin(name) |
| |
| Initialize the built-in module called *name* and return its module object along |
| with storing it in ``sys.modules``. If the module was already initialized, it |
| will be initialized *again*. Re-initialization involves the copying of the |
| built-in module's ``__dict__`` from the cached module over the module's entry in |
| ``sys.modules``. If there is no built-in module called *name*, ``None`` is |
| returned. |
| |
| |
| .. function:: init_frozen(name) |
| |
| Initialize the frozen module called *name* and return its module object. If |
| the module was already initialized, it will be initialized *again*. If there |
| is no frozen module called *name*, ``None`` is returned. (Frozen modules are |
| modules written in Python whose compiled byte-code object is incorporated |
| into a custom-built Python interpreter by Python's :program:`freeze` |
| utility. See :file:`Tools/freeze/` for now.) |
| |
| |
| .. function:: is_builtin(name) |
| |
| Return ``1`` if there is a built-in module called *name* which can be |
| initialized again. Return ``-1`` if there is a built-in module called *name* |
| which cannot be initialized again (see :func:`init_builtin`). Return ``0`` if |
| there is no built-in module called *name*. |
| |
| |
| .. function:: is_frozen(name) |
| |
| Return ``True`` if there is a frozen module (see :func:`init_frozen`) called |
| *name*, or ``False`` if there is no such module. |
| |
| |
| .. function:: load_compiled(name, pathname, [file]) |
| |
| .. index:: pair: file; byte-code |
| |
| Load and initialize a module implemented as a byte-compiled code file and return |
| its module object. If the module was already initialized, it will be |
| initialized *again*. The *name* argument is used to create or access a module |
| object. The *pathname* argument points to the byte-compiled code file. The |
| *file* argument is the byte-compiled code file, open for reading in binary mode, |
| from the beginning. It must currently be a real file object, not a user-defined |
| class emulating a file. |
| |
| |
| .. function:: load_dynamic(name, pathname[, file]) |
| |
| Load and initialize a module implemented as a dynamically loadable shared |
| library and return its module object. If the module was already initialized, it |
| will be initialized *again*. Re-initialization involves copying the ``__dict__`` |
| attribute of the cached instance of the module over the value used in the module |
| cached in ``sys.modules``. The *pathname* argument must point to the shared |
| library. The *name* argument is used to construct the name of the |
| initialization function: an external C function called ``initname()`` in the |
| shared library is called. The optional *file* argument is ignored. (Note: |
| using shared libraries is highly system dependent, and not all systems support |
| it.) |
| |
| |
| .. function:: load_source(name, pathname[, file]) |
| |
| Load and initialize a module implemented as a Python source file and return its |
| module object. If the module was already initialized, it will be initialized |
| *again*. The *name* argument is used to create or access a module object. The |
| *pathname* argument points to the source file. The *file* argument is the |
| source file, open for reading as text, from the beginning. It must currently be |
| a real file object, not a user-defined class emulating a file. Note that if a |
| properly matching byte-compiled file (with suffix :file:`.pyc` or :file:`.pyo`) |
| exists, it will be used instead of parsing the given source file. |
| |
| |
| .. class:: NullImporter(path_string) |
| |
| The :class:`NullImporter` type is a :pep:`302` import hook that handles |
| non-directory path strings by failing to find any modules. Calling this type |
| with an existing directory or empty string raises :exc:`ImportError`. |
| Otherwise, a :class:`NullImporter` instance is returned. |
| |
| Python adds instances of this type to ``sys.path_importer_cache`` for any path |
| entries that are not directories and are not handled by any other path hooks on |
| ``sys.path_hooks``. Instances have only one method: |
| |
| |
| .. method:: NullImporter.find_module(fullname [, path]) |
| |
| This method always returns ``None``, indicating that the requested module could |
| not be found. |
| |
| |
| .. _examples-imp: |
| |
| Examples |
| -------- |
| |
| The following function emulates what was the standard import statement up to |
| Python 1.4 (no hierarchical module names). (This *implementation* wouldn't work |
| in that version, since :func:`find_module` has been extended and |
| :func:`load_module` has been added in 1.4.) :: |
| |
| import imp |
| import sys |
| |
| def __import__(name, globals=None, locals=None, fromlist=None): |
| # Fast path: see if the module has already been imported. |
| try: |
| return sys.modules[name] |
| except KeyError: |
| pass |
| |
| # If any of the following calls raises an exception, |
| # there's a problem we can't handle -- let the caller handle it. |
| |
| fp, pathname, description = imp.find_module(name) |
| |
| try: |
| return imp.load_module(name, fp, pathname, description) |
| finally: |
| # Since we may exit via an exception, close fp explicitly. |
| if fp: |
| fp.close() |
| |
| .. index:: module: knee |
| |
| A more complete example that implements hierarchical module names and includes a |
| :func:`reload` function can be found in the module :mod:`knee`. The :mod:`knee` |
| module can be found in :file:`Demo/imputil/` in the Python source distribution. |
| |