36.2. imp
— Access the import internals¶
Source code: Lib/imp.py
This module provides an interface to the mechanisms used to implement the
import
statement. It defines the following constants and functions:
-
imp.
get_magic
()¶ Return the magic string value used to recognize byte-compiled code files (
.pyc
files). (This value may be different for each Python version.)Obsoleto desde la versión 3.4: Use
importlib.util.MAGIC_NUMBER
instead.
-
imp.
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-inopen()
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 valuesPY_SOURCE
,PY_COMPILED
, orC_EXTENSION
, described below.Obsoleto desde la versión 3.3: Use the constants defined on
importlib.machinery
instead.
-
imp.
find_module
(name[, path])¶ Try to find the module name. If path is omitted or
None
, the list of directory names given bysys.path
is searched, but first a few special places are searched: the function tries to find a built-in module with the given name (C_BUILTIN
), then a frozen module (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).Otherwise, path must be a list of directory names; each directory is searched for files with any of the suffixes returned by
get_suffixes()
above. Invalid names in the list are silently ignored (but all list items must be strings).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
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,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 isPKG_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
find_module()
andload_module()
to find and load package P, and then usefind_module()
with the path argument set toP.__path__
. When P itself has a dotted name, apply this recipe recursively.Obsoleto desde la versión 3.3: Use
importlib.util.find_spec()
instead unless Python 3.3 compatibility is required, in which case useimportlib.find_loader()
.
-
imp.
load_module
(name, file, pathname, description)¶ Load a module that was previously found by
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 beNone
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 byget_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
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 atry
…finally
statement.Obsoleto desde la versión 3.3: If previously used in conjunction with
imp.find_module()
then consider usingimportlib.import_module()
, otherwise use the loader returned by the replacement you chose forimp.find_module()
. If you calledimp.load_module()
and related functions directly then use the classes inimportlib.machinery
, e.g.importlib.machinery.SourceFileLoader(name, path).load_module()
.
-
imp.
new_module
(name)¶ Return a new empty module object called name. This object is not inserted in
sys.modules
.Obsoleto desde la versión 3.4: Use
types.ModuleType
instead.
-
imp.
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:
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
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
sys
,__main__
andbuiltins
. 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
from
…import
…, callingreload()
for the other module does not redefine the objects imported from it — one way around this is to re-execute thefrom
statement, another is to useimport
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.
Distinto en la versión 3.3: Relies on both
__name__
and__loader__
being defined on the module being reloaded instead of just__name__
.Obsoleto desde la versión 3.4: Use
importlib.reload()
instead.- 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
The following functions are conveniences for handling PEP 3147 byte-compiled file paths.
Nuevo en la versión 3.2.
-
imp.
cache_from_source
(path, debug_override=None)¶ Return the PEP 3147 path to the byte-compiled file associated with the source path. For example, if path is
/foo/bar/baz.py
the return value would be/foo/bar/__pycache__/baz.cpython-32.pyc
for Python 3.2. Thecpython-32
string comes from the current magic tag (seeget_tag()
; ifsys.implementation.cache_tag
is not defined thenNotImplementedError
will be raised). By passing inTrue
orFalse
for debug_override you can override the system’s value for__debug__
, leading to optimized bytecode.path need not exist.
Distinto en la versión 3.3: If
sys.implementation.cache_tag
isNone
, thenNotImplementedError
is raised.Obsoleto desde la versión 3.4: Use
importlib.util.cache_from_source()
instead.Distinto en la versión 3.5: The debug_override parameter no longer creates a
.pyo
file.
-
imp.
source_from_cache
(path)¶ Given the path to a PEP 3147 file name, return the associated source code file path. For example, if path is
/foo/bar/__pycache__/baz.cpython-32.pyc
the returned path would be/foo/bar/baz.py
. path need not exist, however if it does not conform to PEP 3147 format, aValueError
is raised. Ifsys.implementation.cache_tag
is not defined,NotImplementedError
is raised.Distinto en la versión 3.3: Raise
NotImplementedError
whensys.implementation.cache_tag
is not defined.Obsoleto desde la versión 3.4: Use
importlib.util.source_from_cache()
instead.
-
imp.
get_tag
()¶ Return the PEP 3147 magic tag string matching this version of Python’s magic number, as returned by
get_magic()
.Obsoleto desde la versión 3.4: Use
sys.implementation.cache_tag
directly starting in Python 3.3.
The following functions help interact with the import system’s internal locking mechanism. Locking semantics of imports are an implementation detail which may vary from release to release. However, Python ensures that circular imports work without any deadlocks.
-
imp.
lock_held
()¶ Return
True
if the global import lock is currently held, elseFalse
. On platforms without threads, always returnFalse
.On platforms with threads, a thread executing an import first holds a global import lock, then sets up a per-module lock for the rest of the import. This blocks other threads from importing the same module until the original import completes, preventing other threads from seeing incomplete module objects constructed by the original thread. An exception is made for circular imports, which by construction have to expose an incomplete module object at some point.
Distinto en la versión 3.3: The locking scheme has changed to per-module locks for the most part. A global import lock is kept for some critical tasks, such as initializing the per-module locks.
Obsoleto desde la versión 3.4.
-
imp.
acquire_lock
()¶ Acquire the interpreter’s global import lock for the current thread. This lock should be used by import hooks to ensure thread-safety when importing modules.
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.
Distinto en la versión 3.3: The locking scheme has changed to per-module locks for the most part. A global import lock is kept for some critical tasks, such as initializing the per-module locks.
Obsoleto desde la versión 3.4.
-
imp.
release_lock
()¶ Release the interpreter’s global import lock. On platforms without threads, this function does nothing.
Distinto en la versión 3.3: The locking scheme has changed to per-module locks for the most part. A global import lock is kept for some critical tasks, such as initializing the per-module locks.
Obsoleto desde la versión 3.4.
The following constants with integer values, defined in this module, are used
to indicate the search result of find_module()
.
-
imp.
PY_SOURCE
¶ The module was found as a source file.
Obsoleto desde la versión 3.3.
-
imp.
PY_COMPILED
¶ The module was found as a compiled code object file.
Obsoleto desde la versión 3.3.
-
imp.
C_EXTENSION
¶ The module was found as dynamically loadable shared library.
Obsoleto desde la versión 3.3.
-
imp.
PKG_DIRECTORY
¶ The module was found as a package directory.
Obsoleto desde la versión 3.3.
-
imp.
C_BUILTIN
¶ The module was found as a built-in module.
Obsoleto desde la versión 3.3.
-
imp.
PY_FROZEN
¶ The module was found as a frozen module.
Obsoleto desde la versión 3.3.
-
class
imp.
NullImporter
(path_string)¶ The
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 raisesImportError
. Otherwise, aNullImporter
instance is returned.Instances have only one method:
-
find_module
(fullname[, path])¶ This method always returns
None
, indicating that the requested module could not be found.
Distinto en la versión 3.3:
None
is inserted intosys.path_importer_cache
instead of an instance ofNullImporter
.Obsoleto desde la versión 3.4: Insert
None
intosys.path_importer_cache
instead.-
36.2.1. 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 find_module()
has been extended and
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()