imp — Access the import internals
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.)
-
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-in 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
PY_SOURCE, PY_COMPILED, or C_EXTENSION, described
below.
-
imp.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 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 (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).
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 is 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
find_module() and load_module() to find and load package P, and
then use find_module() with the path argument set to P.__path__.
When P itself has a dotted name, apply this recipe recursively.
-
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 is
equivalent to a reload()! 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 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 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 try ... finally statement.
-
imp.new_module(name)
- Return a new empty module object called name. This object is not inserted
in sys.modules.
-
imp.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).
-
imp.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.
New in version 2.3.
-
imp.release_lock()
Release the interpreter’s import lock. On platforms without threads, this
function does nothing.
New in version 2.3.
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.
-
imp.PY_COMPILED
- The module was found as a compiled code object file.
-
imp.C_EXTENSION
- The module was found as dynamically loadable shared library.
-
imp.PKG_DIRECTORY
- The module was found as a package directory.
-
imp.C_BUILTIN
- The module was found as a built-in module.
-
imp.PY_FROZEN
- The module was found as a frozen module (see init_frozen()).
The following constant and functions are obsolete; their functionality is
available through find_module() or load_module(). They are kept
around for backward compatibility:
-
imp.SEARCH_ERROR
- Unused.
-
imp.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.
-
imp.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 freeze
utility. See Tools/freeze/ for now.)
-
imp.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 init_builtin()). Return 0 if
there is no built-in module called name.
-
imp.is_frozen(name)
- Return True if there is a frozen module (see init_frozen()) called
name, or False if there is no such module.
-
imp.load_compiled(name, pathname[, file])
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.
-
imp.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.)
-
imp.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 .pyc or .pyo)
exists, it will be used instead of parsing the given source file.
-
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 raises ImportError.
Otherwise, a 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:
-
find_module(fullname[, path])
- This method always returns None, indicating that the requested module could
not be found.
New in version 2.5.
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()
A more complete example that implements hierarchical module names and includes a
reload() function can be found in the module knee. The knee
module can be found in Demo/imputil/ in the Python source distribution.
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