29.1. imp — Access the import internals — Python v3.1.5 documentation

29.1. imp — Access the import internals — Python v3.1.5 documentation
index
modules
next
previous
Python v3.1.5 documentation
The Python Standard Library
29. Importing Modules
29.1.
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
. If
path
is omitted or
None
, the list of
directory names given by
sys.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 is
PKG_DIRECTORY
This function does not handle hierarchical module names (names containing
dots). In order to find
.*M*, that is, submodule
of package
, use
find_module()
and
load_module()
to find and load package
, and
then use
find_module()
with the
path
argument set to
P.__path__
When
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 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
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.
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.
imp.
release_lock
Release the interpreter’s import lock. On platforms without threads, this
function does nothing.
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:
If a module is syntactically correct but its initialization fails, the first
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
import
it again (this will bind the name to the
partially initialized module object) before you can
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
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__
and
__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
from
...
import
..., calling
reload()
for the other module does not
redefine the objects imported from it — one way around this is to re-execute
the
from
statement, another is to use
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.
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()
).
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.
29.1.1. 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
()
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.
Table Of Contents
29.1.
imp
— Access the
import
internals
29.1.1. Examples
Previous topic
29. Importing Modules
Next topic
29.2.
zipimport
— Import modules from Zip archives
This Page
Report a Bug
Show Source
Quick search
Enter search terms or a module, class or function name.
index
modules
next
previous
Python v3.1.5 documentation
The Python Standard Library
29. Importing Modules
1990-2012, Python Software Foundation.
The Python Software Foundation is a non-profit corporation.
Please donate.
Found a bug
Created using
Sphinx
0.6.5.