Import hook support.
Consistent use of this module will make it possible to change the
different mechanisms involved in loading modules independently.
While the built-in module imp exports interfaces to the built-in
module searching and loading algorithm, and it is possible to replace
the built-in function __import__ in order to change the semantics of
the import statement, until now it has been difficult to combine the
effect of different __import__ hacks, like loading modules from URLs
by rimport.py, or restricted execution by rexec.py.
This module defines three new concepts:
1) A "file system hooks" class provides an interface to a filesystem.
One hooks class is defined (Hooks), which uses the interface provided
by standard modules os and os.path. It should be used as the base
class for other hooks classes.
2) A "module loader" class provides an interface to to search for a
module in a search path and to load it. It defines a method which
searches for a module in a single directory; by overriding this method
one can redefine the details of the search. If the directory is None,
built-in and frozen modules are searched instead.
Two module loader class are defined, both implementing the search
strategy used by the built-in __import__ function: ModuleLoader uses
the imp module's find_module interface, while HookableModuleLoader
uses a file system hooks class to interact with the file system. Both
use the imp module's load_* interfaces to actually load the module.
3) A "module importer" class provides an interface to import a
module, as well as interfaces to reload and unload a module. It also
provides interfaces to install and uninstall itself instead of the
default __import__ and reload (and unload) functions.
One module importer class is defined (ModuleImporter), which uses a
module loader instance passed in (by default HookableModuleLoader is
The classes defined here should be used as base classes for extended
functionality along those lines.
If a module mporter class supports dotted names, its import_module()
must return a different value depending on whether it is called on
behalf of a "from ... import ..." statement or not. (This is caused
by the way the __import__ hook is used by the Python interpreter.) It
would also do wise to install a different version of reload().
BUILTIN_MODULE = 6
C_BUILTIN = 6
C_EXTENSION = 3
FROZEN_MODULE = 7
PKG_DIRECTORY = 5
PY_COMPILED = 2
PY_FROZEN = 7
PY_SOURCE = 1
VERBOSE = 0