New in version 3.1.
The purpose of the importlib package is two-fold. One is to provide an implementation of the import statement (and thus, by extension, the __import__() function) in Python source code. This provides an implementation of import which is portable to any Python interpreter. This also provides a reference implementation which is easier to comprehend than one in a programming language other than Python.
Two, the components to implement import can be exposed in this package, making it easier for users to create their own custom objects (known generically as an importer) to participate in the import process. Details on providing custom importers can be found in PEP 302.
See also
Import a module. The name argument specifies what module to import in absolute or relative terms (e.g. either pkg.mod or ..mod). If the name is specified in relative terms, then the package argument must be set to the name of the package which is to act as the anchor for resolving the package name (e.g. import_module('..mod', 'pkg.subpkg') will import pkg.mod).
The import_module() function acts as a simplifying wrapper around importlib.__import__(). This means all semantics of the function are derived from importlib.__import__(), including requiring the package from which an import is occurring to have been previously imported (i.e., package must already be imported). The most important difference is that import_module() returns the most nested package or module that was imported (e.g. pkg.mod), while __import__() returns the top-level package or module (e.g. pkg).
The importlib.abc module contains all of the core abstract base classes used by import. Some subclasses of the core abstract base classes are also provided to help in implementing the core ABCs.
An abstract base class representing a finder. See PEP 302 for the exact definition for a finder.
An abstract base class for a loader. See PEP 302 for the exact definition for a loader.
An abstract method for loading a module. If the module cannot be loaded, ImportError is raised, otherwise the loaded module is returned.
If the requested module already exists in sys.modules, that module should be used and reloaded. Otherwise the loader should create a new module and insert it into sys.modules before any loading begins, to prevent recursion from the import. If the loader inserted a module and the load fails, it must be removed by the loader from sys.modules; modules already in sys.modules before the loader began execution should be left alone. The importlib.util.module_for_loader() decorator handles all of these details.
The loader should set several attributes on the module. (Note that some of these attributes can change when a module is reloaded.)
The name of the module.
The path to where the module data is stored (not set for built-in modules).
A list of strings specifying the search path within a package. This attribute is not set on modules.
The parent package for the module/package. If the module is top-level then it has a value of the empty string. The importlib.util.set_package() decorator can handle the details for __package__.
The loader used to load the module. (This is not set by the built-in import machinery, but it should be set whenever a loader is used.)
An abstract base class for a loader which implements the optional PEP 302 protocol for loading arbitrary resources from the storage back-end.
An abstract base class for a loader which implements the optional PEP 302 protocol for loaders that inspect modules.
An abstract base class inheriting from importlib.abc.InspectLoader and importlib.abc.ResourceLoader designed to ease the loading of Python source modules (bytecode is not handled; see importlib.abc.PyPycLoader for a source/bytecode ABC). A subclass implementing this ABC will only need to worry about exposing how the source code is stored; all other details for loading Python source code will be handled by the concrete implementations of key methods.
An abstract base class inheriting from importlib.abc.PyLoader. This ABC is meant to help in creating loaders that support both Python source and bytecode.
This module contains the various objects that help import find and load modules.
An importer for built-in modules. All known built-in modules are listed in sys.builtin_module_names. This class implements the importlib.abc.Finder and importlib.abc.InspectLoader ABCs.
Only class methods are defined by this class to alleviate the need for instantiation.
An importer for frozen modules. This class implements the importlib.abc.Finder and importlib.abc.InspectLoader ABCs.
Only class methods are defined by this class to alleviate the need for instantiation.
Finder for sys.path. This class implements the importlib.abc.Finder ABC.
This class does not perfectly mirror the semantics of import in terms of sys.path. No implicit path hooks are assumed for simplification of the class and its semantics.
Only class method are defined by this class to alleviate the need for instantiation.
This module contains the various objects that help in the construction of an importer.
A decorator for a loader method, to handle selecting the proper module object to load with. The decorated method is expected to have a call signature taking two positional arguments (e.g. load_module(self, module)) for which the second argument will be the module object to be used by the loader. Note that the decorator will not work on static methods because of the assumption of two arguments.
The decorated method will take in the name of the module to be loaded as expected for a loader. If the module is not found in sys.modules then a new one is constructed with its __name__ attribute set. Otherwise the module found in sys.modules will be passed into the method. If an exception is raised by the decorated method and a module was added to sys.modules it will be removed to prevent a partially initialized module from being in left in sys.modules. If the module was already in sys.modules then it is left alone.
Use of this decorator handles all the details of which module object a loader should initialize as specified by PEP 302.
A decorator for a loader to set the __package__ attribute on the module returned by the loader. If __package__ is set and has a value other than None it will not be changed. Note that the module returned by the loader is what has the attribute set on and not the module found in sys.modules.
Reliance on this decorator is discouraged when it is possible to set __package__ before the execution of the code is possible. By setting it before the code for the module is executed it allows the attribute to be used at the global level of the module during initialization.
Below is an example meta path importer that uses a dict for back-end storage for source code. While not an optimal solution – manipulations of __path__ on packages does not influence import – it does illustrate what little is required to implement an importer.
"""An importer where source is stored in a dict."""
from importlib import abc
class DictImporter(abc.Finder, abc.PyLoader):
"""A meta path importer that stores source code in a dict.
The keys are the module names -- packages must end in ``.__init__``.
The values must be something that can be passed to 'bytes'.
"""
def __init__(self, memory):
"""Store the dict."""
self.memory = memory
def contains(self, name):
"""See if a module or package is in the dict."""
if name in self.memory:
return name
package_name = '{}.__init__'.format(name)
if package_name in self.memory:
return package_name
return False
__contains__ = contains # Convenience.
def find_module(self, fullname, path=None):
"""Find the module in the dict."""
if fullname in self:
return self
return None
def source_path(self, fullname):
"""Return the module name if the module is in the dict."""
if not fullname in self:
raise ImportError
return fullname
def get_data(self, path):
"""Return the bytes for the source.
The value found in the dict is passed through 'bytes' before being
returned.
"""
name = self.contains(path)
if not name:
raise IOError
return bytes(self.memory[name])
def is_package(self, fullname):
"""Tell if module is a package based on whether the dict contains the
name with ``.__init__`` appended to it."""
if fullname not in self:
raise ImportError
if fullname in self.memory:
return False
# If name is in this importer but not as it is then it must end in
# ``__init__``.
else:
return True