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# 29.6. `importlib` – An implementation of `import`

New in version 3.1.

## 29.6.1. Introduction

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 implementaiton 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

The import statement
  The language reference for the `import` statement.
Packages specification
  Original specification of packages. Some semantics have changed since the writing of this document (e.g. redirecting based on `None` in `sys.modules`).
The `__import__()` function
  The built-in function for which the `import` statement is syntactic sugar.
PEP 235
  Import on Case-Insensitive Platforms
PEP 263
  Defining Python Source Code Encodings
PEP 302
  New Import Hooks.
PEP 328
  Imports: Multi-Line and Absolute/Relative
PEP 366
  Main module explicit relative imports
PEP 3128
  Using UTF-8 as the Default Source Encoding

## 29.6.2. Functions

`importlib.``__import__`(name, globals={}, locals={}, fromlist=list(), level=0)
  An implementation of the built-in `__import__()` function. See the built-in function’s documentation for usage instructions.

`importlib.``import_module`(name, package=None)

  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`).

## 29.6.3. `importlib.abc` – Abstract base classes related to import

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.

class `importlib.abc.``Finder`

  An abstract base class representing a finder. See PEP 302 for the exact definition for a finder.

  `find_module`(fullname, path=None)
    An abstract method for finding a loader for the specified module. If the finder is found on `sys.meta_path` and the module to be searched for is a subpackage or module then path will be the value of `__path__` from the parent package. If a loader cannot be found, `None` is returned.

class `importlib.abc.``Loader`

  An abstract base class for a loader. See PEP 302 for the exact definition for a loader.

  `load_module`(fullname)

    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.)

    - `__name__`

        The name of the module.

    - `__file__`

        The path to where the module data is stored (not set for built-in modules).

    - `__path__`

        A list of strings specifying the search path within a package. This attribute is not set on modules.

    - `__package__`

        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__`.

    - `__loader__`

        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.)

class `importlib.abc.``ResourceLoader`

  An abstract base class for a loader which implements the optional PEP 302 protocol for loading arbitrary resources from the storage back-end.

  `get_data`(path)
    An abstract method to return the bytes for the data located at path. Loaders that have a file-like storage back-end that allows storing arbitrary data can implement this abstract method to give direct access to the data stored. `IOError` is to be raised if the path cannot be found. The path is expected to be constructed using a module’s `__file__` attribute or an item from a package’s `__path__`.

class `importlib.abc.``InspectLoader`

  An abstract base class for a loader which implements the optional PEP 302 protocol for loaders that inspect modules.

  `get_code`(fullname)
    An abstract method to return the `code` object for a module. `None` is returned if the module does not have a code object (e.g. built-in module). `ImportError` is raised if loader cannot find the requested module.
  `get_source`(fullname)
    An abstract method to return the source of a module. It is returned as a text string with universal newlines. Returns `None` if no source is available (e.g. a built-in module). Raises `ImportError` if the loader cannot find the module specified.
  `is_package`(fullname)
    An abstract method to return a true value if the module is a package, a false value otherwise. `ImportError` is raised if the loader cannot find the module.

class `importlib.abc.``PyLoader`

  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.

  `source_path`(fullname)
    An abstract method that returns the path to the source code for a module. Should return `None` if there is no source code. `ImportError` if the module cannot be found.
  `load_module`(fullname)
    A concrete implementation of `importlib.abc.Loader.load_module()` that loads Python source code. All needed information comes from the abstract methods required by this ABC. The only pertinent assumption made by this method is that when loading a package `__path__` is set to `[os.path.dirname(__file__)]`.
  `get_code`(fullname)
    A concrete implementation of `importlib.abc.InspectLoader.get_code()` that creates code objects from Python source code, by requesting the source code (using `source_path()` and `get_data()`), converting it to standard newlines, and compiling it with the built-in `compile()` function.
  `get_source`(fullname)
    A concrete implementation of `importlib.abc.InspectLoader.get_source()`. Uses `importlib.abc.InspectLoader.get_data()` and `source_path()` to get the source code. It tries to guess the source encoding using `tokenize.detect_encoding()`.

class `importlib.abc.``PyPycLoader`

  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.

  `source_mtime`(fullname)
    An abstract method which returns the modification time for the source code of the specified module. The modification time should be an integer. If there is no source code, return `None. If the module cannot be found then :exc:`ImportError` is raised.
  `bytecode_path`(fullname)
    An abstract method which returns the path to the bytecode for the specified module, if it exists. It returns `None` if no bytecode exists (yet). Raises `ImportError` if the module is not found.
  `write_bytecode`(fullname, bytecode)
    An abstract method which has the loader write bytecode for future use. If the bytecode is written, return `True`. Return `False` if the bytecode could not be written. This method should not be called if `sys.dont_write_bytecode` is true. The bytecode argument should be a bytes string or bytes array.

## 29.6.4. `importlib.machinery` – Importers and path hooks

This module contains the various objects that help `import` find and load modules.

class `importlib.machinery.``BuiltinImporter`

  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.

class `importlib.machinery.``FrozenImporter`

  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.

class `importlib.machinery.``PathFinder`

  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.

  classmethod `find_module`(fullname, path=None)
    Class method that attempts to find a loader for the module specified by fullname on `sys.path` or, if defined, on path. For each path entry that is searched, `sys.path_importer_cache` is checked. If an non-false object is found then it is used as the finder to look for the module being searched for. If no entry is found in `sys.path_importer_cache`, then `sys.path_hooks` is searched for a finder for the path entry and, if found, is stored in `sys.path_importer_cache` along with being queried about the module. If no finder is ever found then `None` is returned.

## 29.6.5. `importlib.util` – Utility code for importers

This module contains the various objects that help in the construction of an importer.

`importlib.util.``module_for_loader`(method)

  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.

`importlib.util.``set_loader`(fxn)
  A decorator for a loader method, to set the `__loader__` attribute on loaded modules. If the attribute is already set the decorator does nothing. It is assumed that the first positional argument to the wrapped method is what `__loader__` should be set to.

`importlib.util.``set_package`(fxn)

  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.

## 29.6.6. Example

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

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