29.6. importlib – An implementation of import

New in version 3.1:

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 implementation of import which is portable to any Python interpreter. This also provides a reference implementation which is easier to comprehend than one implemented in a programming language other than Python.

Two, the components to implement import are 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 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 import statement is syntactic sugar for this function.

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 3120

Using UTF-8 as the Default Source Encoding

PEP 3147

PYC Repository Directories

29.6.2. Functions

importlib.__import__(name, globals={}, locals={}, fromlist=list(), level=0)

An implementation of the built-in __import__() function.

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.ExecutionLoader

An abstract base class which inherits from InspectLoader that, when implemented, helps a module to be executed as a script. The ABC represents an optional PEP 302 protocol.

get_filename(fullname)

An abstract method that is to return the value of __file__ for the specified module. If no path is available, ImportError is raised.

If source code is available, then the method should return the path to the source file, regardless of whether a bytecode was used to load the module.

class importlib.abc.SourceLoader

An abstract base class for implementing source (and optionally bytecode) file loading. The class inherits from both ResourceLoader and ExecutionLoader, requiring the implementation of:

• ResourceLoader.get_data()

• ExecutionLoader.get_filename()

  Should only return the path to the source file; sourceless loading is not supported.

The abstract methods defined by this class are to add optional bytecode file support. Not implementing these optional methods causes the loader to only work with source code. Implementing the methods allows the loader to work with source and bytecode files; it does not allow for sourceless loading where only bytecode is provided. Bytecode files are an optimization to speed up loading by removing the parsing step of Python’s compiler, and so no bytecode-specific API is exposed.

path_mtime(self, path)

Optional abstract method which returns the modification time for the specified path.

set_data(self, path, data)

Optional abstract method which writes the specified bytes to a file path. Any intermediate directories which do not exist are to be created automatically.

When writing to the path fails because the path is read-only (errno.EACCES), do not propagate the exception.

get_code(self, fullname)

Concrete implementation of InspectLoader.get_code().

load_module(self, fullname)

Concrete implementation of Loader.load_module().

get_source(self, fullname)

Concrete implementation of InspectLoader.get_source().

is_package(self, fullname)

Concrete implementation of InspectLoader.is_package(). A module is determined to be a package if its file path is a file named __init__ when the file extension is removed.

class importlib.abc.PyLoader

An abstract base class inheriting from ExecutionLoader and ResourceLoader designed to ease the loading of Python source modules (bytecode is not handled; see SourceLoader 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.

Deprecated since version 3.2:

Deprecated since version 3.2: This class has been deprecated in favor of SourceLoader and is slated for removal in Python 3.4. See below for how to create a subclass that is compatible with Python 3.1 onwards.

If compatibility with Python 3.1 is required, then use the following idiom to implement a subclass that will work with Python 3.1 onwards (make sure to implement ExecutionLoader.get_filename()):

try:
    from importlib.abc import SourceLoader
except ImportError:
    from importlib.abc import PyLoader as SourceLoader


class CustomLoader(SourceLoader):
    def get_filename(self, fullname):
        """Return the path to the source file."""
        # Implement ...

    def source_path(self, fullname):
        """Implement source_path in terms of get_filename."""
        try:
            return self.get_filename(fullname)
        except ImportError:
            return None

    def is_package(self, fullname):
        """Implement is_package by looking for an __init__ file
        name as returned by get_filename."""
        filename = os.path.basename(self.get_filename(fullname))
        return os.path.splitext(filename)[0] == '__init__'

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. Raises ImportError if the loader knows it cannot handle the module.

get_filename(fullname)

A concrete implementation of importlib.abc.ExecutionLoader.get_filename() that relies on source_path(). If source_path() returns None, then ImportError is raised.

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()) and compiling it with the built-in compile() function.

get_source(fullname)

A concrete implementation of importlib.abc.InspectLoader.get_source(). Uses importlib.abc.ResourceLoader.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 PyLoader. This ABC is meant to help in creating loaders that support both Python source and bytecode.

Deprecated since version 3.2:

Deprecated since version 3.2: This class has been deprecated in favor of SourceLoader and to properly support PEP 3147. If compatibility is required with Python 3.1, implement both SourceLoader and PyLoader; instructions on how to do so are included in the documentation for PyLoader. Do note that this solution will not support sourceless/bytecode-only loading; only source and bytecode loading.