Using importlib.metadata — Python 3.11.1 documentation

# Using `importlib.metadata`

New in version 3.8.

Changed in version 3.10: `importlib.metadata` is no longer provisional.

Source code: Lib/importlib/metadata/__init__.py

`importlib_metadata` is a library that provides access to the metadata of an installed Distribution Package, such as its entry points or its top-level names (Import Packages, modules, if any). Built in part on Python’s import system, this library intends to replace similar functionality in the entry point API and metadata API of `pkg_resources`. Along with `importlib.resources`, this package can eliminate the need to use the older and less efficient `pkg_resources` package.

`importlib_metadata` operates on third-party distribution packages installed into Python’s `site-packages` directory via tools such as pip. Specifically, it works with distributions with discoverable `dist-info` or `egg-info` directories, and metadata defined by the Core metadata specifications.

Important

These are not necessarily equivalent to or correspond 1:1 with the top-level import package names that can be imported inside Python code. One distribution package can contain multiple import packages (and single modules), and one top-level import package may map to multiple distribution packages if it is a namespace package. You can use package_distributions() to get a mapping between them.

By default, distribution metadata can live on the file system or in zip archives on `sys.path`. Through an extension mechanism, the metadata can live almost anywhere.

See also

https://importlib-metadata.readthedocs.io/

  The documentation for `importlib_metadata`, which supplies a backport of `importlib.metadata`. This includes an API reference for this module’s classes and functions, as well as a migration guide for existing users of `pkg_resources`.

## Overview

Let’s say you wanted to get the version string for a Distribution Package you’ve installed using `pip`. We start by creating a virtual environment and installing something into it:

    $ python3 -m venv example
    $ source example/bin/activate
    (example) $ python -m pip install wheel

You can get the version string for `wheel` by running the following:

    (example) $ python
    >>> from importlib.metadata import version
    >>> version('wheel')
    '0.32.3'

You can also get a collection of entry points selectable by properties of the EntryPoint (typically ‘group’ or ‘name’), such as `console_scripts`, `distutils.commands` and others. Each group contains a collection of EntryPoint objects.

You can get the metadata for a distribution:

    >>> list(metadata('wheel'))
    ['Metadata-Version', 'Name', 'Version', 'Summary', 'Home-page', 'Author', 'Author-email', 'Maintainer', 'Maintainer-email', 'License', 'Project-URL', 'Project-URL', 'Project-URL', 'Keywords', 'Platform', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Requires-Python', 'Provides-Extra', 'Requires-Dist', 'Requires-Dist']

You can also get a distribution’s version number, list its constituent files, and get a list of the distribution’s Distribution requirements.

## Functional API

This package provides the following functionality via its public API.

### Entry points

The `entry_points()` function returns a collection of entry points. Entry points are represented by `EntryPoint` instances; each `EntryPoint` has a `.name`, `.group`, and `.value` attributes and a `.load()` method to resolve the value. There are also `.module`, `.attr`, and `.extras` attributes for getting the components of the `.value` attribute.

Query all entry points:

    >>> eps = entry_points()

The `entry_points()` function returns an `EntryPoints` object, a collection of all `EntryPoint` objects with `names` and `groups` attributes for convenience:

    >>> sorted(eps.groups)
    ['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation']

`EntryPoints` has a `select` method to select entry points matching specific properties. Select entry points in the `console_scripts` group:

    >>> scripts = eps.select(group='console_scripts')

Equivalently, since `entry_points` passes keyword arguments through to select:

    >>> scripts = entry_points(group='console_scripts')

Pick out a specific script named “wheel” (found in the wheel project):

    >>> 'wheel' in scripts.names
    True
    >>> wheel = scripts['wheel']

Equivalently, query for that entry point during selection:

    >>> (wheel,) = entry_points(group='console_scripts', name='wheel')
    >>> (wheel,) = entry_points().select(group='console_scripts', name='wheel')

Inspect the resolved entry point:

    >>> wheel
    EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts')
    >>> wheel.module
    'wheel.cli'
    >>> wheel.attr
    'main'
    >>> wheel.extras
    []
    >>> main = wheel.load()
    >>> main
    <function main at 0x103528488>

The `group` and `name` are arbitrary values defined by the package author and usually a client will wish to resolve all entry points for a particular group. Read the setuptools docs for more information on entry points, their definition, and usage.

Compatibility Note

The “selectable” entry points were introduced in `importlib_metadata` 3.6 and Python 3.10. Prior to those changes, `entry_points` accepted no parameters and always returned a dictionary of entry points, keyed by group. For compatibility, if no parameters are passed to entry_points, a `SelectableGroups` object is returned, implementing that dict interface. In the future, calling `entry_points` with no parameters will return an `EntryPoints` object. Users should rely on the selection interface to retrieve entry points by group.

### Distribution metadata

Every Distribution Package includes some metadata, which you can extract using the `metadata()` function:

    >>> wheel_metadata = metadata('wheel')

The keys of the returned data structure, a `PackageMetadata`, name the metadata keywords, and the values are returned unparsed from the distribution metadata:

    >>> wheel_metadata['Requires-Python']
    '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'

`PackageMetadata` also presents a `json` attribute that returns all the metadata in a JSON-compatible form per PEP 566:

    >>> wheel_metadata.json['requires_python']
    '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'

Note

The actual type of the object returned by `metadata()` is an implementation detail and should be accessed only through the interface described by the PackageMetadata protocol.

Changed in version 3.10: The `Description` is now included in the metadata when presented through the payload. Line continuation characters have been removed.

New in version 3.10: The `json` attribute was added.

### Distribution versions

The `version()` function is the quickest way to get a Distribution Package’s version number, as a string:

    >>> version('wheel')
    '0.32.3'

### Distribution files

You can also get the full set of files contained within a distribution. The `files()` function takes a Distribution Package name and returns all of the files installed by this distribution. Each file object returned is a `PackagePath`, a `pathlib.PurePath` derived object with additional `dist`, `size`, and `hash` properties as indicated by the metadata. For example:

    >>> util = [p for p in files('wheel') if 'util.py' in str(p)][0]
    >>> util
    PackagePath('wheel/util.py')
    >>> util.size
    859
    >>> util.dist
    <importlib.metadata._hooks.PathDistribution object at 0x101e0cef0>
    >>> util.hash
    <FileHash mode: sha256 value: bYkw5oMccfazVCoYQwKkkemoVyMAFoR34mmKBx8R1NI>

Once you have the file, you can also read its contents:

    >>> print(util.read_text())
    import base64
    import sys
    ...
    def as_bytes(s):
        if isinstance(s, text_type):
            return s.encode('utf-8')
        return s

You can also use the `locate` method to get a the absolute path to the file:

    >>> util.locate()
    PosixPath('/home/gustav/example/lib/site-packages/wheel/util.py')

In the case where the metadata file listing files (RECORD or SOURCES.txt) is missing, `files()` will return `None`. The caller may wish to wrap calls to `files()` in always_iterable or otherwise guard against this condition if the target distribution is not known to have the metadata present.

### Distribution requirements

To get the full set of requirements for a Distribution Package, use the `requires()` function:

    >>> requires('wheel')
    ["pytest (>=3.0.0) ; extra == 'test'", "pytest-cov ; extra == 'test'"]

### Mapping import to distribution packages

A convenience method to resolve the Distribution Package name (or names, in the case of a namespace package) that provide each importable top-level Python module or Import Package:

    >>> packages_distributions()
    {'importlib_metadata': ['importlib-metadata'], 'yaml': ['PyYAML'], 'jaraco': ['jaraco.classes', 'jaraco.functools'], ...}

New in version 3.10.

## Distributions

While the above API is the most common and convenient usage, you can get all of that information from the `Distribution` class. A `Distribution` is an abstract object that represents the metadata for a Python Distribution Package. You can get the `Distribution` instance:

    >>> from importlib.metadata import distribution
    >>> dist = distribution('wheel')

Thus, an alternative way to get the version number is through the `Distribution` instance:

    >>> dist.version
    '0.32.3'

There are all kinds of additional metadata available on the `Distribution` instance:

    >>> dist.metadata['Requires-Python']
    '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
    >>> dist.metadata['License']
    'MIT'

The full set of available metadata is not described here. See the Core metadata specifications for additional details.

## Distribution Discovery

By default, this package provides built-in support for discovery of metadata for file system and zip file Distribution Packages. This metadata finder search defaults to `sys.path`, but varies slightly in how it interprets those values from how other import machinery does. In particular:

- `importlib.metadata` does not honor `bytes` objects on `sys.path`.

- `importlib.metadata` will incidentally honor `pathlib.Path` objects on `sys.path` even though such values will be ignored for imports.

## Extending the search algorithm

Because Distribution Package metadata is not available through `sys.path` searches, or package loaders directly, the metadata for a distribution is found through import system finders. To find a distribution package’s metadata, `importlib.metadata` queries the list of meta path finders on `sys.meta_path`.

By default `importlib_metadata` installs a finder for distribution packages found on the file system. This finder doesn’t actually find any distributions, but it can find their metadata.

The abstract class `importlib.abc.MetaPathFinder` defines the interface expected of finders by Python’s import system. `importlib.metadata` extends this protocol by looking for an optional `find_distributions` callable on the finders from `sys.meta_path` and presents this extended interface as the `DistributionFinder` abstract base class, which defines this abstract method:

    @abc.abstractmethod
    def find_distributions(context=DistributionFinder.Context()):
        """Return an iterable of all Distribution instances capable of
        loading the metadata for packages for the indicated ``context``.
        """

The `DistributionFinder.Context` object provides `.path` and `.name` properties indicating the path to search and name to match and may supply other relevant context.

What this means in practice is that to support finding distribution package metadata in locations other than the file system, subclass `Distribution` and implement the abstract methods. Then from a custom finder, return instances of this derived `Distribution` in the `find_distributions()` method.