importlib_metadata is a library that provides for access to installed
package metadata. 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 in Python 3.7
and newer (backported as importlib_resources for older versions of
Python), this can eliminate the need to use the older and less efficient
By “installed package” we generally mean a third-party package installed into
site-packages directory via tools such as pip. Specifically,
it means a package with either a discoverable
directory, and metadata defined by PEP 566 or its older specifications.
By default, package metadata can live on the file system or in zip archives on
sys.path. Through an extension mechanism, the metadata can live almost
Let’s say you wanted to get the version string for a package you’ve installed
pip. We start by creating a virtual environment and installing
something into it:
$ python3 -m venv example $ source example/bin/activate (example) $ pip install importlib_metadata (example) $ 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 the set of entry points keyed by group, such as
distutils.commands and others. Each group contains a
sequence 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']
This package provides the following functionality via its public API.
entry_points() function returns a dictionary of all entry points,
keyed by group. Entry points are represented by
EntryPoint has a
.value attributes and
.load() method to resolve the value:
>>> eps = entry_points() >>> list(eps) ['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation'] >>> scripts = eps['console_scripts'] >>> wheel = [ep for ep in scripts if ep.name == 'wheel'] >>> wheel EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts') >>> main = wheel.load() >>> main <function main at 0x103528488>
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 entrypoints, their definition, and usage.
Every distribution includes some metadata, which you can extract using the
>>> wheel_metadata = metadata('wheel')
The keys of the returned data structure 1 name the metadata keywords, and their values are returned unparsed from the distribution metadata:
>>> wheel_metadata['Requires-Python'] '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
version() function is the quickest way to get a distribution’s version
number, as a string:
>>> version('wheel') '0.32.3'
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.Path derived object with additional
hash properties as indicated by the metadata. For example:
>>> util = [p for p in files('wheel') if 'util.py' in str(p)] >>> 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
To get the full set of requirements for a distribution, use the
function. Note that this returns an iterator:
>>> list(requires('wheel')) ["pytest (>=3.0.0) ; extra == 'test'"]
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 package. You can
>>> from importlib_metadata import distribution >>> dist = distribution('wheel')
Thus, an alternative way to get the version number is through the
>>> dist.version '0.32.3'
There are all kinds of additional metadata available on the
>>> d.metadata['Requires-Python'] '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*' >>> d.metadata['License'] 'MIT'
The full set of available metadata is not described here. See PEP 566 for additional details.
Extending the search algorithm¶
Because package metadata is not available through
sys.path searches, or
package loaders directly, the metadata for a package is found through import
system finders. To find a distribution package’s metadata,
importlib_metadata queries the list of meta path finders on
importlib_metadata installs a finder for distribution packages
found on the file system. This finder doesn’t actually find any packages,
but it can find the packages’ 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. If the finder has this method, it must return
an iterator over instances of the
Distribution abstract class. This
method must have the signature:
def find_distributions(name=None, path=None): """Return an iterable of all Distribution instances capable of loading the metadata for packages matching the name (or all names if not supplied) along the paths in the list of directories ``path`` (defaults to sys.path). """
What this means in practice is that to support finding distribution package
metadata in locations other than the file system, you should derive from
Distribution and implement the
load_metadata() method. This takes a
single argument which is the name of the package whose metadata is being
found. This instance of the
Distribution base abstract class is what your
find_distributions() method should return.