Using importlib.metadata

注解

This functionality is provisional and may deviate from the usualversion semantics of the standard library.

importlib.metadata is a library that provides for access to installedpackage metadata. Built in part on Python's import system, this libraryintends to replace similar functionality in the entry pointAPI and metadata API of pkg_resources. Along withimportlib.resources in Python 3.7and newer (backported as importlib_resources for older versions ofPython), this can eliminate the need to use the older and less efficientpkg_resources package.

By "installed package" we generally mean a third-party package installed intoPython's site-packages directory via tools such as pip. Specifically,it means a package with either a discoverable dist-info or egg-infodirectory, and metadata defined by PEP 566 or its older specifications.By default, package metadata can live on the file system or in zip archives onsys.path. Through an extension mechanism, the metadata can live almostanywhere.

概述

Let's say you wanted to get the version string for a package you've installedusing pip. We start by creating a virtual environment and installingsomething into it:

$ python3 -m venv example
$ source example/bin/activate
(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 asconsole_scripts, distutils.commands and others. Each group contains asequence 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 itsconstituent files, and get a list of the distribution'sDistribution requirements.

可用 API

This package provides the following functionality via its public API.

Entry points

The entry_points() function returns a dictionary of all entry points,keyed by group. Entry points are represented by EntryPoint instances;each EntryPoint has a .name, .group, and .value attributes anda .load() method to resolve the value.

>>> eps = entry_points()  # doctest: +SKIP
>>> list(eps)  # doctest: +SKIP
['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation']
>>> scripts = eps['console_scripts']  # doctest: +SKIP
>>> wheel = [ep for ep in scripts if ep.name == 'wheel'][0]  # doctest: +SKIP
>>> wheel  # doctest: +SKIP
EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts')
>>> main = wheel.load()  # doctest: +SKIP
>>> main  # doctest: +SKIP
<function main at 0x103528488>

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

Distribution metadata

Every distribution includes some metadata, which you can extract using themetadata() function:

>>> wheel_metadata = metadata('wheel')  

The keys of the returned data structure 1 name the metadata keywords, andtheir values are returned unparsed from the distribution metadata:

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

Distribution versions

The version() function is the quickest way to get a distribution's versionnumber, as a string:

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

Distribution files

You can also get the full set of files contained within a distribution. Thefiles() function takes a distribution package name and returns all of thefiles installed by this distribution. Each file object returned is aPackagePath, a pathlib.Path 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

In the case where the metadata file listing files(RECORD or SOURCES.txt) is missing, files() willreturn None. The caller may wish to wrap calls tofiles() in always_iterableor otherwise guard against this condition if the targetdistribution is not known to have the metadata present.

Distribution requirements

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

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

Distributions

While the above API is the most common and convenient usage, you can get allof that information from the Distribution class. A Distribution is anabstract object that represents the metadata for a Python package. You canget the Distribution instance:

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

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

>>> dist.version  
'0.32.3'

There are all kinds of additional metadata available on the Distributioninstance:

>>> 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, orpackage loaders directly, the metadata for a package is found through importsystem finders. To find a distribution package's metadata,importlib.metadata queries the list of meta path finders onsys.meta_path.

By default importlib.metadata installs a finder for distribution packagesfound 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 theinterface expected of finders by Python's import system.importlib.metadata extends this protocol by looking for an optionalfind_distributions callable on the finders fromsys.meta_path and presents this extended interface as theDistributionFinder abstract base class, which defines this abstractmethod:

@abc.abstractmethoddef 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 .nameproperties indicating the path to search and names to match and maysupply other relevant context.

What this means in practice is that to support finding distribution packagemetadata in locations other than the file system, you should derive fromDistribution and implement the load_metadata() method. Then fromyour finder, return instances of this derived Distribution in thefind_distributions() method.

备注

• 1
• Technically, the returned distribution metadata object is anemail.message.Messageinstance, but this is an implementation detail, and not part of thestable API. You should only use dictionary-like methods and syntaxto access the metadata contents.