使用 importlib.metadata

3.8 新版功能.

在 3.10 版更改: importlib.metadata 不再是暂定的。

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

importlib_metadata 是一个库，它提供对已安装 分发包 的元数据的访问，例如它的入口点或它的顶级名称（导入包 、模块，如果有的话）。这个库部分构建在 Python 的 import 系统上，它打算取代 pkg_resources 的 entry point API 和 metadata API 中的类似功能。连同 importlib.resources，这个包可以消除使用旧的和效率较低的 pkg_resources 包的需要。

importlib_metadata 操作通过 pip 等工具安装到 Python 的 site-packages 目录中的第三方分发包。具体来说，它适用于具有可发现的 dist-info 或 egg-info 目录的发行版，以及由 Core 元数据规范定义的元数据。

重要

它们不一定等同于或与可在 Python 代码中导入的顶级导入包名 1:1 对应。一个发布包可以包含多个导入包(和单个模块)，如果是命名空间包，一个顶级导入包可以映射到多个发布包。您可以使用 package_distributions() 来获取它们之间的映射。

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.

参见

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.

概述

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.

功能性 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. With importlib_metadata 5.0 and Python 3.12, entry_points always returns an EntryPoints object. See backports.entry_points_selectable for compatibility options.

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.*'

备注

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 <https://importlib-metadata.readthedocs.io/en/latest/api.html#importlib_metadata.PackageMetadata>.

在 3.10 版更改: The Description is now included in the metadata when presented through the payload. Line continuation characters have been removed.

3.10 新版功能: 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'], ...}

3.10 新版功能.

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.