importlib.metadata
¶
源代码： Lib/importlib/metadata.py
3.8 版新增。
注意
This functionality is provisional and may deviate from the usual version semantics of the standard library.
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
pkg_resources
包。
By “installed package” we generally mean a third-party package installed into Python’s
site-packages
directory via tools such as
pip
. Specifically, it means a package with either a discoverable
dist-info
or
egg-info
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 anywhere.
Let’s say you wanted to get the version string for a 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) $ 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
console_scripts
,
distutils.commands
and others. Each group contains a sequence of
EntryPoint
对象。
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 分发要求 .
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
instances; each
EntryPoint
拥有
.name
,
.group
，和
.value
attributes and a
.load()
method to resolve the value. There are also
.module
,
.attr
，和
.extras
attributes for getting the components of the
.value
属性：
>>> 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'][0]
>>> 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>
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.
Every distribution includes some metadata, which you can extract using the
metadata()
函数：
>>> 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
，
pathlib.PurePath
derived object with additional
dist
,
size
，和
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()
将返回
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.
To get the full set of requirements for a distribution, use the
requires()
函数：
>>> requires('wheel')
["pytest (>=3.0.0) ; extra == 'test'", "pytest-cov ; 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 get the
Distribution
实例：
>>> from importlib.metadata import distribution
>>> dist = distribution('wheel')
Thus, an alternative way to get the version number is through the
Distribution
实例：
>>> dist.version
'0.32.3'
There are all kinds of additional metadata available on the
Distribution
实例：
>>> 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 PEP 566 for additional details.
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
元路径查找器
on
sys.meta_path
.
默认
PathFinder
for Python includes a hook that calls into
importlib.metadata.MetadataPathFinder
for finding distributions loaded from typical file-system-based paths.
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``.
"""
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
在
find_distributions()
方法。
脚注
Technically, the returned distribution metadata object is an
email.message.EmailMessage
instance, but this is an implementation detail, and not part of the stable API. You should only use dictionary-like methods and syntax to access the metadata contents.