Using importlib.metadata

Source code: Lib/importlib/metadata.py

Novo na versão 3.8.

Nota

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 package.

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.

Visão Geral

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

Você pode obter a string de versão para wheel executando o seguinte:

(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 objects.

Você pode obter os metadados para uma distribuição:

>>> 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']

Você também pode obter uma número da versão da distribuição, listar seus arquivos constituintes e obter uma lista dos Requisitos de distribuição da distribuição.

API funcional

Este pacote fornece a seguinte funcionalidade por meio de sua API pública.

Pontos de entrada

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 and a .load() method to resolve the value. There are also .module, .attr, and .extras attributes for getting the components of the .value attribute:

>>> 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>

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.

Metadados de distribuição

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

>>> 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.*'

Versões de distribuição

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

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

Arquivos de distribuição

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>

Uma vez que tenha o arquivo, você também pode ler seu conteúdo:

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

Você também pode usar o método locate para obter o caminho absoluto para o arquivo:

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

No caso em que o arquivo de metadados que lista os arquivos (RECORD ou SOURCES.txt) estiver faltando, files() retornará None. O chamador pode querer agrupar chamadas para files() em always_iterable ou de outra forma se proteger contra isso condição se a distribuição de destino não for conhecida por ter os metadados presentes.

Requisitos de distribuição

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'"]

Distribuições

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 instance:

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

Assim, uma forma alternativa de obter o número da versão é através da instância Distribution:

>>> dist.version  
'0.32.3'

Existem todos os tipos de metadados adicionais disponíveis na instância Distribution:

>>> 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.

Estendendo o algoritmo de pesquisa

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 sys.meta_path.

The default PathFinder for Python includes a hook that calls into importlib.metadata.MetadataPathFinder for finding distributions loaded from typical file-system-based paths.

A classe abstrata importlib.abc.MetaPathFinder define a interface esperada dos localizadores pelo sistema de importação do Python. importlib.metadata estende este protocolo procurando por um chamável find_distributions opcional nos localizadores de sys.meta_path e apresenta esta interface estendida como a classe base abstrata DistributionFinder, que define este método abstrato:

@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``.
    """

O objeto DistributionFinder.Context fornece propriedades .path e .name indicando o caminho para pesquisar e o nome a ser correspondido e pode fornecer outro contexto relevante.

O que isso significa na prática é que para prover suporte à localização de metadados de pacotes de distribuição em outros locais fora do sistema de arquivos, crie uma subclasse Distribution e implemente os métodos abstratos. Então, a partir de um localizador personalizado, retorne instâncias deste derivado de Distribution no método find_distributions().

Notas de rodapé

1

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.