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annotate planemo/lib/python3.7/site-packages/importlib_metadata/docs/using.rst @ 1:56ad4e20f292 draft

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"planemo upload commit 6eee67778febed82ddd413c3ca40b3183a3898f1"
author guerler
date Fri, 31 Jul 2020 00:32:28 -0400
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1 .. _using:
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2
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3 =================================
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4 Using :mod:`!importlib_metadata`
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5 =================================
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6
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7 ``importlib_metadata`` is a library that provides for access to installed
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8 package metadata. Built in part on Python's import system, this library
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9 intends to replace similar functionality in the `entry point
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10 API`_ and `metadata API`_ of ``pkg_resources``. Along with
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11 :mod:`importlib.resources` in Python 3.7
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12 and newer (backported as :doc:`importlib_resources <importlib_resources:index>` for older versions of
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13 Python), this can eliminate the need to use the older and less efficient
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14 ``pkg_resources`` package.
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15
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16 By "installed package" we generally mean a third-party package installed into
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17 Python's ``site-packages`` directory via tools such as `pip
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18 <https://pypi.org/project/pip/>`_. Specifically,
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19 it means a package with either a discoverable ``dist-info`` or ``egg-info``
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20 directory, and metadata defined by :pep:`566` or its older specifications.
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21 By default, package metadata can live on the file system or in zip archives on
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22 :data:`sys.path`. Through an extension mechanism, the metadata can live almost
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23 anywhere.
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25
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26 Overview
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27 ========
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28
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29 Let's say you wanted to get the version string for a package you've installed
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30 using ``pip``. We start by creating a virtual environment and installing
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31 something into it::
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32
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33 $ python3 -m venv example
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34 $ source example/bin/activate
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35 (example) $ pip install importlib_metadata
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36 (example) $ pip install wheel
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37
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38 You can get the version string for ``wheel`` by running the following::
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39
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40 (example) $ python
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41 >>> from importlib_metadata import version
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42 >>> version('wheel')
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43 '0.32.3'
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44
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45 You can also get the set of entry points keyed by group, such as
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46 ``console_scripts``, ``distutils.commands`` and others. Each group contains a
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47 sequence of :ref:`EntryPoint <entry-points>` objects.
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48
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49 You can get the :ref:`metadata for a distribution <metadata>`::
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50
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51 >>> list(metadata('wheel'))
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52 ['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']
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53
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54 You can also get a :ref:`distribution's version number <version>`, list its
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55 :ref:`constituent files <files>`, and get a list of the distribution's
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56 :ref:`requirements`.
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58
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59 Functional API
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60 ==============
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61
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62 This package provides the following functionality via its public API.
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63
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64
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65 .. _entry-points:
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66
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67 Entry points
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68 ------------
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69
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70 The ``entry_points()`` function returns a dictionary of all entry points,
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71 keyed by group. Entry points are represented by ``EntryPoint`` instances;
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72 each ``EntryPoint`` has a ``.name``, ``.group``, and ``.value`` attributes and
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73 a ``.load()`` method to resolve the value. There are also ``.module``,
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74 ``.attr``, and ``.extras`` attributes for getting the components of the
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75 ``.value`` attribute::
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76
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77 >>> eps = entry_points()
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78 >>> list(eps)
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79 ['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation']
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80 >>> scripts = eps['console_scripts']
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81 >>> wheel = [ep for ep in scripts if ep.name == 'wheel'][0]
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82 >>> wheel
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83 EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts')
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84 >>> wheel.module
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85 'wheel.cli'
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86 >>> wheel.attr
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87 'main'
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88 >>> wheel.extras
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89 []
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90 >>> main = wheel.load()
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91 >>> main
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92 <function main at 0x103528488>
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93
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94 The ``group`` and ``name`` are arbitrary values defined by the package author
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95 and usually a client will wish to resolve all entry points for a particular
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96 group. Read `the setuptools docs
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97 <https://setuptools.readthedocs.io/en/latest/setuptools.html#dynamic-discovery-of-services-and-plugins>`_
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98 for more information on entry points, their definition, and usage.
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99
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100
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101 .. _metadata:
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102
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103 Distribution metadata
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104 ---------------------
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105
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106 Every distribution includes some metadata, which you can extract using the
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107 ``metadata()`` function::
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108
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109 >>> wheel_metadata = metadata('wheel')
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110
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111 The keys of the returned data structure [#f1]_ name the metadata keywords, and
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112 their values are returned unparsed from the distribution metadata::
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113
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114 >>> wheel_metadata['Requires-Python']
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115 '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
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116
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117
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118 .. _version:
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119
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120 Distribution versions
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121 ---------------------
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122
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123 The ``version()`` function is the quickest way to get a distribution's version
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124 number, as a string::
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125
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126 >>> version('wheel')
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127 '0.32.3'
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128
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129
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130 .. _files:
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131
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132 Distribution files
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133 ------------------
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134
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135 You can also get the full set of files contained within a distribution. The
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136 ``files()`` function takes a distribution package name and returns all of the
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137 files installed by this distribution. Each file object returned is a
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138 ``PackagePath``, a :class:`pathlib.Path` derived object with additional ``dist``,
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139 ``size``, and ``hash`` properties as indicated by the metadata. For example::
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140
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141 >>> util = [p for p in files('wheel') if 'util.py' in str(p)][0]
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142 >>> util
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143 PackagePath('wheel/util.py')
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144 >>> util.size
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145 859
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146 >>> util.dist
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147 <importlib_metadata._hooks.PathDistribution object at 0x101e0cef0>
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148 >>> util.hash
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149 <FileHash mode: sha256 value: bYkw5oMccfazVCoYQwKkkemoVyMAFoR34mmKBx8R1NI>
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150
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151 Once you have the file, you can also read its contents::
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152
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153 >>> print(util.read_text())
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154 import base64
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155 import sys
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156 ...
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157 def as_bytes(s):
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158 if isinstance(s, text_type):
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159 return s.encode('utf-8')
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160 return s
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161
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162 In the case where the metadata file listing files
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163 (RECORD or SOURCES.txt) is missing, ``files()`` will
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164 return ``None``. The caller may wish to wrap calls to
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165 ``files()`` in `always_iterable
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166 <https://more-itertools.readthedocs.io/en/stable/api.html#more_itertools.always_iterable>`_
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167 or otherwise guard against this condition if the target
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168 distribution is not known to have the metadata present.
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169
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170 .. _requirements:
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171
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172 Distribution requirements
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173 -------------------------
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174
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175 To get the full set of requirements for a distribution, use the ``requires()``
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176 function::
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177
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178 >>> requires('wheel')
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179 ["pytest (>=3.0.0) ; extra == 'test'", "pytest-cov ; extra == 'test'"]
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180
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181
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182 Distributions
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183 =============
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184
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185 While the above API is the most common and convenient usage, you can get all
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186 of that information from the ``Distribution`` class. A ``Distribution`` is an
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187 abstract object that represents the metadata for a Python package. You can
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188 get the ``Distribution`` instance::
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189
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190 >>> from importlib_metadata import distribution
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191 >>> dist = distribution('wheel')
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192
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193 Thus, an alternative way to get the version number is through the
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194 ``Distribution`` instance::
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195
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196 >>> dist.version
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197 '0.32.3'
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198
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199 There are all kinds of additional metadata available on the ``Distribution``
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200 instance::
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201
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202 >>> d.metadata['Requires-Python']
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203 '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
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204 >>> d.metadata['License']
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205 'MIT'
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206
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207 The full set of available metadata is not described here. See :pep:`566`
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208 for additional details.
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209
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210
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211 Extending the search algorithm
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212 ==============================
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213
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214 Because package metadata is not available through :data:`sys.path` searches, or
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215 package loaders directly, the metadata for a package is found through import
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216 system `finders`_. To find a distribution package's metadata,
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217 ``importlib.metadata`` queries the list of :term:`meta path finders <meta path finder>` on
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218 :data:`sys.meta_path`.
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219
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220 By default ``importlib_metadata`` installs a finder for distribution packages
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221 found on the file system. This finder doesn't actually find any *packages*,
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222 but it can find the packages' metadata.
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223
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224 The abstract class :py:class:`importlib.abc.MetaPathFinder` defines the
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225 interface expected of finders by Python's import system.
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226 ``importlib_metadata`` extends this protocol by looking for an optional
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227 ``find_distributions`` callable on the finders from
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228 :data:`sys.meta_path` and presents this extended interface as the
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229 ``DistributionFinder`` abstract base class, which defines this abstract
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230 method::
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231
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232 @abc.abstractmethod
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233 def find_distributions(context=DistributionFinder.Context()):
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234 """Return an iterable of all Distribution instances capable of
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235 loading the metadata for packages for the indicated ``context``.
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236 """
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237
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238 The ``DistributionFinder.Context`` object provides ``.path`` and ``.name``
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239 properties indicating the path to search and name to match and may
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240 supply other relevant context.
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241
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242 What this means in practice is that to support finding distribution package
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243 metadata in locations other than the file system, subclass
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244 ``Distribution`` and implement the abstract methods. Then from
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245 a custom finder, return instances of this derived ``Distribution`` in the
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246 ``find_distributions()`` method.
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247
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248
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249 .. _`entry point API`: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points
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250 .. _`metadata API`: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#metadata-api
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251 .. _`finders`: https://docs.python.org/3/reference/import.html#finders-and-loaders
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252
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253
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254 .. rubric:: Footnotes
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255
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256 .. [#f1] Technically, the returned distribution metadata object is an
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257 :class:`email.message.EmailMessage`
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258 instance, but this is an implementation detail, and not part of the
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259 stable API. You should only use dictionary-like methods and syntax
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260 to access the metadata contents.