Merge pull request #706 from jwodder/importlib-resource

Describe using importlib-metadata as an alternative to pkg_resources for single-sourcing versions

source/guides/single-sourcing-package-version.rst

@@ -75,17 +75,47 @@ number of your project:
         :file:`MANIFEST.in`).
 
 #.  Set the value in :file:`setup.py`, and have the project code use the
-    ``pkg_resources`` API.
+    ``importlib.metadata`` API to fetch the value at runtime.
+    (``importlib.metadata`` was introduced in Python 3.8 and is available to
+    older versions as the ``importlib-metadata`` project.)  An installed
+    project's version can be fetched with the API as follows::
 
-    ::
+        try:
+            from importlib import metadata
+        except ImportError:
+            # Running on pre-3.8 Python; use importlib-metadata package
+            import importlib_metadata as metadata
 
-        import pkg_resources
-        assert pkg_resources.get_distribution('pip').version == '1.2.0'
+        assert metadata.version('pip') == '1.2.0'
 
-    Be aware that the ``pkg_resources`` API only knows about what's in the
+    Be aware that the ``importlib.metadata`` API only knows about what's in the
     installation metadata, which is not necessarily the code that's currently
     imported.
 
+    If a project uses this method to fetch its version at runtime, then its
+    ``install_requires`` value needs to be edited to install
+    ``importlib-metadata`` on pre-3.8 versions of Python like so::
+
+        setup(
+            ...
+            install_requires=[
+                ...
+                'importlib-metadata ~= 1.0 ; python_version < "3.8"',
+                ...
+            ],
+            ...
+        )
+
+    An older (and less efficient) alternative to ``importlib.metadata`` is the
+    ``pkg_resources`` API provided by ``setuptools``::
+
+        import pkg_resources
+        assert pkg_resources.get_distribution('pip').version == '1.2.0'
+
+    If a project uses ``pkg_resources`` to fetch its own version at runtime,
+    then ``setuptools`` must be added to the project's ``install_requires``
+    list.
+
     Example using this technique: `setuptools <https://github.com/pypa/setuptools/blob/master/setuptools/version.py>`_.