bpo-38010 Sync importlib.metadata with importlib_metadata 0.20. (GH-15646)

Sync importlib.metadata with importlib_metadata 0.20.

Doc/library/importlib.metadata.rst

@@ -158,6 +158,13 @@ Once you have the file, you can also read its contents::
             return s.encode('utf-8')
         return s
 
+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
+<https://more-itertools.readthedocs.io/en/stable/api.html#more_itertools.always_iterable>`_
+or otherwise guard against this condition if the target
+distribution is not known to have the metadata present.
 
 .. _requirements:
 

Lib/importlib/metadata.py

@@ -213,6 +213,15 @@ class Distribution:
 
     @property
     def files(self):
+        """Files in this distribution.
+
+        :return: Iterable of PackagePath for this distribution or None
+
+        Result is `None` if the metadata file that enumerates files
+        (i.e. RECORD for dist-info or SOURCES.txt for egg-info) is
+        missing.
+        Result may be empty if the metadata exists but is empty.
+        """
         file_lines = self._read_files_distinfo() or self._read_files_egginfo()
 
         def make_file(name, hash=None, size_str=None):
@@ -245,8 +254,7 @@ class Distribution:
         return self._read_dist_info_reqs() or self._read_egg_info_reqs()
 
     def _read_dist_info_reqs(self):
-        spec = self.metadata['Requires-Dist']
-        return spec and filter(None, spec.splitlines())
+        return self.metadata.get_all('Requires-Dist')
 
     def _read_egg_info_reqs(self):
         source = self.read_text('requires.txt')
@@ -318,7 +326,11 @@ class DistributionFinder(MetaPathFinder):
 
 class PathDistribution(Distribution):
     def __init__(self, path):
-        """Construct a distribution from a path to the metadata directory."""
+        """Construct a distribution from a path to the metadata directory.
+
+        :param path: A pathlib.Path or similar object supporting
+                     .joinpath(), __div__, .parent, and .read_text().
+        """
         self._path = path
 
     def read_text(self, filename):

Lib/test/test_importlib/test_metadata_api.py

@@ -109,6 +109,8 @@ class APITests(
     def test_requires_dist_info(self):
         deps = list(requires('distinfo-pkg'))
         assert deps and all(deps)
+        assert 'wheel >= 1.0' in deps
+        assert "pytest; extra == 'test'" in deps
 
     def test_more_complex_deps_requires_text(self):
         requires = textwrap.dedent("""

Misc/NEWS.d/next/Library/2019-09-02-14-30-39.bpo-38010.JOnz9Z.rst

@@ -0,0 +1 @@
+In ``importlib.metadata`` sync with ``importlib_metadata`` 0.20, clarifying behavior of ``files()`` and fixing issue where only one requirement was returned for ``requires()`` on ``dist-info`` packages.