Start patching DistributionMetadata.read_pkg_file

This turns get_metadata_version into a method on DistributionMetadata,
populated either by inferrence (in the case of package metadata
specified in `setup`) or from the data in a specified PKG-INFO file.

To populate metadata_version from PKG-INFO, we need to monkey patch
read_pkg_file in addition to write_pkg_file.

setuptools/dist.py

@@ -43,18 +43,25 @@ def _get_unpatched(cls):
     return get_unpatched(cls)
 
 
-def get_metadata_version(dist_md):
-    if dist_md.long_description_content_type or dist_md.provides_extras:
-        return StrictVersion('2.1')
-    elif (dist_md.maintainer is not None or
-          dist_md.maintainer_email is not None or
-          getattr(dist_md, 'python_requires', None) is not None):
-        return StrictVersion('1.2')
-    elif (dist_md.provides or dist_md.requires or dist_md.obsoletes or
-            dist_md.classifiers or dist_md.download_url):
-        return StrictVersion('1.1')
+def get_metadata_version(self):
+    mv = getattr(self, 'metadata_version', None)
+
+    if mv is None:
+        if self.long_description_content_type or self.provides_extras:
+            mv = StrictVersion('2.1')
+        elif (self.maintainer is not None or
+              self.maintainer_email is not None or
+              getattr(self, 'python_requires', None) is not None):
+            mv = StrictVersion('1.2')
+        elif (self.provides or self.requires or self.obsoletes or
+                self.classifiers or self.download_url):
+            mv = StrictVersion('1.1')
+        else:
+            mv = StrictVersion('1.0')
+
+        self.metadata_version = mv
 
-    return StrictVersion('1.0')
+    return mv
 
 
 def read_pkg_file(self, file):
@@ -100,7 +107,7 @@ def read_pkg_file(self, file):
     self.classifiers = _read_list('classifier')
 
     # PEP 314 - these fields only exist in 1.1
-    if metadata_version == StrictVersion('1.1'):
+    if self.metadata_version == StrictVersion('1.1'):
         self.requires = _read_list('requires')
         self.provides = _read_list('provides')
         self.obsoletes = _read_list('obsoletes')
@@ -114,7 +121,7 @@ def read_pkg_file(self, file):
 def write_pkg_file(self, file):
     """Write the PKG-INFO format data to a file object.
     """
-    version = get_metadata_version(self)
+    version = self.get_metadata_version()
 
     file.write('Metadata-Version: %s\n' % version)
     file.write('Name: %s\n' % self.get_name())

setuptools/monkey.py

@@ -84,7 +84,7 @@ def patch_all():
         warehouse = 'https://upload.pypi.org/legacy/'
         distutils.config.PyPIRCCommand.DEFAULT_REPOSITORY = warehouse
 
-    _patch_distribution_metadata_write_pkg_file()
+    _patch_distribution_metadata()
 
     # Install Distribution throughout the distutils
     for module in distutils.dist, distutils.core, distutils.cmd:
@@ -101,11 +101,11 @@ def patch_all():
     patch_for_msvc_specialized_compiler()
 
 
-def _patch_distribution_metadata_write_pkg_file():
-    """Patch write_pkg_file to also write Requires-Python/Requires-External"""
-    distutils.dist.DistributionMetadata.write_pkg_file = (
-        setuptools.dist.write_pkg_file
-    )
+def _patch_distribution_metadata():
+    """Patch write_pkg_file and read_pkg_file for higher metadata standards"""
+    for attr in ('write_pkg_file', 'read_pkg_file', 'get_metadata_version'):
+        new_val = getattr(setuptools.dist, attr)
+        setattr(distutils.dist.DistributionMetadata, attr, new_val)
 
 
 def patch_func(replacement, target_mod, func_name):