Deprecate construction of Distribution and subclasses without impleme…

…nting abstract methods. Fixes #422.

importlib_metadata/__init__.py

@@ -348,7 +348,26 @@ def __repr__(self):
         return f'<FileHash mode: {self.mode} value: {self.value}>'
 
 
-class Distribution(metaclass=abc.ABCMeta):
+class DeprecatedNonAbstract:
+    def __new__(cls, *args, **kwargs):
+        all_names = {
+            name for subclass in inspect.getmro(cls) for name in vars(subclass)
+        }
+        abstract = {
+            name
+            for name in all_names
+            if getattr(getattr(cls, name), '__isabstractmethod__', False)
+        }
+        if abstract:
+            warnings.warn(
+                f"Unimplemented abstract methods {abstract}",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+        return super().__new__(cls)
+
+
+class Distribution(DeprecatedNonAbstract):
     """A Python distribution package."""
 
     @abc.abstractmethod

tests/_context.py

@@ -0,0 +1,13 @@
+import contextlib
+
+
+# from jaraco.context 4.3
+class suppress(contextlib.suppress, contextlib.ContextDecorator):
+    """
+    A version of contextlib.suppress with decorator support.
+
+    >>> @suppress(KeyError)
+    ... def key_error():
+    ...     {}['']
+    >>> key_error()
+    """

tests/test_main.py

@@ -1,12 +1,15 @@
 import re
 import pickle
 import unittest
+import warnings
 import importlib
 import importlib_metadata
+import contextlib
 import itertools
 import pyfakefs.fake_filesystem_unittest as ffs
 
 from . import fixtures
+from ._context import suppress
 from importlib_metadata import (
     Distribution,
     EntryPoint,
@@ -20,6 +23,13 @@
 )
 
 
+@contextlib.contextmanager
+def suppress_known_deprecation():
+    with warnings.catch_warnings(record=True) as ctx:
+        warnings.simplefilter('default', category=DeprecationWarning)
+        yield ctx
+
+
 class BasicTests(fixtures.DistInfoPkg, unittest.TestCase):
     version_pattern = r'\d+\.\d+(\.\d)?'
 
@@ -44,6 +54,9 @@ def test_package_not_found_mentions_metadata(self):
 
         assert "metadata" in str(ctx.exception)
 
+    # expected to fail until ABC is enforced
+    @suppress(AssertionError)
+    @suppress_known_deprecation()
     def test_abc_enforced(self):
         with self.assertRaises(TypeError):
             type('DistributionSubclass', (Distribution,), {})()