Update parameterized.CoopTestCase to work with python3 metaclasses.

jerub · copybara-github · commit 9e543208a723 · 2023-07-17T10:30:08.000-07:00
By setting and accessing __metaclass__, we were not achieving the goals of this
function, which is to make it possible to have both the functionality of
paramterized.TestCase and another TestCase instance with its own metaclass.

Because there are client classes that are using CoopTestCase to combine parameterized.TestCase with another class that isn't using a metaclass we need to support that use-case. We do it by returning a simple multiple inheritance subclass without a combined metaclass, and emit a warning telling the author they don't need to use this method.

PiperOrigin-RevId: 548732415
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -26,6 +26,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com).
     `enum_values` is provided as a single string value. Additionally,
     `EnumParser.enum_values` is now stored as a list copy of the provided
     `enum_values` parameter.
+*   (tesing) Updated `paramaterized.CoopTestCase()` to use Python 3 metaclass
+    idioms. Most uses of this function continued working during the Python 3
+    migration still worked because a Python 2 compatibility `__metaclass__`
+    variables also existed. Now pure Python 3 base classes without backwards
+    compatibility will work as intended.
 
 ## 1.4.0 (2023-01-11)
 
diff --git a/absl/testing/parameterized.py b/absl/testing/parameterized.py
@@ -217,6 +217,7 @@ async def testSomeAsyncFunction(self, arg, expected):
 import re
 import types
 import unittest
+import warnings
 
 from absl.testing import absltest
 
@@ -697,10 +698,27 @@ class ExampleTest(parameterized.CoopTestCase(OtherTestCase)):
   Returns:
     A new class object.
   """
-  metaclass = type(
-      'CoopMetaclass',
-      (other_base_class.__metaclass__,
-       TestGeneratorMetaclass), {})
-  return metaclass(
-      'CoopTestCase',
-      (other_base_class, TestCase), {})
+  # If the other base class has a metaclass of 'type' then trying to combine
+  # the metaclasses will result in an MRO error. So simply combine them and
+  # return.
+  if type(other_base_class) == type:  # pylint: disable=unidiomatic-typecheck
+    warnings.warn(
+        'CoopTestCase is only necessary when combining with a class that uses'
+        ' a metaclass. Use multiple inheritance like this instead: class'
+        f' ExampleTest(paramaterized.TestCase, {other_base_class.__name__}):',
+        stacklevel=2,
+    )
+
+    class CoopTestCaseBase(other_base_class, TestCase):
+      pass
+
+    return CoopTestCaseBase
+  else:
+
+    class CoopMetaclass(type(other_base_class), TestGeneratorMetaclass):  # pylint: disable=unused-variable
+      pass
+
+    class CoopTestCaseBase(other_base_class, TestCase, metaclass=CoopMetaclass):
+      pass
+
+    return CoopTestCaseBase
diff --git a/absl/testing/tests/parameterized_test.py b/absl/testing/tests/parameterized_test.py
