Don't use metaclasses when class decorators can do the job.

Thanks to Nick Coghlan for pointing out that I'd forgotten about class
decorators.

Lib/email/_policybase.py

@@ -91,31 +91,25 @@ class _PolicyBase:
         return self.clone(**other.__dict__)
 
 
-# Conceptually this isn't a subclass of ABCMeta, but since we want Policy to
-# use ABCMeta as a metaclass *and* we want it to use this one as well, we have
-# to make this one a subclas of ABCMeta.
-class _DocstringExtenderMetaclass(abc.ABCMeta):
-
-    def __new__(meta, classname, bases, classdict):
-        if classdict.get('__doc__') and classdict['__doc__'].startswith('+'):
-            classdict['__doc__'] = meta._append_doc(bases[0].__doc__,
-                                                    classdict['__doc__'])
-        for name, attr in classdict.items():
-            if attr.__doc__ and attr.__doc__.startswith('+'):
-                for cls in (cls for base in bases for cls in base.mro()):
-                    doc = getattr(getattr(cls, name), '__doc__')
-                    if doc:
-                        attr.__doc__ = meta._append_doc(doc, attr.__doc__)
-                        break
-        return super().__new__(meta, classname, bases, classdict)
-
-    @staticmethod
-    def _append_doc(doc, added_doc):
+def _append_doc(doc, added_doc):
+    doc = doc.rsplit('\n', 1)[0]
     added_doc = added_doc.split('\n', 1)[1]
     return doc + '\n' + added_doc
 
+def _extend_docstrings(cls):
+    if cls.__doc__ and cls.__doc__.startswith('+'):
+        cls.__doc__ = _append_doc(cls.__bases__[0].__doc__, cls.__doc__)
+    for name, attr in cls.__dict__.items():
+        if attr.__doc__ and attr.__doc__.startswith('+'):
+            for c in (c for base in cls.__bases__ for c in base.mro()):
+                doc = getattr(getattr(c, name), '__doc__')
+                if doc:
+                    attr.__doc__ = _append_doc(doc, attr.__doc__)
+                    break
+    return cls
 
-class Policy(_PolicyBase, metaclass=_DocstringExtenderMetaclass):
+
+class Policy(_PolicyBase, metaclass=abc.ABCMeta):
 
     r"""Controls for how messages are interpreted and formatted.
 
@@ -264,6 +258,7 @@ class Policy(_PolicyBase, metaclass=_DocstringExtenderMetaclass):
         raise NotImplementedError
 
 
+@_extend_docstrings
 class Compat32(Policy):
 
     """+

Lib/email/policy.py

@@ -2,7 +2,7 @@
 code that adds all the email6 features.
 """
 
-from email._policybase import Policy, Compat32, compat32
+from email._policybase import Policy, Compat32, compat32, _extend_docstrings
 from email.utils import _has_surrogates
 from email.headerregistry import HeaderRegistry as HeaderRegistry
 
@@ -17,6 +17,7 @@ __all__ = [
     'HTTP',
     ]
 
+@_extend_docstrings
 class EmailPolicy(Policy):
 
     """+

Lib/test/test_email/__init__.py

@@ -73,10 +73,8 @@ class TestEmailBase(unittest.TestCase):
                                     'item {}'.format(i))
 
 
-# Metaclass to allow for parameterized tests
-class Parameterized(type):
-
-    """Provide a test method parameterization facility.
+def parameterize(cls):
+    """A test method parameterization class decorator.
 
     Parameters are specified as the value of a class attribute that ends with
     the string '_params'.  Call the portion before '_params' the prefix.  Then
@@ -92,9 +90,10 @@ class Parameterized(type):
     In a _params dictioanry, the keys become part of the name of the generated
     tests.  In a _params list, the values in the list are converted into a
     string by joining the string values of the elements of the tuple by '_' and
-    converting any blanks into '_'s, and this become part of the name.  The
-    full name of a generated test is the portion of the _params name before the
-    '_params' portion, plus an '_', plus the name derived as explained above.
+    converting any blanks into '_'s, and this become part of the name.
+    The  full name of a generated test is a 'test_' prefix, the portion of the
+    test function name after the  '_as_' separator, plus an '_', plus the name
+    derived as explained above.
 
     For example, if we have:
 
@@ -123,10 +122,8 @@ class Parameterized(type):
     be used to select the test individually from the unittest command line.
 
     """
-
-    def __new__(meta, classname, bases, classdict):
     paramdicts = {}
-        for name, attr in classdict.items():
+    for name, attr in cls.__dict__.items():
         if name.endswith('_params'):
             if not hasattr(attr, 'keys'):
                 d = {}
@@ -138,7 +135,7 @@ class Parameterized(type):
                 attr = d
             paramdicts[name[:-7] + '_as_'] = attr
     testfuncs = {}
-        for name, attr in classdict.items():
+    for name, attr in cls.__dict__.items():
         for paramsname, paramsdict in paramdicts.items():
             if name.startswith(paramsname):
                 testnameroot = 'test_' + name[len(paramsname):]
@@ -148,5 +145,6 @@ class Parameterized(type):
                     testname = testnameroot + '_' + paramname
                     test.__name__ = testname
                     testfuncs[testname] = test
-        classdict.update(testfuncs)
-        return super().__new__(meta, classname, bases, classdict)
+    for key, value in testfuncs.items():
+        setattr(cls, key, value)
+    return cls

Lib/test/test_email/test_generator.py

@@ -4,10 +4,11 @@ import unittest
 from email import message_from_string, message_from_bytes
 from email.generator import Generator, BytesGenerator
 from email import policy
-from test.test_email import TestEmailBase, Parameterized
+from test.test_email import TestEmailBase, parameterize
 
 
-class TestGeneratorBase(metaclass=Parameterized):
+@parameterize
+class TestGeneratorBase:
 
     policy = policy.default
 

Lib/test/test_email/test_headerregistry.py

@@ -4,7 +4,7 @@ import unittest
 from email import errors
 from email import policy
 from email.message import Message
-from test.test_email import TestEmailBase, Parameterized
+from test.test_email import TestEmailBase, parameterize
 from email import headerregistry
 from email.headerregistry import Address, Group
 
@@ -175,7 +175,8 @@ class TestDateHeader(TestHeaderBase):
         self.assertEqual(m['Date'].datetime, self.dt)
 
 
-class TestAddressHeader(TestHeaderBase, metaclass=Parameterized):
+@parameterize
+class TestAddressHeader(TestHeaderBase):
 
     example_params = {
 

Lib/test/test_email/test_pickleable.py

@@ -6,9 +6,11 @@ import email
 import email.message
 from email import policy
 from email.headerregistry import HeaderRegistry
-from test.test_email import TestEmailBase, Parameterized
+from test.test_email import TestEmailBase, parameterize
 
-class TestPickleCopyHeader(TestEmailBase, metaclass=Parameterized):
+
+@parameterize
+class TestPickleCopyHeader(TestEmailBase):
 
     header_factory = HeaderRegistry()
 
@@ -33,7 +35,8 @@ class TestPickleCopyHeader(TestEmailBase, metaclass=Parameterized):
         self.assertEqual(str(h), str(header))
 
 
-class TestPickleCopyMessage(TestEmailBase, metaclass=Parameterized):
+@parameterize
+class TestPickleCopyMessage(TestEmailBase):
 
     # Message objects are a sequence, so we have to make them a one-tuple in
     # msg_params so they get passed to the parameterized test method as a