Improve Counter.__repr__() to not fail with unorderable values

Lib/collections.py

@@ -583,8 +583,12 @@ class Counter(dict):
     def __repr__(self):
         if not self:
             return '%s()' % self.__class__.__name__
-        items = ', '.join(map('%r: %r'.__mod__, self.most_common()))
-        return '%s({%s})' % (self.__class__.__name__, items)
+        try:
+            items = ', '.join(map('%r: %r'.__mod__, self.most_common()))
+            return '%s({%s})' % (self.__class__.__name__, items)
+        except TypeError:
+            # handle case where values are not orderable
+            return '{0}({1!r})'.format(self.__class__.__name__, dict(self))
 
     # Multiset-style mathematical operations discussed in:
     #       Knuth TAOCP Volume II section 4.6.3 exercise 19

Lib/test/test_collections.py

@@ -893,6 +893,12 @@ class TestCounter(unittest.TestCase):
         c.subtract('aaaabbcce')
         self.assertEqual(c, Counter(a=-1, b=0, c=-1, d=1, e=-1))
 
+    def test_repr_nonsortable(self):
+        c = Counter(a=2, b=None)
+        r = repr(c)
+        self.assertIn("'a': 2", r)
+        self.assertIn("'b': None", r)
+
     def test_helper_function(self):
         # two paths, one for real dicts and one for other mappings
         elems = list('abracadabra')