Fixes #164, with test-case, as requested by Julian Berman.

jsonschema/exceptions.py

@@ -48,9 +48,9 @@ def __str__(self):
         return unicode(self).encode("utf-8")
 
     def __unicode__(self):
-        if _unset in (
-            self.validator, self.validator_value, self.instance, self.schema,
-        ):
+        if any(m is _unset for m in (
+            self.validator, self.validator_value, self.instance, self.schema
+        )):
             return self.message
 
         pschema = pprint.pformat(self.schema, width=72)

jsonschema/tests/test_exceptions.py

@@ -268,3 +268,23 @@ def test_if_its_in_the_tree_anyhow_it_does_not_raise_an_error(self):
         )
         tree = exceptions.ErrorTree([error])
         self.assertIsInstance(tree["foo"], exceptions.ErrorTree)
+
+
+    def test_str_works_with_instances_having_overriden_eq_operator(self):
+        """
+        Checks for https://github.com./Julian/jsonschema/issues/164 which
+        rendered exceptions unusable when a `ValidationError` involved classes
+        withthe `eq` operator overridden (such as pandas.DataFrame),
+        caused by a `XX in YYY` check within `__unicode__`() method.
+        """
+
+        class InstanceWithOverridenEq(object):
+            def __eq__(self, other):
+                raise Exception("Instance's __eq__()hould not have been called!")
+        inst = InstanceWithOverridenEq()
+        error = exceptions.ValidationError(
+            "a message", validator="foo", instance=inst, validator_value='some', schema='schema',
+        )
+
+        ex_str = str(error)
+        self.assertTrue(str(exceptions).find(type(inst).__name__), ex_str)