Ignore exception classes which are not types

Fixes #135

bugbear.py

@@ -130,6 +130,16 @@ def _to_name_str(node):
     return _to_name_str(node.value) + "." + node.attr
 
 
+def _typesafe_issubclass(cls, class_or_tuple):
+    try:
+        return issubclass(cls, class_or_tuple)
+    except TypeError:
+        # User code specifies a type that is not a type in our current run. Might be
+        # their error, might be a difference in our environments. We don't know so we
+        # ignore this
+        return False
+
+
 @attr.s
 class BugBearVisitor(ast.NodeVisitor):
     filename = attr.ib()
@@ -190,7 +200,7 @@ def visit_ExceptHandler(self, node):
                     good = list(filter(lambda e: e not in aliases, good))
 
                 for name, other in itertools.permutations(tuple(good), 2):
-                    if issubclass(
+                    if _typesafe_issubclass(
                         getattr(builtins, name, type), getattr(builtins, other, ())
                     ):
                         if name in good:

tests/b014.py

@@ -59,3 +59,11 @@ class MyError(Exception):
     # socket.error and select.error are aliases of OSError. See PEP 3151 for
     # more info.
     pass
+
+
+try:
+    pass
+except (MyException, NotImplemented):
+    # NotImplemented is not an exception, let's not crash on it.
+    pass
+