Use compat.string_types to check for visit name string

Th visitor optimization looking for plain string needs to
accommodate for u"" in Python 2 due to the unicode_literals
future.

Fixes: #4800
Change-Id: I5e3136f2f2cbd5f24d89186b599f59ea2e7f1550

diff --git a/lib/sqlalchemy/sql/visitors.py b/lib/sqlalchemy/sql/visitors.py
index c06c954596e866f0ac042004ef167cb008df4422..0a3cb8c1cfecdccb88cd42393cc2578a1f28621f 100644 (file)
--- a/lib/sqlalchemy/sql/visitors.py
+++ b/lib/sqlalchemy/sql/visitors.py
@@ -76,7 +76,8 @@ def _generate_dispatch(cls):
     """
     if "__visit_name__" in cls.__dict__:
         visit_name = cls.__visit_name__
-        if isinstance(visit_name, str):
+
+        if isinstance(visit_name, util.compat.string_types):
             # There is an optimization opportunity here because the
             # the string name of the class's __visit_name__ is known at
             # this early stage (import time) so it can be pre-constructed.