FIX: only warn getattr when key is str

Author: deniederhut

pandas/core/generic.py

@@ -1787,7 +1787,7 @@ def _slice(self, slobj, axis=0, kind=None):
         return result
 
     def _set_item(self, key, value):
-        if callable(getattr(self, str(key), None)):
+        if isinstance(key, string_types) and callable(getattr(self, key, None)):
             warnings.warn("Column name '{key}' collides with a built-in "
                           "method, which will cause unexpected attribute "
                           "behavior".format(key=key), stacklevel=3)