PERF: increase performance of string split when expand=True

Author: cgevans

pandas/core/strings.py

@@ -1090,7 +1090,11 @@ def _wrap_result_expand(self, result, expand=False):
         else:
             index = self.series.index
             if expand:
-                cons_row = self.series._constructor
+                def cons_row(x):
+                    if x is np.nan:
+                        return [ x ]
+                    else:
+                        return x
                 cons = self.series._constructor_expanddim
                 data = [cons_row(x) for x in result]
                 return cons(data, index=index)