Use isinstance(foo, string_types).

FIAT/reference_element.py

@@ -36,6 +36,7 @@
 from math import factorial
 
 from six import iteritems, itervalues
+from six import string_types
 from six.moves import reduce
 import numpy
 from numpy import ravel_multi_index, transpose
@@ -905,7 +906,7 @@ def ufc_cell(cell):
     """Handle incoming calls from FFC."""
 
     # celltype could be a string or a cell.
-    if isinstance(cell, str):
+    if isinstance(cell, string_types):
         celltype = cell
     else:
         celltype = cell.cellname()

FIAT/restricted.py

@@ -21,6 +21,7 @@
 
 import numpy
 import six
+from six import string_types
 from six import iteritems
 from FIAT.dual_set import DualSet
 from FIAT.finite_element import FiniteElement
@@ -38,7 +39,7 @@ def __init__(self, element, indices=None, restriction_domain=None):
         if not indices:
             indices = _get_indices(element, restriction_domain)
 
-        if isinstance(indices, str):
+        if isinstance(indices, string_types):
             raise RuntimeError("variable 'indices' was a string; did you forget to use a keyword?")
 
         if len(indices) == 0: