diff --git a/trytond/model/fields/field.py b/trytond/model/fields/field.py
index 93e74edcd3..72f3af404d 100644
--- a/trytond/model/fields/field.py
+++ b/trytond/model/fields/field.py
@@ -11,11 +11,12 @@ def domain_validate(value):
             if isinstance(arg, basestring):
                 if arg not in ('AND', 'OR'):
                     return False
-            elif isinstance(arg, tuple) or \
-                    (isinstance(arg, list) and len(arg) > 2 and \
-                    (arg[1] in OPERATORS) or \
-                    (isinstance(arg[1], PYSON) and \
-                    arg[1].types() == set([str]))):
+            elif (isinstance(arg, tuple)
+                or (isinstance(arg, list)
+                    and len(arg) > 2
+                    and ((arg[1] in OPERATORS)
+                        or (isinstance(arg[1], PYSON)
+                            and arg[1].types() == set([str]))))):
                 pass
             elif isinstance(arg, list):
                 if not test_domain(arg):