Allow lists to be generated from any non-string iterable

The type check on the list field requires a list, though many
objects can be trivially iterated as lists, like sqlalchemy
lazy query objects. Relax the check slightly to fit a broader
range of "lists".

Change-Id: I514a0e6d445e9a14e3c0279b4e9bebfa0940337d

oslo_versionedobjects/fields.py

@@ -13,6 +13,7 @@
 #    under the License.
 
 import abc
+import collections
 import datetime
 from distutils import versionpredicate
 import re
@@ -642,7 +643,9 @@ class CompoundFieldType(FieldType):
 
 class List(CompoundFieldType):
     def coerce(self, obj, attr, value):
-        if not isinstance(value, list):
+
+        if (not isinstance(value, collections.Iterable) or
+           isinstance(value, six.string_types + (collections.Mapping,))):
             raise ValueError(_('A list is required in field %(attr)s, '
                                'not a %(type)s') %
                              {'attr': attr, 'type': type(value).__name__})

oslo_versionedobjects/tests/test_fields.py

@@ -1229,3 +1229,41 @@ class TestIPV6Network(TestField):
         invalid_vals = [x for x in self.coerce_bad_values]
         for invalid_val in invalid_vals:
             self.assertNotRegex(str(invalid_val), pattern)
+
+
+class FakeCounter(six.Iterator):
+    def __init__(self):
+        self.n = 0
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        if self.n <= 4:
+            self.n += 1
+            return self.n
+        else:
+            raise StopIteration
+
+
+class TestListTypes(test.TestCase):
+
+    def test_regular_list(self):
+        fields.List(fields.Integer).coerce(None, None, [1, 2])
+
+    def test_non_iterable(self):
+        self.assertRaises(ValueError,
+                          fields.List(fields.Integer).coerce, None, None, 2)
+
+    def test_string_iterable(self):
+        self.assertRaises(ValueError,
+                          fields.List(fields.Integer).coerce, None, None,
+                          'hello')
+
+    def test_mapping_iterable(self):
+        self.assertRaises(ValueError,
+                          fields.List(fields.Integer).coerce, None, None,
+                          {'a': 1, 'b': 2})
+
+    def test_iter_class(self):
+        fields.List(fields.Integer).coerce(None, None, FakeCounter())