Fix AttributeError with Sequence defaults in instant_defaults_listener

sqlalchemy_utils/listeners.py

@@ -24,7 +24,9 @@ def instant_defaults_listener(target, args, kwargs):
 
     for key, column in sa.inspect(target.__class__).columns.items():
         if hasattr(column, 'default') and column.default is not None:
-            if callable(column.default.arg):
+            if not hasattr(column.default, 'arg'):
+                continue
+            elif callable(column.default.arg):
                 kwargs[key] = column.default.arg(target)
             else:
                 kwargs[key] = column.default.arg

tests/test_instant_defaults_listener.py

@@ -28,6 +28,20 @@ def byline(self, value):
     return Article
 
 
+@pytest.fixture
+def Document(Base):
+    class Document(Base):
+        __tablename__ = 'document'
+        id = sa.Column(
+            sa.Integer,
+            sa.Sequence('document_id_seq'),
+            primary_key=True
+        )
+        title = sa.Column(sa.Unicode(255), default='Untitled')
+
+    return Document
+
+
 class TestInstantDefaultListener:
     def test_assigns_defaults_on_object_construction(self, Article):
         article = Article()
@@ -40,3 +54,8 @@ def test_callables_as_defaults(self, Article):
     def test_override_default_with_setter_function(self, Article):
         article = Article(byline='provided byline')
         assert article.byline == 'provided byline'
+
+    def test_handles_sequence_defaults(self, Document):
+        document = Document()
+        assert document.title == 'Untitled'
+        assert document.id is None