Deepcopy defaults to prevent weird references in yaml.

pre_commit/jsonschema_extensions.py

@@ -16,7 +16,9 @@ def extend_with_default(validator_class):
 
         for property, subschema in properties.iteritems():
             if "default" in subschema:
-                instance.setdefault(property, subschema["default"])
+                instance.setdefault(
+                    property, copy.deepcopy(subschema["default"]),
+                )
 
     return jsonschema.validators.extend(
         validator_class, {"properties" : set_defaults},

tests/jsonschema_extensions_test.py

@@ -49,3 +49,10 @@ def test_apply_defaults_deep():
         },
     )
     assert ret == {'foo': {'bar': {'baz': 'herp'}}}
+
+
+def test_apply_defaults_copies():
+    schema = {'properties': {'foo': {'default': []}}}
+    ret1 = apply_defaults({}, schema)
+    ret2 = apply_defaults({}, schema)
+    assert ret1['foo'] is not ret2['foo']