Add jsonschema extension to populate defaults.

pre_commit/jsonschema_extensions.py

@@ -0,0 +1,32 @@
+
+import copy
+import jsonschema
+import jsonschema.validators
+
+
+# From https://github.com/Julian/jsonschema/blob/master/docs/faq.rst
+def extend_with_default(validator_class):
+    validate_properties = validator_class.VALIDATORS["properties"]
+
+    def set_defaults(validator, properties, instance, schema):
+        for error in validate_properties(
+            validator, properties, instance, schema,
+        ):
+            yield error
+
+        for property, subschema in properties.iteritems():
+            if "default" in subschema:
+                instance.setdefault(property, subschema["default"])
+
+    return jsonschema.validators.extend(
+        validator_class, {"properties" : set_defaults},
+    )
+
+
+DefaultingValidator = extend_with_default(jsonschema.Draft4Validator)
+
+
+def apply_defaults(obj, schema):
+    obj = copy.deepcopy(obj)
+    DefaultingValidator(schema).validate(obj)
+    return obj