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

tests/jsonschema_extensions_test.py

@@ -0,0 +1,51 @@
+
+from pre_commit.jsonschema_extensions import apply_defaults
+
+
+def test_apply_defaults_copies_object():
+    input = {}
+    ret = apply_defaults(input, {})
+    assert ret is not input
+
+
+def test_apply_default_does_not_touch_schema_without_defaults():
+    ret = apply_defaults(
+        {'foo': 'bar'},
+        {'type': 'object', 'properties': {'foo': {}, 'baz': {}}},
+    )
+    assert ret == {'foo': 'bar'}
+
+
+def test_apply_defaults_applies_defaults():
+    ret = apply_defaults(
+        {'foo': 'bar'},
+        {
+            'type': 'object',
+            'properties': {
+                'foo': {'default': 'biz'},
+                'baz': {'default': 'herp'},
+            }
+        }
+    )
+    assert ret == {'foo': 'bar', 'baz': 'herp'}
+
+
+def test_apply_defaults_deep():
+    ret = apply_defaults(
+        {'foo': {'bar': {}}},
+        {
+            'type': 'object',
+            'properties': {
+                'foo': {
+                    'type': 'object',
+                    'properties': {
+                        'bar': {
+                            'type': 'object',
+                            'properties': {'baz': {'default': 'herp'}},
+                        },
+                    },
+                },
+            },
+        },
+    )
+    assert ret == {'foo': {'bar': {'baz': 'herp'}}}