Introduce AttributeContainerMeta (#326)

pynamodb/attributes.py

@@ -2,6 +2,7 @@
 PynamoDB attributes
 """
 import six
+from six import add_metaclass
 import json
 from base64 import b64encode, b64decode
 from datetime import datetime
@@ -118,11 +119,13 @@ def _serialize(self, value):
         return {ATTR_TYPE_MAP[self.attribute.attr_type]: self.attribute.serialize(value)}
 
 
-class AttributeContainer(object):
+class AttributeContainerMeta(type):
 
-    _dynamo_to_python_attrs = None
+    def __init__(cls, name, bases, attrs):
+        super(AttributeContainerMeta, cls).__init__(name, bases, attrs)
+        AttributeContainerMeta._initialize_attributes(cls)
 
-    @classmethod
+    @staticmethod
     def _initialize_attributes(cls):
         """
         Initialize attributes on the class.
@@ -157,16 +160,17 @@ def _initialize_attributes(cls):
                 else:
                     instance.attr_name = item_name
 
+
+@add_metaclass(AttributeContainerMeta)
+class AttributeContainer(object):
+
     @classmethod
     def _get_attributes(cls):
         """
         Returns the attributes of this class as a mapping from `python_attr_name` => `attribute`.
 
         :rtype: dict[str, Attribute]
         """
-        if '_attributes' not in cls.__dict__:
-            # Each subclass of AttributeContainer needs its own attributes map.
-            cls._initialize_attributes()
         return cls._attributes
 
     @classmethod
@@ -176,8 +180,6 @@ def _dynamo_to_python_attr(cls, dynamo_key):
 
         This covers cases where an attribute name has been overridden via "attr_name".
         """
-        if cls._attributes is None:
-            cls._initialize_attributes()
         return cls._dynamo_to_python_attrs.get(dynamo_key, dynamo_key)
 
     def _set_defaults(self):
@@ -473,6 +475,13 @@ def deserialize(self, value):
         return None
 
 
+class MapAttributeMeta(AttributeContainerMeta):
+    """
+    This is only here for backwards compatibility: i.e. so type(MapAttribute) == MapAttributeMeta
+    """
+
+
+@add_metaclass(MapAttributeMeta)
 class MapAttribute(AttributeContainer, Attribute):
     attr_type = MAP
 
@@ -482,7 +491,6 @@ def __init__(self, hash_key=False, range_key=False, null=None, default=None, att
                                            null=null,
                                            default=default,
                                            attr_name=attr_name)
-        self._get_attributes()  # Ensure attributes are always inited
         self.attribute_values = {}
         self._set_defaults()
         self._set_attributes(**attrs)

pynamodb/models.py

@@ -11,7 +11,7 @@
 from six import add_metaclass
 from pynamodb.exceptions import DoesNotExist, TableDoesNotExist, TableError
 from pynamodb.throttle import NoThrottle
-from pynamodb.attributes import Attribute, AttributeContainer, MapAttribute, ListAttribute
+from pynamodb.attributes import Attribute, AttributeContainer, AttributeContainerMeta, MapAttribute, ListAttribute
 from pynamodb.connection.base import MetaTable
 from pynamodb.connection.table import TableConnection
 from pynamodb.connection.util import pythonic
@@ -153,14 +153,15 @@ def __iter__(self):
         return iter(self.results)
 
 
-class MetaModel(type):
+class MetaModel(AttributeContainerMeta):
     """
     Model meta class
 
     This class is just here so that index queries have nice syntax.
     Model.index.query()
     """
     def __init__(cls, name, bases, attrs):
+        super(MetaModel, cls).__init__(name, bases, attrs)
         if isinstance(attrs, dict):
             for attr_name, attr_obj in attrs.items():
                 if attr_name == META_CLASS_NAME:

pynamodb/tests/test_attributes.py

@@ -19,7 +19,7 @@
 from pynamodb.attributes import (
     BinarySetAttribute, BinaryAttribute, NumberSetAttribute, NumberAttribute,
     UnicodeAttribute, UnicodeSetAttribute, UTCDateTimeAttribute, BooleanAttribute, LegacyBooleanAttribute,
-    MapAttribute, ListAttribute, Attribute,
+    MapAttribute, MapAttributeMeta, ListAttribute, Attribute,
     JSONAttribute, DEFAULT_ENCODING, NUMBER, STRING, STRING_SET, NUMBER_SET, BINARY_SET,
     BINARY, MAP, LIST, BOOLEAN, _get_value_for_deserialize)
 
@@ -716,6 +716,9 @@ class ThingModel(Model):
         with pytest.raises(KeyError):
             bad = t.nested['something_else']
 
+    def test_metaclass(self):
+        assert type(MapAttribute) == MapAttributeMeta
+
 
 class TestValueDeserialize:
     def test__get_value_for_deserialize(self):