Use six for metaclass and iters in Python 2/3.

ecs/managers.py

@@ -1,5 +1,7 @@
 """Entity and System Managers."""
 
+import six
+
 from ecs.exceptions import (
     NonexistentComponentTypeForEntity, DuplicateSystemTypeError)
 from ecs.models import Entity
@@ -68,11 +70,11 @@ def remove_component(self, entity, component_type):
             pass
 
     def pairs_for_type(self, component_type):
-        """Return a list of ``(entity, component_instance)`` tuples for all
-        entities in the database possessing a component of ``component_type``.
-        Return an empty list if there are no components of this type in the
-        database. Can use in a loop like this, where ``Renderable`` is a
-        component type:
+        """Return an iterator over ``(entity, component_instance)`` tuples for
+        all entities in the database possessing a component of
+        ``component_type``. Return an empty iterator if there are no components
+        of this type in the database. It should be used in a loop like this,
+        where ``Renderable`` is a component type:
 
         .. code-block:: python
 
@@ -83,15 +85,14 @@ def pairs_for_type(self, component_type):
         :param component_type: a type of created component
         :type component_type: :class:`type` which is :class:`Component`
             subclass
-        :return: list of ``(entity, component_instance)`` tuples
-        :rtype: :class:`tuple` of
+        :return: iterator on ``(entity, component_instance)`` tuples
+        :rtype: :class:`iter` on
             (:class:`ecs.models.Entity`, :class:`ecs.models.Component`)
         """
         try:
-            # Return a copy of the items for Python 3.
-            return list(self._database[component_type].items())
+            return six.iteritems(self._database[component_type])
         except KeyError:
-            return []
+            return six.iteritems({})
 
     def component_for_entity(self, entity, component_type):
         """Return the instance of ``component_type`` for the entity from the

ecs/models.py

@@ -3,6 +3,8 @@
 from __future__ import print_function
 from abc import ABCMeta, abstractmethod
 
+import six
+
 
 class Entity(object):
     """Encapsulation of a GUID to use in the entity database."""
@@ -27,12 +29,11 @@ class Component(object):
     pass
 
 
+@six.add_metaclass(ABCMeta)
 class System(object):
     """An object that represents an operation on a set of objects from the game
     database. The :meth:`update` method must be implemented.
     """
-    __metaclass__ = ABCMeta
-
     @abstractmethod
     def update(self, entity_manager, dt):
         """Run the system for this frame. This method is called by the system
@@ -44,5 +45,5 @@ def update(self, entity_manager, dt):
         :param dt: delta time, or elapsed time for this frame
         :type dt: :class:`float`
         """
-        print("System's update() method was called: "
-              "entity_manager={0}, dt={1}".format(entity_manager, dt))
+        six.print_("System's update() method was called: "
+                   'entity_manager={0}, dt={1}'.format(entity_manager, dt))

requirements-dev.txt

@@ -1,3 +1,5 @@
+--requirement requirements.txt
+
 # Testing
 pytest==2.5.1
 py==1.4.19

requirements.txt

@@ -0,0 +1,2 @@
+# For Python 2/3 compatibility
+six==1.5.2

setup.py

@@ -221,7 +221,7 @@ def run_tests(self):
         'Topic :: Software Development :: Libraries :: Application Frameworks',
     ],
     packages=find_packages(),
-    install_requires=[],
+    install_requires=['six==1.5.2'],
     # Allow tests to be run with `python setup.py test'.
     tests_require=[
         'pytest==2.5.1',

tests/test_managers.py

@@ -48,14 +48,14 @@ def test_create_new_entities(self, manager):
     class TestPairsForType(object):
         def test_existing_component_type(
                 self, manager, entities, components, component_types):
-            assert manager.pairs_for_type(component_types[0]) == [
+            assert list(manager.pairs_for_type(component_types[0])) == [
                 (entities[0], components[0]),
                 (entities[1], components[5]),
                 (entities[3], components[0])]
 
         def test_nonexistent_component_type(
                 self, manager, entities, component_types):
-            assert manager.pairs_for_type(component_types[2]) == []
+            assert list(manager.pairs_for_type(component_types[2])) == []
 
     class TestRemoveComponent(object):
         def test_remove_some_of_a_component(