add priority keyword to add_system() method

    Add a `priority` attribute to `ecs.models.Systems` and sort
    `ecs.managers.SystemManager._systems` by it every time a system is added.
    Also add a new test case to ensure that system with the same priority
    end up being executed in the order they were added and adjust
    `test_systems_added` and `test_remove_system`.

ecs/managers.py

@@ -157,11 +157,13 @@ def systems(self):
         """
         return self._systems
 
-    def add_system(self, system_instance):
+    def add_system(self, system_instance, priority=0):
         """Add a :class:`ecs.models.System` instance to the manager.
 
         :param system_instance: instance of a system
+        :param priority: non-negative integer (default: 0)
         :type system_instance: :class:`ecs.models.System`
+        :type priority: :class:`int`
         :raises: :class:`ecs.exceptions.DuplicateSystemTypeError` when the
             system type is already present in this manager
         :raises: :class:`ecs.exceptions.SystemAlreadyAddedToManagerError` when
@@ -178,6 +180,9 @@ def add_system(self, system_instance):
         self._system_types[system_type] = system_instance
         self._systems.append(system_instance)
 
+        system_instance.priority = priority
+        self._systems.sort(key=lambda x: x.priority)
+
     def remove_system(self, system_type):
         """Tell the manager to no longer run the system of this type.
 

ecs/models.py

@@ -43,6 +43,10 @@ def __init__(self):
         self.system_manager = None
         """The system manager to which this system belongs. Again, a system is
         only allowed to belong to one at a time."""
+        self.priority = None
+        """The priority for this system when the system manager runs
+        :meth:`ecs.managers.SystemManager.update()`. Must be a non-negative
+        integer with 0 being the highest priority."""
 
     @abstractmethod
     def update(self, dt):

tests/test_managers.py

@@ -1,4 +1,5 @@
 import re
+import random
 
 from pytest import fixture, raises
 import pytest
@@ -128,7 +129,7 @@ def system_types(self):
         return [
             type('System' + str(i),
                  (System,),
-                 {'update': MagicMock()})
+                 {'update': MagicMock(), 'order': i})
             for i in range(5)]
 
     @fixture
@@ -139,13 +140,15 @@ def systems(self, system_types):
     def manager(self, systems):
         sm = SystemManager(sentinel.entity_manager)
         for system in systems:
-            sm.add_system(system)
+            sm.add_system(system, priority=random.randrange(4))
         return sm
 
     class TestAddSystem(object):
         def test_systems_added(self, manager, systems):
             # add_system was called normally in the fixture.
-            assert manager.systems == systems
+            # since add_system sorts the manager.systems
+            # we switch to sets for easy comparing
+            assert set(manager.systems) == set(systems)
 
         def test_entity_manager_set(self, manager):
             for system in manager.systems:
@@ -155,6 +158,20 @@ def test_system_manager_set(self, manager):
             for system in manager.systems:
                 assert system.system_manager == manager
 
+        class TestPriority(object):
+            def test_obeys_order(self, manager, systems):
+                old_priority = -1
+                old_order = -1
+                for s in manager.systems:
+                    priority = s.priority
+                    order = s.order
+                    assert old_priority <= priority
+                    if priority == old_priority:
+                        assert old_order < order
+
+                    old_priority = priority
+                    old_order = old_order
+
         class TestDuplicate(object):
             def test_raises_error(self, manager, systems):
                 with raises(DuplicateSystemTypeError) as exc_info:
@@ -194,7 +211,8 @@ def test_system_and_entity_managers_not_changed(
     class TestRemoveSystem(object):
         def test_remove_system(self, manager, systems, system_types):
             manager.remove_system(system_types[0])
-            assert manager.systems == systems[1:]
+            # using sets for the same reason as in test_systems_added
+            assert set(manager.systems) == set(systems[1:])
 
         def test_entity_manager_unset(self, manager, systems, system_types):
             manager.remove_system(system_types[0])