roles: implement required attributes

Implement required attributes for roles via the ``__require__``
attribute.

TODO

@@ -18,7 +18,5 @@
     - inner and augment
 
 - roles
-  - required attributes
-  - required methods
   - method exclusion and aliasing
   - apply role to instance?

elk/meta.py

@@ -59,6 +59,10 @@ def __new__(mcs, name, bases, dict):
 
         cls = type.__new__(mcs, name, bases, dict)
 
+        # check role requirements
+        for role in roles:
+            ElkRoleMeta.check_requirements(cls, role)
+
         # apply method modifiers
         modifiers = sorted(
             v for v in dict.viewvalues()
@@ -132,6 +136,15 @@ def apply_to_class_dict(mcs, dict, role):
             if k not in dict:
                 dict[k] = v
 
+    @classmethod
+    def check_requirements(mcs, cls, role):
+        requires = getattr(role, '__require__', ())
+        if isinstance(requires, str):
+            requires = (requires,)
+        for require in requires:
+            if not hasattr(cls, require):
+                raise TypeError('{} requires {}'.format(role, require))
+
 
 class Elk(object):
     __metaclass__ = ElkMeta

elk/test_roles.py

@@ -136,3 +136,38 @@ def test_car(self):
         car.break_()
         self.assertTrue(car.is_broken)
         self.assertTrue(isinstance(car, Breakable))
+
+
+class SingleRequireRole(elk.ElkRole):
+    __require__ = 'x'
+
+
+class MultiRequireRole(elk.ElkRole):
+    __require__ = 'y', 'z'
+
+
+class RequiresTestCase(unittest.TestCase):
+    def test_missing_requirement_raises_TypeError(self):
+        with self.assertRaises(TypeError):
+            class A(elk.Elk):
+                __with__ = SingleRequireRole
+
+        with self.assertRaises(TypeError):
+            class B(elk.Elk):
+                y = elk.ElkAttribute()
+                __with__ = MultiRequireRole
+
+        with self.assertRaises(TypeError):
+            class C(elk.Elk):
+                z = elk.ElkAttribute()
+                __with__ = MultiRequireRole
+
+    def test_satisfied_requirement_allows_composition(self):
+        class A(elk.Elk):
+            __with__ = SingleRequireRole, MultiRequireRole
+            x = elk.ElkAttribute()
+            y = elk.ElkAttribute()
+            z = elk.ElkAttribute()
+
+        self.assertTrue(issubclass(A, SingleRequireRole))
+        self.assertTrue(issubclass(A, MultiRequireRole))