jdowner · April 28, 2016 10:58 · Apr 28, 2016
diff --git a/reqpro.py b/reqpro.py
@@ -0,0 +1,78 @@
+import functools
+import weakref
+
+class Property(object):
+    instances = weakref.WeakValueDictionary()
+
+    def __new__(cls, name):
+        if name in Property.instances:
+            return Property.instances[name]
+
+        obj = super().__new__(cls)
+        setattr(obj, 'name', name)
+
+        Property.instances[name] = obj
+
+        return obj
+
+
+class requires(object):
+    def __init__(self, *args, dynamic=False):
+        self.requires = set(args)
+        self.dynamic = dynamic
+
+    def __call__(self, func, *args, **kwargs):
+        @functools.wraps(func)
+        def static_wrapper(*args, **kwargs):
+            return func(*args, **kwargs)
+
+        @functools.wraps(func)
+        def dynamic_wrapper(*args, **kwargs):
+            for req in self.requires:
+                assert req in Property.instances, 'missing requirement: {}'.format(req)
+
+            return func(*args, **kwargs)
+
+        if self.dynamic:
+            return dynamic_wrapper
+
+        for req in self.requires:
+            assert req in Property.instances
+
+        return static_wrapper
+
+
+class provides(object):
+    def __init__(self, *args):
+        self.properties = list(args)
+
+    def __call__(self, func, *args, **kwargs):
+        @functools.wraps(func)
+        def impl(*args, **kwargs):
+            return func(*args, **kwargs)
+
+        setattr(impl, '__provides__', [Property(p) for p in self.properties])
+
+        return impl
+
+
+@requires('bar', dynamic=True)
+@provides('foo')
+class Foo(object):
+    def stuff(self):
+        print('foo')
+        provides('baz')
+
+    @requires('baz', dynamic=True)
+    def other(self):
+        print('baz')
+
+
+@provides('bar')
+class Bar(object):
+    pass
+
+f = Foo()
+f.stuff()
+f.other()
+
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