adding context.py.

uranium/lib/context.py

@@ -0,0 +1,63 @@
+from contextlib import contextmanager
+from ..exceptions import UraniumException
+
+
+class Proxy(object):
+    """
+    a proxy object takes a context stack,
+    and proxies requests to whatever the
+    context stack is currently referring to
+    """
+
+    def __init__(self, context_stack):
+        object.__setattr__(self, "_context", context_stack)
+
+    def _get_current_object(self):
+        return getattr(self, "_context").obj
+
+    def __getattr__(self, key):
+        return getattr(self._get_current_object(), key)
+
+    def __setattr__(self, key, value):
+        return self._get_current_object().__setattr__(key, value)
+
+    def __dir__(self):
+        return dir(self._get_current_object())
+
+
+class ContextUnavailable(UraniumException):
+    """ raised when a context is unavailable. """
+
+
+class ContextStack(object):
+    """
+    keeps track of the context of an application
+    object.
+    """
+
+    def __init__(self):
+        self._stack = []
+
+    def create_context(self, obj):
+        """
+        create a contextmanager which will add the
+        object as the current object for this contextstack.
+        """
+        @contextmanager
+        def with_obj_as_context():
+            self.push(obj)
+            yield
+            self.pop()
+        return with_obj_as_context()
+
+    def push(self, obj):
+        self._stack.append(obj)
+
+    def pop(self):
+        return self._stack.pop()
+
+    @property
+    def obj(self):
+        if len(self._stack) == 0:
+            raise ContextUnavailable("")
+        return self._stack[-1]