Add the items to the instance of the environment if they do not exist
        yet.  This is used by :ref:`extensions <writing-extensions>` to register
        callbacks and configuration values without breaking inheritance.

def extend(self, **attributes):
        """Add the items to the instance of the environment if they do not exist
        yet.  This is used by :ref:`extensions <writing-extensions>` to register
        callbacks and configuration values without breaking inheritance.
        """
        for key, value in attributes.iteritems():
            if not hasattr(self, key):
                setattr(self, key, value)