Specify context, i.e. condition that must be met.

        Arguments:
            key (str): Name of the context whose value you want to query.
        Returns:
            Context:

def when(self, key):
        """ Specify context, i.e. condition that must be met.

        Arguments:
            key (str): Name of the context whose value you want to query.
        Returns:
            Context:
        """
        ctx = Context(key, self)
        self.context.append(ctx)
        return ctx