def _avg(value1, value2, weight):
        """Returns the weighted average of two values and handles the case where
            one value is None. If both values are None, None is returned.
        """
        if value1 is None:
            return value2
        if value2 is None:
            return value1
        return value2 * weight + value1 * (1 - weight)
Returns the weighted average of two values and handles the case where
            one value is None. If both values are None, None is returned.