Simple Weighted Average with Python

While I suspect there must already be a built in weighted average tool in some python library ( scipy / numpy), for a project I'm working on I just wrote my own.  I would love to hear what people think and if I can improve the function in any way.

def weightedAverage(valueWeightList):
        '''
        Takes a list of tuples (value, weight) and
        returns weighted average as calculated by
        Sum of all values * weights / Sum of all weights
        '''
        numerator = sum([v * w for v,w in valueWeightList])
        denominator = sum([w for v,w in valueWeightList])
        if(denominator != 0):
            return(float(numerator) / float(denominator))
        else:
            return None