What about

def reduce(function, iterable, initializer=None):
    it = iter(iterable)
    if initializer is None:
        value = next(it)
    else:
        value = initializer
    for element in it:
        value = function(value, element)
    return value

Remember, an equivalent doesn't have to be 100% compatible, it is a way for the user to quickly get an idea what the function does.