由于两个原因，您需要初始化product = 1.一种，简单地说，您会得到错误的答案. evens_product([4])应该返回4，而不是8. 第二，它使您不必将没有偶数的列表作为特例.如果没有偶数，则永远不要更改product的值并将其保持不变.

You need to initialize product = 1, for two reasons. One, simply put, you'll get the wrong answer. evens_product([4]) should return 4, not 8. Two, it saves you from having to treat a list with no even numbers as a special case. If there are no even numbers, you never change the value of product and return it unchanged.

def evens_product(xs):
    product = 1
    for i in xs:
        if i%2 == 0:
            product *= i
    return product