Python：用於匯總滿足條件的列表中的元素的一個班輪

Question

如何在一行中編寫此代碼python？

num_positive_weights = 0
for i in  weights['value']:
    if i >= 0:
        num_positive_weights+=1

Answer 1

好吧，那不是有效的Python代碼（不支持i++語法），但它如下：

num_positive_weights = sum(i>=0 for i in weights['value'])

Answer 2

num_positive_weights = len([i for i in weights['value'] if i >= 0])

Answer 3

num_positive_weights = sum(filter(lambda x: x >= 0, weights['value']))

Answer 4

如果我們忽略import語句，你可以把它寫成

import operator
num_positive_weights = reduce(operator.add, (1 for i in weights['value'] if i >= 0))

如果import語句計數，則可以執行

num_positive_weights = reduce(__import__("operator").add, (1 for i in weights['value'] if i >= 0))

要么

num_positive_weights = reduce(lambda a,b: a+b, (1 for i in weights['value'] if i >= 0))

或者，更深入：

num_positive_weights = reduce(lambda a,b: a+1, filter(lambda i: i >= 0, weights['value']), 0)