Python：用于汇总满足条件的列表中的元素的一个班轮

Question

如何在一行中编写此代码python？

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

Answer 1

嗯，这不是有效的Python代码（i++语法不受支持），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

如果我们忽略import语句，您可以将其写为

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

如果import语句计数，则可以

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

或

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

或者，更进一步：

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