Python: One liner for summing up elements in a list meeting a condition

Question

How can I write this code python in one line?

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代码（不支持i++语法），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

If we ignore import statements, you can write it as

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

If import statements count, you can do

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

or

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

Or, to go deeper:

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