Python：理论为什么我不能做：print（i.extend（j））

Question

Python理论问题 我想学习为什么＆＃34; print（i.extend（j））＆＃34; DOESN＆＃39; T工作。 它的输出是：＆＃34;无＆＃34;。 print（j）DOES work（它的输出是：＆＃34; [4,5,6,7,8,9]＆＃34;）

i = [1, 2, 3]
j = [4, 5, 6]
k = [7, 8, 9]
# I'd like to learn theory as to why following doesn't work
# OUTPUT is: "None"  
print(i.extend(j))
# Following does work (OUTPUT is:  "[1, 2, 3, 4, 5]")
j.extend(k)
print(j)

Answer 1

答案很简单 -

extend不返回任何内容，并且任何不返回值的函数都将返回None。