Python中的Foreach无法按预期工作 [英] Foreach in Python not working as expected

问题描述

  s = [False] * 5 
 
 for s in [[2]：
 item = not item 
 
 print（s）
  

但它不起作用。

  [False，False，False，False，False] 
  

然而，按预期工作：

  0，len（s），2）：
s [i] = not s [i] 
 
 print（s）
 
 [True，False，True， False，True] 
  

解决方案

因为当你做

  item = not item 
  

实际做的是将数组中引用改为另一个引用数组。 item 只是数组内对象引用的副本。

第二个代码按预期工作，它会改变数组本身的引用，而不是引用副本。

I want to toggle every second element of a list:

s = [False] * 5

for item in s[::2]:
    item = not item

print(s)

But it doesn't work.

[False, False, False, False, False]

This however, works as expected:

for i in range(0, len(s), 2):
    s[i] = not s[i]

print(s)

[True, False, True, False, True]

Why is this happening?

解决方案

Because when you do

item = not item

What you're actually doing is to change the reference to an object in the array with another reference to an object outside the array. item is just a copy of a reference to an object inside the array.

The second code works as expected because it changes the reference in the array itself, not in a reference copy.

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