为什么一个循环会修改列表列表，而另一个则不会 [英] Why will one loop modify a list of lists, but the other won't

问题描述

告诉我，我的理解甚至超出了我的想象.

One of the answers in "python way" to parse and conditionally replace every element in 2D list has revealed to me that I understand even less than I thought.

给出包含字符串的列表列表:

Given a list of lists containing strings:

myLists = [['5','cat','23'], 
           ['33','parakeet','scalpel'], 
           ['correct','horse','battery','staple','99']]  

我希望将每个元素替换为:

I wish to replace each element with the return of:

def numParser(string):
    try:
        return int(string)
    except ValueError:
        return string

我的原始代码是这样，但它不会修改原始数组.我认为是因为list是副本，而不是实际列表(我错了吗?):

My original code was this, but it does not modify the original array. I assumed because list is a copy, not the actual list (am I wrong?):

for single_list in myLists:
    single_list = map(numParser, rawData)

修改位置列表的答案之一是:

And one of the answers which modifies the lists in place is:

for single_list in myLists:
    for i, item in enumerate(single_list):
        single_list[i] = numParser(item)

为什么第二个解决方案有效，但第一个解决方案无效?

Why does the second solution work but not the first?

推荐答案

此语句:

single_list = ...

为名为single_list的局部变量分配一个值.它对任何其他数据都没有影响.

assigns a value to the local variable named single_list. It does not have any effect on any other data.

声明:

single_list[i] = ...

修改名为single_list的局部变量引用的对象的值，特别是它设置列表中特定元素的值.

modifies the value of the object referenced by the local variable named single_list, specifically it sets the value of a particular element of the list.

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