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.

Given a list of lists containing strings:

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

I wish to replace each element with the return of:

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

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)

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)

Why does the second solution work but not the first?

This statement:

single_list = ...

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

The statement:

single_list[i] = ...

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.