Why Will One Loop Modify A List Of Lists, But The Other Won't

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

Solution 1:

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.

Solution 2:

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

The loop assigns "list" as a variable which references the list in myLists. when you reassign the "list" variable it now points to a new list, but the list in used to reference in "myLists" is unchanged. (Also, you shouldn't use "list" as a variable name as it is a Python keyword).

In the second example you do overwrite the reference to the list in myLists, so it alters myLists.

Solution 3:

In your first example you are assigning a new list to the name "single_list", in the second example you are modifying the list referenced by single_list.