Question

When I have a for loop:

for row in list:
    row = something_or_other

It seems that sometimes I can assign a value (or append/extend etc.) directly to row and the list changes accordingly, and sometimes I have to do something roundabout like:

for row in list:
    list[list.index(row)] = something_or_other

What gives?!?

Answer 1

You can never reassign the value row (or in general, whatever your iterating variable is) like this:

x = [1, 2, 3]
for x in lst:
    x = # code

because this is reassigning the variable x entirely (it’s saying “forget that x was a member of a list”).

However, if x is mutable, for example if it’s a list, you can do:

lst = [[1, 2], [3, 4]]
for x in lst:
    x.append(10)

and it will actually change the values (to [[1, 2, 10], [3, 4, 10]]). In technical terms, this is the difference between a rebinding and mutating operations.