Change values in a list using a for loop (python)

Question

I currently have some code that reads like this:

letters = {
10 : "A",
11 : "B",
12 : "C",
13 : "D",
14 : "E",
15 : "F"
}
vallist = [rd1, rd2, gd1, gd2, bd1, bd2]
for i in vallist:
    if i >= 10:
        i = letters[i]

What I want to happen is the for loop to iterate through vallist and replace any value that is greater than 10 with its corresponding letter. However, my current code just changes i and not the original value in the list. For example, if rd1 is set to 15, the code runs through and i is set to "F", but rd1 does not change to "F", and instead just stays as 15. How can I fix this?

Solution 1

For each iteration of the for loop the variable i is assigned with just a copy of the value of an item in vallist, so changes made to i won't be reflected in i.

You should update the items of i via index, which you can generate with the enumerate function:

for index, value in enumerate(vallist):
    if value >= 10:
        vallist[index] = letters[value]

Solution 2

rd1, rd2, gd1, gd2, bd1, bd2 = 10, 11, 12, 13, 14, 9
letters = {
10 : "A",
11 : "B",
12 : "C",
13 : "D",
14 : "E",
15 : "F"
}
vallist = [rd1, rd2, gd1, gd2, bd1, bd2]
for index, value in enumerate(vallist):
    if value >= 10 and value <= 15:
        vallist[index] = letters[value]
print(vallist)

As mentioned in the other comment you need both the index and the value while looping over your vallist. so you can replace the value on the index with the value in your dictionary.

Comments

[Splines]

I think this statement is not correct: "[...] the variable i is assigned with just a copy of the value of an item in vallist, [...]". According to Ned Batchelder's guide "Assignment never copies data". A for-loop is an assignment as well; in this case it makes i refer to a value already referenced by an item in vallist. It's the line i = letters[i] where now i refers to another value, in this case the value of letters[i], e.g. the string "A". There is no copying involved in the author's code example.