This is a code I wrote and it works as expected. However, I think conditional statements and loops can be simplified and become more pythonic codes using methods such as list comprehension.

def erase(sts1, sts2):

    erase_lst = []
    for _, letter1 in enumerate(sts1.upper()):
        if letter1.isalpha():
            if letter1 not in erase_lst:
                erase_lst.append(letter1.upper())
                erase_lst.append(chr(ord(letter1.upper()) + 32))

    sts = ''
    for letter2 in sts2:
        letter2 = ' ' if letter2 in erase_lst else letter2
        sts += letter2
    return sts

Question: For the first part, the successive for loop and if statements with 3 indentations can be organized in a better way?

This is what I tried for the second part, but an error occurred in the bold line.
sts = '' ' ' **if letter2 in erase_lst else letter2 for letter2 in sts2** sts += letter2 return sts
According to the pycharm, ‘letter2’ is an unresolved reference and the end of the statement is expected.

l = [1,2,3,4,5] print(**[x+1 if x >= 5 else x+5 for x in l]**)
However, this simple code has the same structure as the above bold line (at least I cannot get the difference) and works without any problem.
Question: Could you explain what I did wrong here?