Why does the python replace function not work?

Question

I am making my own substitution cipher with python, and am trying to use the replace function. It seems as if it does not work though...

print("Scylla\n\n")
w = input("Encode (E) or Decode (D)?\n\n")
x = str(input("What is the message?\n\n"))
y = str(x.upper())
if w == "E":
    for char in y:
        y.replace("A", "N")
        y.replace("B", "O")
        y.replace("C", "P")
        y.replace("D", "Q")
        y.replace("E", "R")
        y.replace("F", "S")
        y.replace("G", "T")
        y.replace("H", "U")
        y.replace("I", "V")
        y.replace("J", "W")
        y.replace("K", "X")
        y.replace("L", "Y")
        y.replace("M", "Z")
        y.replace("N", "A")
        y.replace("O", "B")
        y.replace("P", "C")
        y.replace("Q", "D")
        y.replace("R", "E")
        y.replace("S", "F")
        y.replace("T", "G")
        y.replace("U", "H")
        y.replace("V", "I")
        y.replace("W", "J")
        y.replace("X", "K")
        y.replace("Y", "L")
        y.replace("Z", "M")
    print(y)

But all it does is print the phrase input in caps. What am I doing wrong?

replace returns the replaced value. Try `y = y.replace(....)` — William D. Irons, Apr 23 '21 at 14:45
Strings are immutable, so `replace` does not modify the string in-place — Wondercricket, Apr 23 '21 at 14:45
Also note, that since your replacing A with N, and then replacing N with A, the end result should be same as what you started with. — William D. Irons, Apr 23 '21 at 14:47

score 0 · Answer 1 · answered Apr 23 '21 at 14:46

The replace function works fine. You're just using it wrong. It returns a new string with the replacement made, rather than updating the old one in place, so you need to save its return value. (And in general, your first reaction should always be "I'm using this standard, common thing wrong" rather than "this standard, common thing is broken".)

Why does the python replace function not work?

1 Answers1