Part of an assignment to re-implement a cypher as a function using string.ascii_uppercase and eventually string.digits to iterate through a given input string/statement. Seeing the amount of other versions is cool, but have to use this specific method.
I can get along with doing this outside of a function, but whence added to a function, I yield only a single character and being unfamiliar with the string method, I think there may be a piece missing from what I constructed.

function: cypher(s)
input: s
result: result of function
key: 25-letter_position

import string

def cypher(s):
    s=s.upper()
    result=""
    for ch in s:
        if ch in string.ascii_uppercase():
            letter_position = string.ascii_uppercase.find(ch)
            new_letter = string.ascii_uppercase[25-letter_position]
            result = result + new_letter
        else:
            result = result + ch
        return result

s = input("Input a string: ")
print("Cyphertext: ",result)