def mutate_string(string, position, character): s="" for i,char in enumerate(string): if i==position: s=s+character else: s=s+char
return s

def mutate_string(string, position, character): return s[:int(i)] + str(c) + s[int(i)+1:]

if name == 'main': s = input() i, c = input().split() s_new = mutate_string(s, int(i), c) print(s_new)

I've used python 3.

def mutate_string(string, position, character): string = list(string) for i in range(len(string)): if i == position: string[i]=character formatted_string = ''.join(string) return formatted_string

if name == 'main': s = input() i, c = input().split() s_new = mutate_string(s, int(i), c) print(s_new)

def mutate_string(string, position, character):
        arr = []
        for s in string:
                arr.append(s)
        arr[position] = character
        string = ""
        for t in arr:
                string += t
        return string

Slicing is an easy way to update immutable strings.

def mutate_string(string, position, character):
    string= string[:position]+ character + string[position+1:] 
    return string