Replacing a substring in a string with another string using python without built-in function

def REPLACE(txt, p1, p2): if p1 not in txt: return "Pattern not found" # getting index of the pattern idx = txt.find(p1) # initialize variables to store new string temp_txt = "" new_txt = "" # deleting substring from the text for i in range(len(txt)): if i < idx or i >= idx + len(p1): temp_txt += txt[i] # adding p2 in txt as replacement for i in range(len(temp_txt)): if i == idx-1: new_txt += temp_txt[i] for j in range(len(p2)): new_txt += p2[j] else: new_txt += temp_txt[i] return new_txt