Python classic programming questions: string replacement

**Function prototype strreplace(str, oldString, newString) **
For example: >>> pstr = "Hello World!"; >>> afterReplaceStr = strreplace(pstr, "World"," Tom");
Then the value of afterReplaceStr is "Hello Tom!"

Method 1: Use string replacement

 def strreplace_v1(old_str, key, value):
 # Replace one or several strings of a string
  new_str = old_str.replace(key, value)return new_str

Method 2: Use regular replacement

import re
def strreplace_v2(msg, key, value):'''Replace one or several strings of a string'''
 m = re.compile(key)
 ret = m.sub(value, msg)return ret

Method 3: Write your own replacement method

def strreplace_v3(msg, key, value):'''Replace one or several strings of a string'''
 n =len(key)

 # Use python's str.index()
 # i = msg.index(key)

 # Use handwritten functions
 i =str_index(msg, key)

 j = i + n
 ret = msg[:i]+ value + msg[j:]return ret

def str_index(msg, key):'''Find the position of the string key in the string msg
 Handwriting str.index()function
    '''
 i = j =0
 n =len(msg)
 m =len(key)
 flag = False
 while i < n and not flag:
  # Check the 0th letter
  if msg[i]!= key[0]:
   i +=1else:
   # Same, detect subsequent letters
   for k inrange(1, m):if msg[i + k]!= key[j + k]:
     i += k +1breakelse:
    # turn up
    return i
 raise ValueError('substring not found')

pstr ="Hello World!"
afterReplaceStr =strreplace_v3(pstr," World"," Tom")print(afterReplaceStr)