input: 
  from typing import List, Tuple
  
  def replace_string(input_string: str, target_string: str, replacement_string: str) -> str:
      """
      Given an input string, this function replaces all occurrences of a specified 
      target string with a replacement string.
  
      >>> replace_string("Hello World!", "World", "Python")
      'Hello Python!'
      >>> replace_string("I like apples and oranges.", "apples", "bananas")
      'I like bananas and oranges.'
      """
      return input_string.replace(target_string, replacement_string)
  
output:     return input_string.replace(target_string, replacement_string)