input: 
  def replace_string(input_string, target_string, replacement_string):
      """ Replace all occurrences of the target_string with replacement_string in input_string.
      >>> replace_string("Hello World!", "World", "Python")
      'Hello Python!'
      >>> replace_string("Programming in Python is fun!", "Python", "Java")
      'Programming in Java is fun!'
      """
      return input_string.replace(target_string, replacement_string)
  
  print(replace_string("Hello World!", "World", "Python"))
  
output:     return input_string.replace(target_string, replacement_string)