input: 
  def replace_string(input_string: str, target_string: str, replacement_string: str) -> str:
      """
      Replace all occurrences of a given string with an alternate string.
      >>> replace_string("Hello World!", "World", "Python")
      'Hello Python!'
      >>> replace_string("One fish, two fish, red fish, blue fish", "fish", "cat")
      'One cat, two cat, red cat, blue cat'
      """
      return input_string.replace(target_string, replacement_string)
  
output:     return input_string.replace(target_string, replacement_string)