input: from typing import Tuple
  
  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("The quick brown fox jumps over the lazy dog.", " ", "_")
      'The_quick_brown_fox_jumps_over_the_lazy_dog.'
      """
output:     return input_string.replace(target_string, replacement_string)