input: Write a function to replace all occurrences of a given string with an alternate string.
  
  def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)
  
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Hello World! Hello World!", "World", "Python")=="Hello Python! Hello Python!"
  assert replace_string("No change here", "World", "Python")=="No change here"
  assert replace_string("", "World", "Python")==""
  assert replace_string("World is amazing", "World", "Python")=="Python is amazing"
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)