input: Write a function to replace all occurrences of a given string with an alternate string. The function should return the modified 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("Hello Hello", "Hello", "Hi")=="Hi Hi"
  assert replace_string("", "World", "Python")==""
  assert replace_string("Hello World!", "Earth", "Python")=="Hello World!"
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)