input: Write a function to replace all occurrences of a given string with an alternate string in a provided input 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("Python is fun!", "Python", "Java")=="Java is fun!"
  assert replace_string("I love Python!", "Python", "Java")=="I love Java!"
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)