input: Write a function to replace all occurrences of a given string with an alternate string in a given input string. The function should take three parameters: the input string, the target string to be replaced, and the replacement string. Return the modified string after all occurrences of the target string have been replaced with the replacement 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("This is a test", "is", "was")=="Thwas was a test"
  assert replace_string("Python is great", "Python", "Java")=="Java is great"
  assert replace_string("No replacements needed", "xyz", "abc")=="No replacements needed"
  assert replace_string("", "any", "thing")==""
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)