input: Write a function to replace all occurrences of a target string with a replacement string in a given input string. Return the modified input string.
  
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Python is fun, Python is great!", "Python", "Java")=="Java is fun, Java is great!"
  assert replace_string("No replacements needed here.", "replacement", "change")=="No replacements needed here."
  assert replace_string("", "empty", "full")==""
  assert replace_string("Same same same", "same", "different")=="Same different different"
output: def replace_string(input_str, target, replacement):
      return input_str.replace(target, replacement)