input: Write a function to return a list of all prefixes from shortest to longest of the given input string.
  assert all_prefixes('') == []
  assert all_prefixes('asdfgh') == ['a', 'as', 'asd', 'asdf', 'asdfg', 'asdfgh']
  assert all_prefixes('WWW') == ['W', 'WW', 'WWW']
  
output: def all_prefixes(input_string):
      return [input_string[:i] for i in range(1, len(input_string) + 1)]