input: Write a python function to convert each character in a string to uppercase.
  assert func("hello") == ['H', 'E', 'L', 'L', 'O']
  assert func("world") == ['W', 'O', 'R', 'L', 'D']
  assert func("Python") == ['P', 'Y', 'T', 'H', 'O', 'N']
output: def func(s):
      return [char.upper() for char in s]