input: Write a function to check if a given string contains only digits. If the string contains only digits, return True; otherwise, return False.
  
  assert is_digit_only("0123") == True
  assert is_digit_only("012a") == False
  assert is_digit_only("") == True
  assert is_digit_only("123456") == True
  assert is_digit_only("abc123") == False
output: def is_digit_only(s):
      return s.isdigit()