input: Write a function to check if a given string contains only digits. The function should return True if all characters in the string are digits, otherwise it should return False.
  assert is_digit_only("0123")==True
  assert is_digit_only("a123")==False
  assert is_digit_only("123a")==False
  assert is_digit_only("123.45")==False
  assert is_digit_only("")==True
output: def is_digit_only(s):
      return s.isdigit()