input: Write a function to determine if a given string contains only digits. The function should return True if all characters in the string are digits, and False otherwise.
  assert is_digit_only("0123")==True
  assert is_digit_only("0a23")==False
  assert is_digit_only("12345")==True
  assert is_digit_only("")==True
  assert is_digit_only("a1234")==False
output: def is_digit_only(s):
      return s.isdigit()