input: Write a function to check whether a given string contains only digits. The function should return True if the string contains only digits and False otherwise. If the string is empty, it should also return False.
  
  assert is_digit_only("0123")==True
  assert is_digit_only("01a3")==False
  assert is_digit_only("")==False
  assert is_digit_only("1234567890")==True
  assert is_digit_only("123 456")==False
output: def is_digit_only(s):
      return s.isdigit()