input: Write a function to check whether a 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("123abc")==False
  assert is_digit_only("12.34")==False
  assert is_digit_only("")==True
  assert is_digit_only("1234567890")==True
output: def is_digit_only(s):
      return s.isdigit()