input: Write a function to check whether a string contains only digits. If the string contains only digits return True else return False. If the string is empty return False.
  def is_digit_only(string):
      if not string:
          return False
      for char in string:
          if not char.isdigit():
              return False
      return True
  
  assert is_digit_only("0123")==True
  assert is_digit_only("012a3")==False
  assert is_digit_only("")==False
  assert is_digit_only("9876543210")==True
  assert is_digit_only("123 456")==False
output: def is_digit_only(string):
      if not string:
          return False
      for char in string:
          if not char.isdigit():
              return False
      return True