input: Write a function to determine if a given string contains only digits. If the string is empty, return False.
  def is_digit_only(string):
      if 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("abc123")==False
  assert is_digit_only("")==False
  assert is_digit_only("456")==True
  assert is_digit_only("123a")==False
output: def is_digit_only(string):
      if string == "":
          return False
      for char in string:
          if not char.isdigit():
              return False
      return True