python 函数名作为另一函数的参数传入，还没有给这个函数名实参？

Question

player.arrange(get_key)

这句代码怎么能成立呢？

get_key的形参不需要传入实参吗？

求大佬给解释一下....

import random

class Card(object):
    """一张牌"""

    def __init__(self, suite, face):
        self._suite = suite
        self._face = face

    @property
    def face(self):
        return self._face
    @property
    def suite(self):
        return self._suite

    def __str__(self):
        if self._face == 1:
            face_str = 'A'
        elif self._face == 11:
            face_str = 'J'
        elif self._face == 12:
            face_str = 'Q'
        elif self._face == 13:
            face_str = 'K'
        else:
            face_str = str(self._face)
        return '%s%s' % (self._suite, face_str)

    def __repr__(self):
        return self.__str__()
class Poker(object):
    """一副牌"""
    def __init__(self):
        self._cards = [Card(suite, face)
                       for suite in '♠♥♣♦'
                       for face in range(1, 14)]
        self._current = 0

    @property
    def cards(self):
        return self._cards

    def shuffle(self):
        """洗牌（随机乱序）"""
        self._current = 0
        random.shuffle(self._cards)

    @property
    def next(self):
        """发牌"""
        card = self._cards[self._current]
        self._current += 1
        return card

    @property
    def has_next(self):
        """还有没有牌"""
        return self._current < len(self._cards)

class Player(object):
    """玩家"""
    def __init__(self, name):
        self._name = name
        self._cards_on_hand = []

    @property
    def name(self):
            return self._name

    @property
    def card_on_hand(self):
        return self._cards_on_hand

    def get(self, card):
        """摸牌"""
        self._cards_on_hand.append(card)

    def arrange(self, card_key):
        """玩家整理手上的牌"""
        self._cards_on_hand.sort(key=card_key)

#排序规则-先根据花色再根据点数排序
def get_key(card):
    return (card.suite, card.face)

def main():
    p = Poker()
    p.shuffle()
    players = [Player('东邪'), Player('西毒'),
              Player('南帝'), Player('北丐')]
    for _ in range(13):
        for player in players:
            player.get(p.next)
    for player in players:
        print(player.name + ':', end = ' ')
        player.arrange(get_key)
        print(player._cards_on_hand)

if __name__ == '__main__':
    main()

Answer 1

player.arrange(get_key)

到
def arrange(self, card_key):的时候

player->self
get_key->card_key

card_key是一个函数类型，这里不是执行函数，所以不需要传参数。它是由sort函数执行的，而sort调用的时候，自然会传它需要的参数。