习题 42: 物以类聚

虽说将函数放到字典里是很有趣的一件事情，你应该也会想到“如果 Python 能自动为你做这件事情该多好”。事实上也的确有，那就是 class 这个关键字。你可以使用 class 创建更棒的“函数字典”，比你在上节练习中做的强大多了。Class（类）有着各种各样强大的功能和用法，但本书不会深入讲这些内容，在这里，你只要你学会把它们当作高级的“函数字典”使用就可以了。

用到“class”的编程语言被称作“Object Oriented Programming（面向对象编程）”语言。这是一种传统的编程方式，你需要做出“东西”来，然后你“告诉”这些东西去完成它们的工作。类似的事情你其实已经做过不少了，只不过还没有意识到而已。记得你做过的这个吧：

stuff = ['Test', 'This', 'Out']
print ' '.join(stuff)

其实你这里已经使用了 class。``stuff`` 这个变量其实是一个 list class （列表类）。而 ' '.join(stuff) 里调用函数 join 的字符串 ' ' （就是一个空格）也是一个 class —— 它是一个 string class (字符串类)。到处都是 class！

还有一个概念是 object（物件），不过我们暂且不提。当你创建过几个 class 后就会学到了。你怎样创建 class 呢？和你创建 ROOMS 的方法差不多，但其实更简单：

class TheThing(object):

    def __init__(self):
        self.number = 0

    def some_function(self):
        print "I got called."

    def add_me_up(self, more):
        self.number += more
        return self.number

# two different things
a = TheThing()
b = TheThing()

a.some_function()
b.some_function()

print a.add_me_up(20)
print a.add_me_up(20)
print b.add_me_up(30)
print b.add_me_up(30)

print a.number
print b.number

Warning

嗯，你开始看到 Python 的“疣子”了。Python 是一门比较旧的语言，其中包含很多丑陋的设计决定。为了将这些丑陋设计掩盖过去，他们就做了一些新的丑陋设计，然后告诉人们让他们习惯这些新的坏设计。``class TheThing(object)`` 就是其中一个例子。这里我就不展开讲了，不过你也不必操心为什么你的 class 要在后面添一个(object) ，只要跟着这样做就可以了，否则将来总有一天别的 Python 程序员会吼你让你这样做。后面我们再讲为什么。

你看到参数里的 self 了吧？你知道它是什么东西吗？对了，它就是 Python 创建的额外的一个参数，有了它你才能实现 a.some_function()` 这种用法，这时它就会把\ 前者翻译成 ``some_function(a) 执行下去。为什么用 self 呢？因为你的函数并不知道你的这个“实例”是来自叫 TheThing 或者别的名字的 class。所以你只要使用一个通用的名字 self 。这样你写出来的函数就会在任何情况下都能正常工作。

其实你可以使用 self 以外的别的字眼，不过如果你这样做的话，你就会成为所有Python 程序员的众之矢的，所以还是随大流的好。只有变态才会在这里乱改，我教你的没错。对以后会读到你的代码的人好点儿，因为你现在的代码10年以后所有的代码都会是一团糟。

接下来，看到 __init__ 函数了吗？这就是你为 Python class 设置内部变量的方式。你可以使用 . 将它们设置到 self 上面。另外看到我们使用了 add_me_up() 为你创建的 self.number 加值。后面你可以看到我们怎样可以使用这种方法为数字加值，然后打印出来。

Class 是很强大的东西，你应该好好读读相关的东西。尽可能多找些东西读并且多多实验。你其实知道它们该怎么用，只要试试就知道了。其实我马上就要去练吉他了，所以我不会让你写练习了。你将使用 class 写一个练习。

接下来我们将把习题41的内容重写，不过这回我们将使用 class：

from sys import exit
from random import randint

class Game(object):

    def __init__(self, start):
        self.quips = [
            "You died.  You kinda suck at this.",
             "Your mom would be proud. If she were smarter.",
             "Such a luser.",
             "I have a small puppy that's better at this."
        ]
        self.start = start

    def play(self):
        next = self.start

        while True:
            print "\n--------"
            room = getattr(self, next)
            next = room()


    def death(self):
        print self.quips[randint(0, len(self.quips)-1)]
        exit(1)

    def central_corridor(self):
        print "The Gothons of Planet Percal #25 have invaded your ship and destroyed"
        print "your entire crew.  You are the last surviving member and your last"
        print "mission is to get the neutron destruct bomb from the Weapons Armory,"
        print "put it in the bridge, and blow the ship up after getting into an "
        print "escape pod."
        print "\n"
        print "You're running down the central corridor to the Weapons Armory when"
        print "a Gothon jumps out, red scaly skin, dark grimy teeth, and evil clown costume"
        print "flowing around his hate filled body.  He's blocking the door to the"
        print "Armory and about to pull a weapon to blast you."

        action = raw_input("> ")

        if action == "shoot!":
            print "Quick on the draw you yank out your blaster and fire it at the Gothon."
            print "His clown costume is flowing and moving around his body, which throws"
            print "off your aim.  Your laser hits his costume but misses him entirely.  This"
            print "completely ruins his brand new costume his mother bought him, which"
            print "makes him fly into an insane rage and blast you repeatedly in the face until"
            print "you are dead.  Then he eats you."
            return 'death'

        elif action == "dodge!":
            print "Like a world class boxer you dodge, weave, slip and slide right"
            print "as the Gothon's blaster cranks a laser past your head."
            print "In the middle of your artful dodge your foot slips and you"
            print "bang your head on the metal wall and pass out."
            print "You wake up shortly after only to die as the Gothon stomps on"
            print "your head and eats you."
            return 'death'

        elif action == "tell a joke":
            print "Lucky for you they made you learn Gothon insults in the academy."
            print "You tell the one Gothon joke you know:"
            print "Lbhe zbgure vf fb sng, jura fur fvgf nebhaq gur ubhfr, fur fvgf nebhaq gur ubhfr."
            print "The Gothon stops, tries not to laugh, then busts out laughing and can't move."
            print "While he's laughing you run up and shoot him square in the head"
            print "putting him down, then jump through the Weapon Armory door."
            return 'laser_weapon_armory'

        else:
            print "DOES NOT COMPUTE!"
            return 'central_corridor'

    def laser_weapon_armory(self):
        print "You do a dive roll into the Weapon Armory, crouch and scan the room"
        print "for more Gothons that might be hiding.  It's dead quiet, too quiet."
        print "You stand up and run to the far side of the room and find the"
        print "neutron bomb in its container.  There's a keypad lock on the box"
        print "and you need the code to get the bomb out.  If you get the code"
        print "wrong 10 times then the lock closes forever and you can't"
        print "get the bomb.  The code is 3 digits."
        code = "%d%d%d" % (randint(1,9), randint(1,9), randint(1,9))
        guess = raw_input("[keypad]> ")
        guesses = 0

        while guess != code and guesses < 10:
            print "BZZZZEDDD!"
            guesses += 1
            guess = raw_input("[keypad]> ")

        if guess == code:
            print "The container clicks open and the seal breaks, letting gas out."
            print "You grab the neutron bomb and run as fast as you can to the"
            print "bridge where you must place it in the right spot."
            return 'the_bridge'
        else:
            print "The lock buzzes one last time and then you hear a sickening"
            print "melting sound as the mechanism is fused together."
            print "You decide to sit there, and finally the Gothons blow up the"
            print "ship from their ship and you die."
            return 'death'


    def the_bridge(self):
        print "You burst onto the Bridge with the netron destruct bomb"
        print "under your arm and surprise 5 Gothons who are trying to"
        print "take control of the ship.  Each of them has an even uglier"
        print "clown costume than the last.  They haven't pulled their"
        print "weapons out yet, as they see the active bomb under your"
        print "arm and don't want to set it off."
        
        action = raw_input("> ")

        if action == "throw the bomb":
            print "In a panic you throw the bomb at the group of Gothons"
            print "and make a leap for the door.  Right as you drop it a"
            print "Gothon shoots you right in the back killing you."
            print "As you die you see another Gothon frantically try to disarm"
            print "the bomb. You die knowing they will probably blow up when"
            print "it goes off."
            return 'death'

        elif action == "slowly place the bomb":
            print "You point your blaster at the bomb under your arm"
            print "and the Gothons put their hands up and start to sweat."
            print "You inch backward to the door, open it, and then carefully"
            print "place the bomb on the floor, pointing your blaster at it."
            print "You then jump back through the door, punch the close button"
            print "and blast the lock so the Gothons can't get out."
            print "Now that the bomb is placed you run to the escape pod to"
            print "get off this tin can."
            return 'escape_pod'
        else:
            print "DOES NOT COMPUTE!"
            return "the_bridge"

    def escape_pod(self):
        print "You rush through the ship desperately trying to make it to"
        print "the escape pod before the whole ship explodes.  It seems like"
        print "hardly any Gothons are on the ship, so your run is clear of"
        print "interference.  You get to the chamber with the escape pods, and"
        print "now need to pick one to take.  Some of them could be damaged"
        print "but you don't have time to look.  There's 5 pods, which one"
        print "do you take?"

        good_pod = randint(1,5)
        guess = raw_input("[pod #]> ")


        if int(guess) != good_pod:
            print "You jump into pod %s and hit the eject button." % guess
            print "The pod escapes out into the void of space, then"
            print "implodes as the hull ruptures, crushing your body"
            print "into jam jelly."
            return 'death'
        else:
            print "You jump into pod %s and hit the eject button." % guess
            print "The pod easily slides out into space heading to"
            print "the planet below.  As it flies to the planet, you look"
            print "back and see your ship implode then explode like a"
            print "bright star, taking out the Gothon ship at the same"
            print "time.  You won!"
            exit(0)


a_game = Game("central_corridor")
a_game.play()

你应该看到的结果

这个版本的游戏和你的上一版效果应该是一样的，其实有些代码都几乎一样。比较一下两版代码，弄懂其中不同的地方，重点需要理解这些东西：

1. 怎样创建一个 class Game(object) 并且放函数到里边去。
2. __init__ 是一个特殊的初始方法，可以预设重要的变量在里边。
3. 为 class 添加函数的方法是将函数在 class 下再缩进一阶，class 的架构就是通过缩进实现的，这点很重要。
4. 你在函数里的内容又缩进了一阶。
5. 注意冒号的用法。
6. 理解 self 的概念，以及它在 __init__ 、 play 、 death 里是怎样使用的。
7. 研究 play 里的 getattr 的功能，这样你就能明白 play 所做的事情。其实你可以手动在 Python 命令行实验一下，从而弄懂它。
8. 最后我们怎样创建了一个 Game ，然后通过 play() 让所有的东西运行起来。

加分习题

1. 研究一下 __dict__ 是什么东西，应该怎样使用。
2. 再为游戏添加一些房间，确认自己已经学会使用 class 。
3. 创建一个新版本，里边使用两个 class，其中一个是 Map ，另一个是 Engine 。提示: 把 play 放到 Engine 里面。