流程控制

教材举例

from sys import exit

def gold_room():
    print "This room is full of gold. How much do you take?"

    next = raw_input("> ")
    if "0" in next or "1" in next:
        how_much = int(next)

    else:
        dead("Man, learn to type a number.")

    if how_much < 50:
        print "Nice, you're not greedy, you win!"
        exit(0)

    else:
        dead("You greedy bastard!")

def bear_room():
    print "There is a beer here."
    print "The bear has a bunch of honey."
    print "The fat bear is in front of another door."
    print "How are you going to move the bear?"

    bear_moved = False

    while True:
        next = raw_input("> ")

        if next == "take honey":
            dead("The bear looks at you then slaps your face off.")
        elif next == "taunt bear" and not bear_moved:
            print "The bear has moved from the door. You can go through it now."
            bear_moved = True
        elif next == "taunt bear" and bear_moved:
            dead("The bear gets pissed off and chews your leg off.")
        elif next == "open door" and bear_moved:
            gold_room()
        else:
            print "I got no idea what that means."

            
def cthulhu_room():
    print "Here you see the great evil Cthulhu."
    print "He, it, whatever stares at you and you go insane."
    print "Do you flee for your life or eat your head?"

    next = raw_input("> ")

    if "flee" in next:
        start()
    elif "head" in next:
        dead("Well that was tasty!")
    else:
        cthulhu_room()

def dead(why):
    print why,"Good job!"
    exit(0)


def start():
    print "You are in a dark room."
    print "There is a door to your right and left."
    print "Which one do you take?"

    next = raw_input("> ")

    if next == "left":
        bear_room()
    elif next == "right":
        cthulhu_room()
    else:
        dead("You stumble around the room until you starve.")


start()

上面是个小游戏，判断用户输入决定流程走向，用户是否通过熊的房间到达黄金屋，不贪图黄金顺利完成任务？简单的流程图如下

流程图

上例中定义了５个函数

1. start():开始
2. bear_room():熊的房间
3. gold_room():黄金屋
4. cthulhu_room():邪神的房间
5. dead():死亡退出

下面分析程序中的代码

exit(0)

user@ubuntu:~/Documents$ python -m pydoc sys
.......
exit(...)
exit([status])
Exit the interpreter by raising SystemExit(status).
If the status is omitted or None, it defaults to zero (i.e., success).
If the status is an integer, it will be used as the system exit status.
If it is another kind of object, it will be printed and the system
exit status will be one (i.e., failure).

exit(0),退出解释器，同时抛出SystemExit exception
status可以为null，也可以是０，表示成功退出
如果status是一个整数，它将作为系统退出代码，如果它是另外的对象，解释器会将它打印出来，系统退出状态码将是１，表示异常退出。
更多关于SystemExit

in

    next = raw_input("> ")
    if "0" in next or "1" in next:
        how_much = int(next)

raw_input函数返回的是string类型，属于python的序列类型

Sequence Types — str
, unicode
, list
, tuple
, bytearray
, buffer
, xrange

'in'是一个比较运算符(Comparisons operator)，所以可以用于真值判断。

比较运算符还有

<
>
==
<=
>=
!=
<>
is
is not
in
not in

>,<,<>,==,!=,>=,<=通常用于比较两个对象的值，两个对象的类型可以不一样，如果都是数字，它们将被转换成常用类型来比较，如果两个不同对象来比较，通常结果都是不相等。

in和not in用于容器类型的成员判断，如if x in s，如果x是s的成员，将返回True，否则返回False.

is和is not用于对象身份的判断，只有x和y是同一个对象时，x is y 返回True，否则返回False.