【Python入门系列2】Event-driven Programming

上课笔记。
这周讲到了Event-driven Programming。

先举个栗子，这是一个猜数字的小游戏：

# template for "Guess the number" mini-project
# input will come from buttons and an input field
# all output for the game will be printed in the console
import simplegui
import random

secret_number = 0
end = 100
remaining_guess = 7

# helper function to start and restart the game
def new_game():
        # initialize global variables used in your code here
        global secret_number
        global end
        secret_number = random.randrange(0, end)
    
        print "New game. Range is from 0 to", end
        print "Number of remaining guesses is", remaining_guess
        print ""


# define event handlers for control panel
def range100():
        # button that changes the range to [0,100) and starts a new game 
        global remaining_guess
        remaining_guess = 7
    
        global end
        end = 100
    
        new_game()
    

def range1000():
        # button that changes the range to [0,1000) and starts a new game     
        global remaining_guess
        remaining_guess = 10
    
        global end
        end = 1000
    
        new_game()
    
def input_guess(guess):
        # main game logic goes here
        global secret_number
        global remaining_guess
    
        guessi = int(guess)
        print "Guess was", guessi
        remaining_guess -= 1
        print "Number of remaining guesses is", remaining_guess
    
        if guessi < secret_number:
                print "Higher"
        elif guessi > secret_number:
                print "Lower"
        else:
                print "Correct"
                print ""
                range100()
            
    print ""
    
     if remaining_guess == 0:
                print "You lose."
         print ""
                range100()

    
# create frame
frame = simplegui.create_frame("game", 200, 200)

# register event handlers for control elements and start frame
frame.add_button('Range is [0,100)', range100, 200)
frame.add_button('Range is [0,1000)', range1000, 200)

frame.add_input("Input your guess: ", input_guess, 50)



# call new_game 
new_game()

# always remember to check your completed program against the grading rubric
复制代码

http://www.codeskulptor.org/#user41_5azXiELlsx_1.py
这个网址要翻一下墙的哈 - -
而且由于simpleGUI这个API的特殊性，貌似只能在codeskulpter网站运行。
点开运行了之后是这样的：

运行结果是这样的：

New game. Range is from 0 to 100
Number of remaining guesses is 7

Guess was 25
Number of remaining guesses is 6
Higher

Guess was 50
Number of remaining guesses is 5
Lower

Guess was 38
Number of remaining guesses is 4
Lower

Guess was 31
Number of remaining guesses is 3
Higher

Guess was 34
Number of remaining guesses is 2
Lower

Guess was 33
Number of remaining guesses is 1
Lower

Guess was 32
Number of remaining guesses is 0
Correct

New game. Range is from 0 to 100
Number of remaining guesses is 7
复制代码

这个module的重点是，event-driven programming的思想。



我们知道，一般玩个什么游戏、使个什么交互应用，每次有不同的操作，就会产生不同的结果。
如下图所示，初始化之后，根据用户的操作，会产生不同的触发事件，当有input event进来的时候，就会等待handler来进行处理，知道最后程序运行结束。


event的种类常见的以下几种：


一般使用下面的program structure是比较合理而符合人的认知以及写代码结构的：

那么这就是刚才的小program里的代码编写顺序。


几点需要注意的tips：
1. 全局变量与局部变量
Python与其他编程语言比较相异的一点是，Python对于变量的定义区分基本上是没有的- -。
因此就会产生一点需要注意的问题。

num1 = 1
print "num1 =", num1

def fun():
            num2 = num1 + 1
            print "num2 =", num2

fun()

print "num1 =", num1
print "num2 =", num2
复制代码

运行这个Python文件，输出结果是这样的：

num1 = 1
num2 = 2
num1 = 1
num2 =
Traceback (most recent call last):
    Line 11, in <module>
        print "num2 =", num2
NameError: name 'num2' is not defined
复制代码

出现了一个NameError，这是为什么呢？
因为在函数体内部定义的变量是局部变量，当程序执行调用函数体的时候才会产生，当函数体调用结束后这个局部变量也就随之消失了。因此在全局中要求输出num2的时候它已经不在了。
而全局变量则不然，它是随着程序一直存在的。

把上面的例子改一改：

num1 = 1
print "num1 =", num1

def fun():
            num1 = 2
            num2 = num1 + 1
            print "num2 =", num2

fun()

print "num1 =", num1
复制代码

运行结果是：

num1 = 1
num2 = 3
num1 = 1
复制代码

我们看到，最后输出的num1还是全局变量的num1，而非函数体中的那一个，函数体中的那一个早就随函数调用的结束消失了。

那么，如果我们要在函数中用到全局变量，我们应该怎么用呢？
在函数中变量前加global申明即可。
如下例所示：

num = 4

def fun1():
            global num
            num = 5

def fun2():
            global num
            num = 6

print num
fun1()
print num
fun2()
print num
复制代码

运行结果是：

4
5
6
复制代码

2.  输入值的转换
在课上使用的simpleGUI的API中，使用add_input这个接口输入的值都是string类型的，因此如果我们需要数字，必须进行强制类型转换。
在开头的例子中，用的是这行代码：

 guessi = int(guess)
复制代码

3. 代码规范（参见Python官方文档https://docs.python.org/release/2.6.8/tutorial/controlflow.html#intermezzo-coding-style）
提高易读性，减少易错性。

•     Use 4-space indentation, and no tabs.
  

         4 spaces are a good compromise between small indentation (allows greater nesting depth) and large indentation (easier to read). Tabs introduce confusion, and  are best left out.

•     Wrap lines so that they don’t exceed 79 characters.
•     This helps users with small displays and makes it possible to have several code files side-by-side on larger displays.
•     Use blank lines to separate functions and classes, and larger blocks of code inside functions.
•     When possible, put comments on a line of their own.
•     Use docstrings.
•     Use spaces around operators and after commas, but not directly inside bracketing constructs: a = f(1, 2) + g(3, 4).
•     Name your classes and functions consistently; the convention is to use CamelCase for classes and lower_case_with_underscores for functions and methods. Always use self as the name for the first method argument (see A First Look at Classes for more on classes and methods).
•     Don’t use fancy encodings if your code is meant to be used in international environments. Plain ASCII works best in any case.
  

好长~