【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
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
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运行结果是：
num1 = 1
num2 = 3
num1 = 1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运行结果是：
4
5
6
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)
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, andare 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~{:5_169:}
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