PYTHON编程导论群问题汇总(三)

Q1: week2-3, Ex-guess my number

在做week2练习：guess my number时遇到一个小问题。
代码已经写好，结果也和给出的答案一样，但是格式有一点不同，如下：

# 这是我的output
Please think of a number between 0 and 100!
Is your secret number 50?
Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly.
l
# 这是答案expected output
Please think of a number between 0 and 100!
Is your secret number 50?
Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly. l

可以发现区别在于答案的输入l是在“Enter ‘h’ to indicate the guess is too high. Enter ‘l’ to indicate the guess is too low. Enter ‘c’ to indicate I guessed correctly.”的同一行，而我的有一个换行，调整了半天也不知道如何让他们处于同一行，故请教大大们。我的代码如下，比较丑陋…

i = 50
low = 0
high = 99
print('Please think of a number between 0 and 100!')
print('Is your secret number: 50?')
while True: print("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly.")a = input() # 问题出在这里，如何能将input和上面的print内容显示在一行里if a != 'h' or 'l' or 'c':print('Sorry, I did not understand your input.')print('Is your secret number:' + str(i) + '?')print("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly.")a = input()elif a == 'l':low = ii = int((i+high)/2)print('Is your secret number:' + str(i))elif a == 'h':high = ii = int((low+i)/2)print('Is your secret number:' + str(i))else:print('Game over. Your secret number was:' + str(i))

lmz:

a = input("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly.")

此外你的代码如果输入的字符不是‘h，l，c’中的一个，你有没发现，他会要求你输入2次input?
具体实现我倒是做了笔记

bigjing:

你的代码除了在input处存在问题外，运行逻辑上存在一些小问题，在不改变你整体框架的前提下，现做如下修改

第9行 if a !='h' or 'l' or 'c'应改为：a != 'h' and a != 'l' and a !='c'
第13行 a = input() 改为 continue
第22行 else: 后需要加 break，否则循环无法终止

修改后完整代码：

i = 50
low = 0
high = 99
print('Please think of a number between 0 and 100!')
print('Is your secret number: 50?')
while True: a = input("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly.") # 问题出在这里，如何能将input和上面的print内容显示在一行里if a != 'h' and a != 'l' and a !='c':print('Sorry, I did not understand your input.')print('Is your secret number:' + str(i) + '?')continueelif a == 'l':low = ii = int((i+high)/2)print('Is your secret number:' + str(i))elif a == 'h':high = ii = int((low+i)/2)print('Is your secret number:' + str(i))else:print('Game over. Your secret number was:' + str(i))break

根据对题目理解，我重新写了一个，供参考

low = 0
high = 100
guess = 50
print('Please think of a number between 0 and 100!')
print('Is your secret number:{}?'.format(guess))
while True: a = input("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly.") if a == 'c':print('Game over. Your secret number was:{}'.format(guess) )breakelif a in ['l','h']:if a == 'l':low = guesselse:high = guessguess = (high+low)//2print('Is your secret number:{}'.format(guess))continueelse:print('Sorry, I did not understand your input.')  continue

lmz:

# week2-3 Exercise: guess my numberprint('Please think of a number between 0 and 100!')
low = 0
high = 100def guess_wrong_ans(ans):global high, lowif ans == 'c':print('Game over. Your secret number was: %s' % mid)return False elif ans == 'h':high = midelif ans == 'l':low = midelse:print('Sorry, I did not understand your input.')return Truestatus = True
while status:mid = (low + high)//2   print('Is your secret number %s?' % mid)ans = input("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly. ")status = guess_wrong_ans(ans)

Q2: 程序运行方法

程序运行方法有三种，比如说print(‘hello world’)：
1. 在命令行直接运行；
2. 编写一个 Python 文件，将 print hello world 封装为一个函数，通过 main 函数调用它来运行；
3. 编写一个 class，将 print hello world 封装为一个 method，通过 main 函数创建 class 实例来运行 method。
后两种方式怎样实现，能举个例子么？

SY:

我的理解是第一个是直接运行，第二个是通过函数，第三个是通过类的方法。

lmz:

# 2.hello.py
def hello():print("hello world")if __name__ == '__main__':hello()

# 3.hello.py
class hello(object):def helloworld(self):print("hello world")def main():       text = hello()     # 初始化实例text.helloworld()  # 调用类的方法if __name__ == '__main__':main()

bigjing:

能解释下python编程中的if __name__ == 'main': 的作用和原理么？

lmz:

if __name__ == '__main__' 的意思是说，只有直接运行python hello.py时，该if条件下的语句才会执行，可以参考廖雪峰的网站链接。
当我们在命令行运行hello模块文件时，Python解释器把一个特殊变量__name__置为__main__，而如果在其他地方导入该hello模块时，if判断将失败，因此，这种if测试可以让一个模块通过命令行运行时执行一些额外的代码，最常见的就是运行测试。

bigjing:

凡星的解释已经很完整了，附另一个解释比较好的连接，供大家学习

Q3: 数组转换

经常能看到对数组进行reshape(-1,1)操作
我们知道reshape(2,3)是转成2行3列。
那reshape(-1,1)这个操作实现的功能是什么？
虽然从结果上来看好像是转成多行一列，-1的存在是固定写法么?

lmz:

Parameters: a : array_like, Array to be reshaped.newshape : int or tuple of ints
The new shape should be compatible with the original shape. If an integer, then the result will be a 1-D array of that length. One shape dimension can be -1. In this case, the value is inferred from the length of the array and remaining dimensions.

案例的话，知乎

c = np.array([[1,2,3],[4,5,6]])
# python2代码
print '改成2行3列:'
print c.reshape(2,3)
print '改成3行2列:'
print c.reshape(3,2)
print '我也不知道几行，反正是1列:'
print c.reshape(-1,1)
print '我也不知道几列，反正是1行：'
print c.reshape(1,-1)
print '不分行列，改成1串'
print c.reshape(-1)

输出为：改成2行3列:
[[1 2 3]
[4 5 6]]
改成3行2列:
[[1 2]
[3 4]
[5 6]]
我也不知道几行，反正是1列:
[[1]
[2]
[3]
[4]
[5]
[6]]
我也不知道几列，反正是1行：
[[1 2 3 4 5 6]]
不分行列，改成1串
[1 2 3 4 5 6]

Q4.函数参数

P33：关键字参数与默认参数的区别？

bigjing:

首先一个观点：关键字参数是包含默认参数范畴的
当关键字参数作为形参的时候，就是默认参数
当关键字参数作为实参的时候，可以实现不按位置赋值参数的作用

Q5. 入栈顺序（函数运行空间）

P35：入栈顺序，为什么第1列中x在f之后，第2列中h在g之后入栈？

bigjing:

拓展一下问题：
什么是栈
什么是栈帧
栈和栈帧有什么区别
函数调用栈帧的过程

lmz:

我就不从概念上去解释了，而是按照我的理解解释：

栈(stack)——一种FILO(先进后出)的数据结构，python里运行函数就是用的这种结构
python执行函数时，会新开一个局部空间（也就是书中的stack frame, 中译文栈帧），函数执行完后，除了返回的数据，该局部空间也就销毁了（即：你不能再访问该局部空间里的变量了）
由于python每次执行函数，都是先跳到函数内部执行完毕后，再返回原空间继续，所以说这是一种FILO(或LIFO)的执行顺序，然后中间涉及的变量储存就是对应的栈结构了。

书中对应的内容，讲的就是python执行代码过程中，变量的产生和销毁的顺序。