类中的python生成器

Question

我正在从这个 thread 学习生成器.这确实是生成器的一个很好的例子。但是我对其中一个示例代码感到困惑。

>>> class Bank(): # let's create a bank, building ATMs
...    crisis = False
...    def create_atm(self):
...        while not self.crisis:
...            yield "$100"
>>> hsbc = Bank() # when everything's ok the ATM gives you as much as you want
>>> corner_street_atm = hsbc.create_atm()
>>> print(corner_street_atm.next())  
$100  
>>> print(corner_street_atm.next())  
$100  
>>> print([corner_street_atm.next() for cash in range(5)])  
['$100', '$100', '$100', '$100', '$100']  
>>> hsbc.crisis = True # crisis is coming, no more money!  
>>> print(corner_street_atm.next())  
<type 'exceptions.StopIteration'>   
>>> wall_street_atm = hsbc.create_atm() # it's even true for new ATMs  
>>> print(wall_street_atm.next())  
<type 'exceptions.StopIteration'>  

>>> hsbc.crisis = False # trouble is, even post-crisis the ATM remains empty
>>> print(corner_street_atm.next())
<type 'exceptions.StopIteration'>

>>> brand_new_atm = hsbc.create_atm() # build a new one to get back in business
>>> for cash in brand_new_atm:
...    print cash
$100
$100
$100
$100
$100
$100
$100
$100
$100
...

当 hsbc.crisis 重置为 False 时，corner_steet_atm 只能产生 StopIteration。为什么会这样。我认为危机过后 corner_street_atm 并非空无一人。

Answer 1

问题出在当你说

corner_street_atm = hsbc.create_atm()

您只调用一次该函数，这就是它会被调用的所有次数。因此，让我们了解正在发生的事情

  def create_atm(self):
        # Called right away, when you say the above line and only then
        print("Called function") 

        # Loops while crisis = False
        while not self.crisis: 
            yield "$100"

现在，我们可以问当危机为真时我们的方法在寻找什么，我们发现它指向下图所示的位置:

  def create_atm(self):
        # Called right away, when you say the above line and only then
        print("Called function") 

        # Loops while crisis = False
        while not self.crisis: 
            yield "$100"
            #<------ Where you function points after you set crisis to false and get the next

好吧，那里什么也没有，所以你得到一个错误。

您真正要查找的似乎是 atm 中的无限循环。像这样:

class Bank():
    crisis = False

    def create_atm(self):
        print("Called function")
        while True:
            if not self.crisis:
                yield "$100"
            else:
                yield "$0"

小心这个，作为你的最后决定

for cash in brand_new_atm:
    print(cash)

将永远循环，因为您目前看不到自动取款机中有多少现金(因此只要没有危机，它就会喷出美元钞票。