Question

我跑的程序是

class Account(object):
    counter = 0

    def __init__(self, holder, number, balance, credit_line = 1500):
        Account.counter += 1

    def __del__(self):
        Account.counter -= 1

print "Counter\t", Account.counter
a1 = Account("Homer Simpson", 2893002, 2325.21)
print "Counter\t", Account.counter
a2 = Account("Fred Flintstone", 2894117, 755.32)
print "Counter\t", Account.counter
a3 = a2
print "Counter\t", Account.counter
a4 = Account("Bill Gates", 2895007, 5324.32)
print "Counter\t", Account.counter
del a4
print "Counter\t", Account.counter

我第一次运行它时输出是正确的：

Counter 0
Counter 1
Counter 2
Counter 2
Counter 3
Counter 2

但如果我在同一个控制台中再次运行它：

Counter 0
Counter 0
Counter 1
Counter 0
Counter 1
Counter 0

知道输出第二次改变的原因吗？

Answer 1

啊，现在知道了！很好的问题！..

在第二轮中：

print "Counter\t", Account.counter   #Class instantiated. Prints 0 
a1 = Account("Homer Simpson", 2893002, 2325.21) #a1 is dereferenced and referenced again!..

上面发生的是创建一个新的Account对象。因此递增计数器。但是当a1再次引用它时，它所引用的同一个前一个对象就会丢失！因此调用 del 并收集垃圾，减少数量为1。

print "Counter\t", Account.counter #Thus no change as referencing dereferencing happen together.Prints 0.
a2 = Account("Fred Flintstone", 2894117, 755.32)

现在等一下，它接下来打印1。那个怎么样？上面发生了什么？现在记住a2和a3都引用同一个对象。当a2引用新对象时，旧对象不会被垃圾收集，因为a3仍然引用它并因此引用该增量。

print "Counter\t", Account.counter #Gets incremented and prints 1 as explained above.
a3 = a2

现在你可能会得到它;-) ..没有创建新对象但是之前引用的对象的最后一个引用丢失了，因为a3引用了一个新对象，从而减少了计数。

print "Counter\t", Account.counter #Count decremented print 0 as explained above.
a4 = Account("Bill Gates", 2895007, 5324.32)

这里有点棘手，但等等。请记住，您已在上次运行中删除了a4。因此创建了一个全新的对象，增加了计数。

print "Counter\t", Account.counter #count incremented prints 1
del a4

现在你必须得到它。好问题！.. :-D

print "Counter\t", Account.counter

抱歉可怕的编辑。这里相当新，手头上没有多少时间，但无法回答:-P

Python计数器变量在第二次运行时出错了吗？

1 个答案: