世界上什么是python中的属性"__class__"

Question

我__class__在python中有一个问题.

文档说这__class__是类实例所属的类.所以我进行了一系列实验:

class counter:
    count = 0
    def __init__(self):
            self.__class__.count += 1

NewCounter1 = counter()
print NewCounter1.count   #The result is 1
NewCounter2 = counter()
print NewCounter2.count   #The result is 2
print NewCounter2.__class__.count is NewCounter2.count  #result: True

一切顺利.

然后我输入如下代码:

NewCounter2.__class__.count = 3

print NewCounter1.count                    #result:3
print NewCounter1.__class__.count      #result:3
print NewCounter2.count                    #result:3
print NewCounter2.__class__.count      #result:3
print NewCounter2.__class__.count is NewCounter2.count      #result: True

从上面的代码中,我认为可能NewCounter1.count等于NewCounter1,或者__class__.count,但以下代码让我感到惊讶:

NewCounter2.count = 5

print NewCounter1.count                 #result:3
print NewCounter1.__class__.count   #result:3
print NewCounter2.count                 #result:5
print NewCounter2.__class__.count   #result:3
print NewCounter2.__class__.count is NewCounter2.count       #result: False

为什么NewCounter2.count改变但NewCounter2.__class__.count仍保持在3？更重要的是,当我改变时NewCounter2.count,NewCounter2.__class__.count is NewCounter2.count变成了False.这个属性在世界上是__class__什么？

Answer 1

"从上面的代码中,我认为NewCounter1.count可能等于NewCounter1._class_.count"

问题是,在你的问题中这句话的那一刻,在唯一的指示之后:

NewCounter1 = counter()
NewCounter2 = counter()
NewCounter2.__class__.count = 3

创建了NewCounter1和NewCounter2
并修改了类属性counter.count后,
没有对象 NewCounter1.count和NewCounter2.count 存在,然后"equals"没有实际意义.

.

查看NewCounter1的创建,紧接着:

class counter:
    count = 0
    def __init__(self):
        self.__class__.count += 1

print 'counter.count BEFORE ==',counter.count  # The result is 0
NewCounter1 = counter()
print '\nNewCounter1.__dict__ ==',NewCounter1.__dict__  # The result is {}
print 'NewCounter1.count    ==',NewCounter1.count # The result is 1
print 'counter.count AFTER  ==',counter.count  # The result is 1

NewCounter._ 字典 _是实例的名称空间NewCounter1
print NewCounter1.count 打印一样print counter.count
.然而,"计数"(字符串"计数")未在命名空间NewCounter1,也就是说没有属性算在命名空间创建的实例!

这怎么可能 ？

那是因为创建实例时没有分配_ init _中的'count'标识符
- >在NewCounter1中没有真正创建任何属性作为字段,也就是说没有创建INSTANCE属性.

结果是,当
print 'NewCounter1.count ==',NewCounter1.count
评估指令时,解释器在NewCounter1的命名空间中找不到实例属性,然后转到实例的类以在该类的命名空间中搜索键'count'; 在那里它找到'count'作为CLASS属性的键,并且可以将对象counter.count的VALUE作为值来显示以响应该指令.

类实例具有作为字典实现的名称空间,该字典是搜索属性引用的第一个位置.当在那里找不到属性,并且实例的类具有该名称的属性时,搜索继续使用类属性. http://docs.python.org/reference/datamodel.html#the-standard-type-hierarchy

因此,NewCounter1.count equals NewCounter1.__class__.count这意味着NewCounter1.count的VALUE,即使这个实际上不存在,也是类属性NewCounter1的VALUE .class .count.这里的"被"是英文动词,而不是功能是用于测试两个对象的身份的语言,它的意思是"被认为具有"

当NewCounter2.__class__.count = 3被执行时,只有类属性counter.count受到影响.NewCounter1和NewCounter2的名称空间保持为空,并且遵循与查找counter.count值的类相同的机制.

.

最后,当NewCounter2.count = 5执行时,这次将INSTANCE属性 计数创建为NewCounter2对象中的字段,并且'count'出现在NewCounter2的命名空间中 .
它不会覆盖任何内容,因为实例中__dict__
没有任何内容没有其他更改会影响NewCounter1和counter.count

以下代码更明确地显示了执行期间的基础事件:

from itertools import islice

class counter:
    count = 0
    def __init__(self):
        print ('  |  counter.count   first == %d  at  %d\n'
               '  |     self.count   first == %d  at  %d')\
               % (counter.count,id(counter.count),
                  self.count,id(self.count))

        self.__class__.count += 1 # <<=====

        print ('  |  counter.count  second == %d  at  %d\n'
               '  |     self.count  second == %d  at  %d\n'
               '  |  id(counter) == %d   id(self) == %d')\
               % (counter.count,id(counter.count),
                  self.count,id(self.count),
                  id(counter),id(self))



def display(*li):
    it = iter(li)
    for ch in it:
        nn = (len(ch)-len(ch.lstrip('\n')))*'\n'
        x = it.next()
        print '%s ==  %s %s' % (ch,x,'' if '__dict__' in ch else 'at '+str(id(x)))



display('counter.count AT START',counter.count)


print ('\n\n----- C1 = counter() ------------------------')
C1 = counter()
display('C1.__dict__',C1.__dict__,
        'C1.count ',C1.count,
        '\ncounter.count ',counter.count)


print ('\n\n----- C2 = counter() ------------------------')
C2 = counter()
print ('  -------------------------------------------') 
display('C1.__dict__',C1.__dict__,
        'C2.__dict__',C2.__dict__,
        'C1.count ',C1.count,
        'C2.count ',C2.count,
        'C1.__class__.count',C1.__class__.count,
        'C2.__class__.count',C2.__class__.count,
        '\ncounter.count ',counter.count)


print '\n\n------- C2.__class__.count = 3 ------------------------\n'
C2.__class__.count = 3
display('C1.__dict__',C1.__dict__,
        'C2.__dict__',C2.__dict__,
        'C1.count ',C1.count,
        'C2.count ',C2.count,
        'C1.__class__.count',C1.__class__.count,
        'C2.__class__.count',C2.__class__.count,
        '\ncounter.count ',counter.count)


print '\n\n------- C2.count = 5 ------------------------\n'
C2.count = 5
display('C1.__dict__',C1.__dict__,
        'C2.__dict__',C2.__dict__,
        'C1.count ',C1.count,
        'C2.count ',C2.count,
        'C1.__class__.count',C1.__class__.count,
        'C2.__class__.count',C2.__class__.count,
        '\ncounter.count ',counter.count)

结果

counter.count AT START ==  0 at 10021628


----- C1 = counter() ------------------------
  |  counter.count   first == 0  at  10021628
  |     self.count   first == 0  at  10021628
  |  counter.count  second == 1  at  10021616
  |     self.count  second == 1  at  10021616
  |  id(counter) == 11211248   id(self) == 18735712
C1.__dict__ ==  {} 
C1.count  ==  1 at 10021616

counter.count  ==  1 at 10021616


----- C2 = counter() ------------------------
  |  counter.count   first == 1  at  10021616
  |     self.count   first == 1  at  10021616
  |  counter.count  second == 2  at  10021604
  |     self.count  second == 2  at  10021604
  |  id(counter) == 11211248   id(self) == 18736032
  -------------------------------------------
C1.__dict__ ==  {} 
C2.__dict__ ==  {} 
C1.count  ==  2 at 10021604
C2.count  ==  2 at 10021604
C1.__class__.count ==  2 at 10021604
C2.__class__.count ==  2 at 10021604

counter.count  ==  2 at 10021604


------- C2.__class__.count = 3 ------------------------

C1.__dict__ ==  {} 
C2.__dict__ ==  {} 
C1.count  ==  3 at 10021592
C2.count  ==  3 at 10021592
C1.__class__.count ==  3 at 10021592
C2.__class__.count ==  3 at 10021592

counter.count  ==  3 at 10021592


------- C2.count = 5 ------------------------

C1.__dict__ ==  {} 
C2.__dict__ ==  {'count': 5} 
C1.count  ==  3 at 10021592
C2.count  ==  5 at 10021568
C1.__class__.count ==  3 at 10021592
C2.__class__.count ==  3 at 10021592

counter.count  ==  3 at 10021592

.

一个有趣的事情是
self.count = counter.count
在行之前添加一条指令
self.__class__.count += 1 # <<=====
来观察结果的变化

.

总之,关键点并不是关注,__class__而是搜索属性的机制,这种机制在被忽略时会产生误导.

Answer 2

这一行:

NewCounter2.__class__.count = 3

改变静态count的counter,但在这里:

NewCounter2.count = 5

NewCounter2现在有自己的count属性隐藏静态count;
这条线没有影响NewCounter1.
这也是原因NewCounter2.__class__.count != NewCounter2.count.