Python对象持有对象的字典（）

Question

这是我为我发现的问题创建的测试用例。

由于某些原因，B（）中的dict（）'l'似乎不具有正确的值。在我的Linux 11.04 Ubuntu，python 2.7.1+上查看下面的输出。

class A(): 
    name = None 
    b = None 
    def __init__(self, name, bname, cname, dname): 
     self.name = name 
     print "A: name", name 
     self.b = B(name, bname, cname, dname) 
     print "A self.b:", self.b 

class B(): 
    name = None 
    l = dict() 
    c = None 
    def __init__(self, name, bname, cname, dname): 
     self.aname = name 
     self.name = bname 
     print " B: name", bname 
     self.c = C(bname, cname, dname) 
     self.l["bb"] = self.c 
     print " B self:", self 
     print " B self.c:", self.c 
     print " B self.l[bb]:", self.l["bb"], "<<<OK>>>" 

    def dump(self): 
     print " A: name", self.aname 
     print " B: name", self.name 
     for i in self.l: 
      print " B: i=", i, "self.l[i]", self.l[i], "<<<ERROR>>>" 

class C(): 
    name = None 
    l = dict() 
    d = None 
    def __init__(self, bname, cname, dname): 
     self.bname = bname 
     self.cname = cname 
     print " B: name", bname 
     print " C: name", cname 
     print " C self:", self 

    def dump(self): 
     print " B name:", self.bname 
     print " C name:", self.cname 

a1 = A("a1", "b1", "c1", "d1") 
a2 = A("a2", "b2", "c2", "d2") 
a3 = A("a3", "b3", "c3", "d3") 

a1.b.dump() 
a1.b.c.dump() 
a2.b.dump() 
a2.b.c.dump() 
a3.b.dump() 
a3.b.c.dump() 

输出我的机器上：

$ python bedntest.py 
A: name a1 
B: name b1 
    B: name b1 
    C: name c1 
    C self: <__main__.C instance at 0xb76f3a6c> 
B self: <__main__.B instance at 0xb76f388c> 
B self.c: <__main__.C instance at 0xb76f3a6c> 
B self.l[bb]: <__main__.C instance at 0xb76f3a6c> <<<OK>>> 
A self.b: <__main__.B instance at 0xb76f388c> 
A: name a2 
B: name b2 
    B: name b2 
    C: name c2 
    C self: <__main__.C instance at 0xb76f3acc> 
B self: <__main__.B instance at 0xb76f3aac> 
B self.c: <__main__.C instance at 0xb76f3acc> 
B self.l[bb]: <__main__.C instance at 0xb76f3acc> <<<OK>>> 
A self.b: <__main__.B instance at 0xb76f3aac> 
A: name a3 
B: name b3 
    B: name b3 
    C: name c3 
    C self: <__main__.C instance at 0xb76f3b2c> 
B self: <__main__.B instance at 0xb76f3b0c> 
B self.c: <__main__.C instance at 0xb76f3b2c> 
B self.l[bb]: <__main__.C instance at 0xb76f3b2c> <<<OK>>> 
A self.b: <__main__.B instance at 0xb76f3b0c> 
A: name a1 
B: name b1 
B: i= bb self.l[i] <__main__.C instance at 0xb76f3b2c> <<<ERROR>>> 
    B name: b1 
    C name: c1 
A: name a2 
B: name b2 
B: i= bb self.l[i] <__main__.C instance at 0xb76f3b2c> <<<ERROR>>> 
    B name: b2 
    C name: c2 
A: name a3 
B: name b3 
B: i= bb self.l[i] <__main__.C instance at 0xb76f3b2c> <<<ERROR>>> 
    B name: b3 
    C name: c3 

据我了解，以上各行：

B: i= bb self.l[i] <__main__.C instance at 0xb76f3b2c> <<<ERROR>>> 

都应该持有C（唯一的实例），如在初始化时看到 - 不是最后创建的实例（请参阅< < <确定>>>行）。

这里发生了什么事？

Answer 1

看起来你正试图在类级别“声明”实例属性。类属性在Python的自己的具体用途，这是错误的把它们放在那里，如果你不打算使用过的类属性

class A(): 
    name = None # Don't do this 
    b = None  # Don't do this 
    def __init__(self, name, bname, cname, dname): 
     self.name = name 
     print "A: name", name 
     self.b = B(name, bname, cname, dname) 
     print "A self.b:", self.b 

在class B你创建了一个类属性l。由于该实例没有它自己的属性l它使用该类的属性。

你可以只写你的B类这样反而

class B(): 
    def __init__(self, name, bname, cname, dname): 
     self.aname = name 
     self.name = bname 
     self.l = dict() 
     print " B: name", bname 
     self.c = C(bname, cname, dname) 
     self.l["bb"] = self.c 
     print " B self:", self 
     print " B self.c:", self.c 
     print " B self.l[bb]:", self.l["bb"], "<<<OK>>>" 

... 

Answer 2

发生了什么事是您创建了一个类属性。通过在__init__()中实例化来创建实例属性。