持有对象dict()的Python对象

Question

这是我为发现的问题创建的一个测试用例。

由于某些原因，B（）中的dict（）'l'似乎没有正确的值。请参阅下面我的Linux 11.04 Ubuntu，python 2.7.1+.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()

我机器上的输出：

    $ 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

据我所知，上面几行：

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

应该都包含一个C（）的唯一实例，如初始化时所见，而不是创建的最后一个实例（请参见<<<OK>>行）。

这里发生了什么？

Answer 1

发生的事情是创建了一个class属性。而是通过在__init__()中实例化来创建实例属性。

Answer 2

看起来您正试图在类级别"声明"实例属性。类属性在Python中有其特定的用途，如果您不打算使用类属性，那么将它们放在那里是错误的

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中，您已经创建了一个类属性l。由于实例没有自己的属性l，因此它使用类的属性。

你可以这样写你的B类，而不是

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 >>>"
...