请问类的实例可以作为字典的key吗？

wolfg

The object must be immutable

limodou

我试了一下没有问题。你的代码是什么，可能是别的错误。

>;>;>; class A:
...     pass
   
>;>;>; a=A()
>;>;>; b={a:1}
>;>;>; b[a]
1

wolfg

原帖由 "Rcfeng"]p2)] 这样是不行的~[/quote 发表：


不使用内置的__cmp__方法就行了

1. class ip:
   
2.    "A Ip class for finding user's area"
   
3.    def __init__(self, low_ip, up_ip):
   
4.        self.low_ip = low_ip
   
5.        self.up_ip = up_ip
   
6.    def __str__(self):
   
7.        return "Low: %d; Up: %d" % (self.low_ip,self.up_ip)
   
8.    def cmpare(self,ip):
   
9.        if( ip>;=low_ip and ip<=up_ip ):
   
10.            return 0
    
11.        elif( ip<low_ip ):
    
12.            return -1
    
13.        else: return 1

复制代码

可能是有了__cmp__就认为是可变的了
Python文档里找到这么一句
[quote]Instances of a class normally compare as non-equal unless the class defines the __cmp__() method.

__cmp__( self, other)

Called by comparison operations if rich comparison (see above) is not defined. Should return a negative integer if self < other, zero if self == other, a positive integer if self >; other. If no __cmp__(), __eq__() or __ne__() operation is defined, class instances are compared by object identity (``address''). See also the description of __hash__() for some important notes on creating objects which support custom comparison operations and are usable as dictionary keys. (Note: the restriction that exceptions are not propagated by __cmp__() has been removed since Python 1.5.)

看了__hash__的介绍，终于明白了

__hash__( self)

Called for the key object for dictionary operations, and by the built-in function hash(). Should return a 32-bit integer usable as a hash value for dictionary operations. The only required property is that objects which compare equal have the same hash value; it is advised to somehow mix together (e.g., using exclusive or) the hash values for the components of the object that also play a part in comparison of objects. If a class does not define a __cmp__() method it should not define a __hash__() operation either; if it defines __cmp__() or __eq__() but not __hash__(), its instances will not be usable as dictionary keys. If a class defines mutable objects and implements a __cmp__() or __eq__() method, it should not implement __hash__(), since the dictionary implementation requires that a key's hash value is immutable (if the object's hash value changes, it will be in the wrong hash bucket).

If a class does not define a __cmp__() method it should not define a __hash__() operation either; if it defines __cmp__() or __eq__() but not __hash__(), its instances will not be usable as dictionary keys.

wolfg

根据文档里的讲解，用__cmp__也可以，再实现__hash__就行了

我随便试了一下

1. def __hash__(self):
   
2.         return hash(self.low_ip + self.up_ip)

复制代码

是可以的，只是不知道这样hash会不会有问题

wolfg

你可以这样做

不过要想用dict的话，就得自己实现__hash__方法，保证low_ip和up_ip值都一样的ip的instance其hash值一样，这样就可以做dict的key了

不知道说得对不对，呵呵