python - 如何使我的类可用作字典键？

Question

class A():
   def __init__(self, data=''):
       self.data = data  

   def __str__(self):
       return str(self.data)

d = {}  
elem = A()  
d[elem] = 'abc'  

elem2 = A()
print d[elem2]    # KeyError  
# actually elem2! was used not elem

我怎样才能不出错地实现它？

我试图用另一个 A() 实例获取 d[elem2](不是 elem)，但内容相同。

Answer 1

答案是肯定的，需要重新定义__hash__()和__eq__():

>>> class A(object):
...   def __init__(self, data=''):
...     self.data = data
...   def __eq__(self, another):
...     return hasattr(another, 'data') and self.data == another.data
...   def __hash__(self):
...     return hash(self.data)
... 
>>> a1, a2, a3 = A('foo'), A('foo'), A('bar')
>>> d = {a1: 'foo'}
>>> d[a1]
'foo'
>>> d[a2]
'foo'
>>> d[a3]
Traceback (most recent call last):
  File "", line 1, in 
KeyError: __main__.A object at 0x927d0>

正如在另一条评论中所解释的，__hash__ 的默认实现只是简单的标识，所以如果你想让它更复杂，你需要明确地定义它。