hash values and equality

[Peter Otten]

Ethan Furman wrote:

> Peter Otten wrote:
>> Ethan Furman wrote:
>> 
>>> Several folk have said that objects that compare equal must hash equal,
>>> and the docs also state this
>>> http://docs.python.org/dev/reference/datamodel.html#object.__hash__
>>>
>>> I'm hoping somebody can tell me what horrible thing will happen if this
>>> isn't the case?  Here's a toy example of a class I'm thinking of writing
>>> that will compare equal with int's, but hash differently:
>>>
>>> --> class Wierd():
>>> ...     def __init__(self, value):
>>> ...         self.value = value
>>> ...     def __eq__(self, other):
>>> ...         return self.value == other
>>> ...     def __hash__(self):
>>> ...         return hash((self.value + 13) ** 3)
>>> ...
>> 
>> Try this:
>> 
>>>>> d = {Wierd(1): 0}
>>>>> 1 in d
>> False
>>>>> 1 in d.keys()
>> True
>> 
> 
> My apologies -- I'm trying this in Python3:

Then you have to convert the keys to a list explicitly:

>>> class Weird:
...     def __init__(self, value):
...             self.value = value
...     def __eq__(self, other):
...             return self.value == other
...     def __hash__(self):
...             return hash((self.value + 13) **3)
...
>>> d = {Weird(1): 0}
>>> 1 in d
False
>>> 1 in d.keys()
False
>>> 1 in list(d.keys())
True