negative numbers are not equal...

[Bruno Desthuilliers]

Terry Reedy a écrit :
> 
> 
> Mel wrote:
>> castironpi wrote:
>>> It would be nice to put together a really canonical case of the use of
>>> the 'is' comparison.  FTSOA for the sake of argument, when do you use
>>> it?  Why is it even in the language?
>>
>> My poster child use case is in a MUDD game.  For instance, the player
>> represented by `this_player` has picked up the yoghurt.  We notify the
>> other players using code that boils down to:
>>
>> for person in this_room.inhabitants:
>>     if person is not this_player:
>>         person.notify ('%s has picked up the %s.'
>>                 % (this_player.name, 'yoghurt'))
>>
>> The `is` test avoids telling this_player something he already knows. 
>> Perhaps the code could be written to make an equality test work, but then
>> again, perhaps the game could have a much more interesting use for 
>> equality
>> between persons.
> 
> Excellent example.  There are three 

make it four.

> uses for 'is'.
> 1. Minor optimization of comparison with None, True, False.

Warning (to any python newbie reading this): x is True is *very* 
different from x == True. IOW ;: don't use 'is' with True and False 
unless you know *exactly* what you're doing.

> 2. Testing the implementation: 'a=1;b=1;a is b' *should* be True, while 
> 'a=257;b=257;a is b' *should* be False.  The CPython test suite has 
> tests like this.
> 3. Comparision of user class objects where identify is important. 
> Objects representing people is certainly such a case ;-).

4. Anywhere you want to test identity.