[Python-Dev] Mundane dict __setitem__...

[David Abrahams]

Guido van Rossum <guido@python.org> writes:

>> I am about to write some code which relies on the following:
>> 
>>     x = (1,2,3)
>>     y = (1,2,3)
>>     d = {}
>>     d[x] = 1
>>     d[y] = 1
>>     assert d.keys()[0] is y
>> 
>> In other words, when you do d[k] = v it replaces both v *and* k in the
>> dict.  I couldn't find it documented anywhere but the source.  I
>> submitted a doc patch which enshrines that behavior, but since Guido
>> doesn't always know what's in the doc I wanted to make sure it was
>> considered reasonable.
>> 
>> Anyone want to tell me it's a bad idea?  It seems like the more useful
>> of the two possible behaviors to me.
>
> Bad idea.  There are no guarantees whether the key will be updated or
> not.

Too bad.  It's better to update the key.

>  (There is also no rule that forbids the compiler from making x
> and y point to the same object, BTW.)

I knew that part.  Note that my test doesn't rely on having distinct
identities.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com