"/a" is not "/a" ?

[Steve Holden]

Gary Herron wrote:
> Robert Kern wrote:
>> On 2009-03-06 14:23, Gary Herron wrote:
>>> Robert Kern wrote:
>>>> On 2009-03-06 13:46, Gary Herron wrote:
>>>>> Emanuele D'Arrigo wrote:
>>>>>> Hi everybody,
>>>>>>
>>>>>> while testing a module today I stumbled on something that I can work
>>>>>> around but I don't quite understand.
>>>>>
>>>>> *Do NOT use "is" to compare immutable types.* **Ever! **
>>>>
>>>> Well, "foo is None" is actually recommended practice....
>>>>
>>>
>>> But since newbies are always falling into this trap, it is still a good
>>> rule to say:
>>>
>>> Newbies: Never use "is" to compare immutable types.
>>>
>>> and then later point out, for those who have absorbed the first rule:
>>>
>>> Experts: Singleton immutable types *may* be compared with "is",
>>> although normal equality with == works just as well.
>>
>> That's not really true. If my object overrides __eq__ in a funny way,
>> "is None" is much safer.
>>
>> Use "is" when you really need to compare by object identity and not
>> value.
> 
> But that definition is the *source* of the trouble.  It is *completely*
> meaningless to newbies.   Until one has experience in programming in
> general and experience in Python in particular, the difference between
> "object identity" and "value" is a mystery.  
> So in order to lead newbies away from this *very* common trap they often
> fall into, it is still a valid rule to say
> 
>    Newbies: Never use "is" to compare immutable types.
> 
I think this is addressing the wrong problem. I;d prefer to say

Newbies: never assume that the interpreter keeps just one copy of any
value. Just because a == b that doesn't mean that a is b. *Sometimes* it
will be, but it isn't usually guaranteed.

> of even better
> 
>    Newbies: Never use "is" to compare anything.
> 
> This will help them avoid traps, and won't hurt their use of the
> language.  If they get to a point that they need to contemplate using
> "is", then almost be definition, they are not a newbie anymore, and the
> rule is still valid.

personally I believe newbies should be allowed the freedom to shoot
themselves in the foot occasionally, and will happily explain the issues
that arise when they do so. It's all good learning.

I think using "is" to compare mutable objects is a difficult topic to
explain, and I think your division of objects into mutable and immutable
types is unhelpful and not to-the-point.

regards
 Steve
-- 
Steve Holden           +1 571 484 6266   +1 800 494 3119
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