[Python-ideas] Make "is" checks on non-singleton literals errors

[Masklinn]

On 2012-10-08, at 21:48 , Guido van Rossum wrote:
> On Mon, Oct 8, 2012 at 12:44 PM, Mike Graham <mikegraham at gmail.com> wrote:
>> I regularly see learners using "is" to check for string equality and
>> sometimes other equality. Due to optimizations, they often come away
>> thinking it worked for them.
>> 
>> There are no cases where
>> 
>>    if x is "foo":
>> 
>> or
>> 
>>   if x is 4:
>> 
>> is actually the code someone intended to write.
>> 
>> Although this has no benefit to anyone but new learners, it also
>> doesn't really do any harm.
> 
> I think the best we can do is to make these SyntaxWarnings. I had the
> same thought recently and I do agree that these are common beginners
> mistakes that can easily hide bugs by succeeding in simple tests.

How would the rather common pattern of using an `object` instance as a
placeholder be handled? An identity test precisely expresses what is
meant and desired in that case, while an equality test does not.

An other one which seems to have some serious usage in the stdlib is
type-testing (e.g. collections.abc, decimal or tests of exception types).
Without type inference, I'm not too sure how that could be handled
as syntactic warnings, and as above an identity test expresses the
purpose of the code better than an equality one.