[Python-Dev] Third and hopefully final post: PEP 557, Data Classes

[Eric V. Smith]

On 11/30/2017 3:35 PM, Carl Meyer wrote:
> On 11/29/2017 05:02 PM, Guido van Rossum wrote:
>> I tried to look up the discussion but didn't find much except that you
>> flagged this as an issue. To repeat, your concern is that isdataclass()
>> applies to *instances*, not classes, which is how Eric has designed it,
>> but you worry that either through the name or just because people don't
>> read the docs it will be confusing. What do you suppose we do? I think
>> making it work for classes as well as for instances would cause another
>> category of bugs (confusion between cases where a class is needed vs. an
>> instance abound in other situations -- we don't want to add to that).
>> Maybe it should raise TypeError when passed a class (unless its
>> metaclass is a dataclass)? Maybe it should be renamed to
>> isdataclassinstance()? That's a mouthful, but I don't know how common
>> the need to call this is, and people who call it a lot can define their
>> own shorter alias.
> 
> Yeah, I didn't propose a specific fix because I think there are several
> options (all mentioned in this thread already), and I don't really have
> strong feelings about them:
> 
> 1) Keep the existing function and name, let it handle either classes or
> instances. I agree that this is probably not the best option available,
> though IMO it's still marginally better than the status quo).
> 
> 2) Punt the problem by removing the function; don't add it to the public
> API at all until we have demonstrated demand.
> 
> 3) Rename it to "is_dataclass_instance" (and maybe also keep a separate
> "is_dataclass" for testing classes directly). (Then there's also the
> choice about raising TypeError vs just returning False if a function is
> given the wrong type; I think TypeError is better.)

In that case, you can spell "is_dataclass_instance":

def isdataclass_instance(obj):
     dataclasses.fields(obj)  # raises TypeError for non-dataclass
                              # classes or instances
     if not isinstance(obj, type):
         raise TypeError('not an instance')
     return True

Since this is easy enough to do in your own code, and I still don't see 
a use case, I'll just add a note to the PEP and omit delete isdataclass().

Plus, you can decide for yourself how to deal with the question of 
returning true for classes or instances or both.

Eric.