On Thu, Oct 11, 2018 at 3:35 PM, Steven D'Aprano <steve@pearwood.info> wrote:
On Thu, Oct 11, 2018 at 12:34:13PM -0700, Chris Barker via Python-ideas wrote:
I don't care what is or isn't a subclass of what -- I don't think that's a Pythonic question. But I do think :
issubclass(frozendict, abc.Mapping) and issubclass(frozendict, abc.Hashable)
would be useful.
So you do care about what is and isn't a subclass :-)
well, kinda -- I don't care whether dict and frozen dict have a subclassing relationship, and I don't care what class a given object is that gets passed to my code. What I *might* care about is what interface it presents, which is what ABCs are for. If Python had a way to check ABCs without issubclass, then I wouldn't care about subclasses at all. I'd actually kind of like to have: hasinterface(an_object, (ABC1, ABC2, ABC3)) Even though, it would be the same as issubclass() (though I'd like an AND relationship with the tuple of ABCs..) Maybe I'm making a distinction that isn't really there, but I see a subclass of an ABC is quite different than a subclass of another concrete class.
It is 2018 and we've had isinstance and issubclass in the language since Python 2.2 in 2002, I really wish that we could get away from the idea that checking types is unPythonic and duck-typing is the One True Way to do things. The old, Python 1.5 form of type-checking:
type(spam) is dict
was not so great, since it tested only for a specific type by identity. But with ABCs and virtual subclasses and inheritance, isinstance is a great way to check that your duck both quacks and swims, rather than watching it sink and drown in the middle of your function :-)
yeah, I'm not so sure -- I"ve been watching Python go more and more in that direction -- and I don't think I'm happy about it. Personally, the only time I check a type is the darn list-of-strings vs single-string issue -- ever since we got true_division, that's the only type error I commonly see (that isn't trivial to debug). And I still wish Python had a Character type, and then I wouldn't need to do that either :-) And actually, if you're talking "modern" Python, isn't static type checking the way to do it now ? Now that I think about it - how does MyPy/Typeshed handle the iterable_of_strings problem? Since a single string IS and iterable of strings? -CHB -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception Chris.Barker@noaa.gov