On Tue, Oct 6, 2020 at 6:50 PM Ricky Teachey <ri...@teachey.org> wrote:
> On Tue, Oct 6, 2020 at 9:35 PM Steven D'Aprano <st...@pearwood.info> > wrote: > >> On Tue, Oct 06, 2020 at 09:30:06AM -0700, aleksiy123 wrote: >> > Currently there are a few Protocols >> > https://docs.python.org/3/library/typing.html#protocols defined in the >> > typing module. One that I recently felt like was missing was a typing >> > annotation for classes with a __str__() method defined. Seems like a >> fairly >> > straightforward implementation. >> > > Why do you care specifically about the `__str__` dunder? Calling str() >> is one of those protocols that should always succeed, whether the >> argument defines `__str__`, `__repr__`, or inherits from object. >> >> Ignoring the possibility of bugs or deliberate raising, I can't think of >> any object that doesn't support the str() protocol. Have I missed >> anything? > > > I was also finding this desire a little puzzling (and thought it might > just be my inexperience). > > And since both `type` and `object` have a __str__ method*, it seems like > the only way any object could NOT support the str() protocol would be to > write a __str__ method and deliberately bust it. Right? Is there even a > way to create a python object that does not have `__str__` in the MRO? > > Seeing as that is the case, and seeing as how the protocol system can't > look inside your __str__ method to find out whether it is busted at type > checking time, isn't `Any` all you'll ever need to type-hint the __str__ > protocol? > > * and upon checking in fact it is the same __str__ method > There's somewhat a precedent -- though it's a questionable one (Luciano discovered this). It shows how tricky these things are though. There's a `typing.SupportsFoat` protocol that checks for the presence of a `__float__` method, and a `typing.SupportsComplex` protocol that checks for `__complex__`. Ironically, complex numbers have a `__float__` method that always fails, and floats don't have a `__complex__` method but complex(x) succeeds if x is a float... -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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