Below I respond to Chris Angelico's post in the python-dev thread. On Sat, Sep 3, 2016 at 2:01 AM, Chris Angelico <ros...@gmail.com> wrote: >>>> >>>> def eggs(cond:bool): >>>> if cond: >>>> x = 1 >>>> else: >>>> x = 1.5 >>>> spam(x) # a good type checker infers that x is of type Union[int, >>>> float] >>> > > I wonder if it would be different if you wrote that as a single expression: > > x = 1 if cond else 1.5 > > x = sum([1] + [0.5] * cond) > > What should type inference decide x is in these cases? Assume an > arbitrarily smart type checker that can implement your ideal; it's > equally plausible to pretend that the type checker can recognize an > if/else block (or even if/elif/else tree of arbitrary length) as a > single "assignment" operation. IMO both of these examples - and by > extension, the if/else of the original - should be assigning a Union > type.
In the first case it would indeed again be Union[int, float] (the code is basically equivalent after all). In the second case, the checker would infer List[Union[int, float]]. No magic involved, assuming that list.__add__ is annotated precisely enough. > Lots of Python code assumes that smallish integers [1] are > entirely compatible with floats. Is Python 4 going to have to deal > with the int/float distinction the way Python 3 did for bytes/text, or > are they fundamentally compatible concepts? Is the "small integer" > like the "ASCII byte/character" as a kind of hybrid beast that people > treat as simultaneously two types? (Personally, I don't think it's > anything like bytes/text. But I'm open to argument.) > > Forcing people to write 1.0 just to be compatible with 1.5 will cause > a lot of annoyance. I leave it to you to decide whether there's a > fundamental difference that needs to be acknowledged, or just > subtleties of representational limitations to be ignored until they > become a problem. Agreed. However, I think it completely depends on the context, which types are "compatible" in the way that 1 and 1.5 often are. Numbers are just a very typical example. But it could as well be Animal and Person. It's just a matter of whether the code base is written in a way that it's ok to pass on either type. > ChrisA > > [1] And by "smallish" I mean less than 2**53. Big enough for a lot of > purposes. Bigger (by definition) than JavaScript's integers, which cap > out at 2**32. > -- + Koos Zevenhoven + http://twitter.com/k7hoven + _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
