[Python-ideas] "Sum" Type hinting [was: Type hinting for path-related functions]

Guido van Rossum guido at python.org
Thu May 19 10:46:23 EDT 2016

On Thu, May 19, 2016 at 12:00 AM, Greg Ewing
<greg.ewing at canterbury.ac.nz> wrote:
> Guido van Rossum wrote:
>> Sometimes you really do want to distinguish between the box and what's
>> in it, and then you can use a bunch of simple classes, or maybe a
>> bunch of namedtuples, to represent the different kinds of boxes. If
>> you declare the unopened box type as a Union you can get mypy to check
>> that you've covered all your bases.
> Well, sort of. It will check that you don't pass anything
> outside of the set of allowed types, but it can't tell
> whether you've handled all the possible cases in the
> code. That's something Haskell can do because (1) it has
> special constructs for case analysis, and (2) its
> algebraic types can't be extended or subclassed.

I'm still not sure I understand why this check that you've handled all
cases is so important (I've met a few people who obsessed about it in
other languages, but I don't really feel the need in my gut yet).

I also don't think that subclasses cause problems (if there's a match
for a particular class, it will match the subclass too).

Anyway, I believe mypy could easily check that you're always returning
a value in situations like this (even though it doesn't do that now):

def foo(arg: Union[X, Y, Z]) -> int:
    if isinstance(arg, X): return arg.counter
    if isinstance(arg, Y): return 0

It should be possible to figure out that Z is  missing here.

>> The one thing that Python doesn't have (and mypy doesn't add) would be
>> a match statement. The design of a Pythonic match statement would be
>> an interesting exercise;
> Yes, if there were dedicated syntax for it, mypy could
> check that you covered all the branches of a Union.
> I've thought about this off and on a bit. Maybe I'll
> write about some ideas in another post.

Would love to hear your thoughts!.

--Guido van Rossum (python.org/~guido)

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