bpo-42128: Add __match_args__ to structseq-based classes

[pablogsal]

https://bugs.python.org/issue42128

[brandtbucher]

Thanks Pablo!

Should we have a test for a structseq with unnamed fields? Asserting that os.stat_result.__match_args__ is expected should do it.

[pablogsal]

I don't think we can handle this in a better way: just exclude them from match_args. The other (much worse option IMHO) is generate some missing names for these.

[pablogsal]

Should we have a test for a structseq with unnamed fields? Asserting that os.stat_result.__match_args__ is expected should do it.

Done in 40bf3b7

[brandtbucher]

Looking a bit closer... should we limit this to only VISIBLE_SIZE_TP(type) members? Since they're not displayed in the repr and can't be iterated over, I don't think there's any way to easily (as a user reading the code) map positional arguments to members like tm_zone and tm_gmtoff.

Plus, we can always extend it later if it turns out to be an unnecessary limitation.

[pablogsal]

Looking a bit closer... should we limit this to only VISIBLE_SIZE_TP(type) members? Since they're not displayed in the repr and can't be iterated over, I don't think there's any way to easily (as a user reading the code) map positional arguments to members like tm_zone and tm_gmtoff.

Plus, we can always extend it later if it turns out to be an unnecessary limitation.

Good point! I agree with that. I have pushed a new commit with this limitation.

[brandtbucher]

Looks good to me!

The only improvement I might suggest is building up a list, then converting it to a tuple (or creating a tuple and shrinking it later with _PyTuple_Resize). That way we only need to loop once.

[pablogsal]

Looks good to me!

The only improvement I might suggest is building up a list, then converting it to a tuple (or creating a tuple and shrinking it later with _PyTuple_Resize). That way we only need to loop once.

I considered that, but I think the code would be harder to read because appending to the list can fail on every iteration and we also need to covert the list to a tuple that can also fail. Also it will be less efficient so I decided with the current approach.

I can do the resize approach, that will be cleaner.

[pablogsal]

(or creating a tuple and shrinking it later with _PyTuple_Resize). That way we only need to loop once.

Done