Structural Pattern Matching (PEP 634)

[brandtbucher]

BPO	42128
Nosy	@gvanrossum, @rhettinger, @willingc, @dmoisset, @lysnikolaou, @pablogsal, @tirkarthi, @brandtbucher, @isidentical, @gousaiyang, @Fidget-Spinner, @akulakov, @freundTech
PRs	bpo-42128: Structural Pattern Matching (PEP 634) #22917 bpo-42128: Add what's new section for PEP 634 (pattern matching) #24588 bpo-42128: Add documentation for pattern matching (PEP 634) #24664 bpo-42128: Add Pattern Matching to What's New #24667 bpo-42128: Add documentation for the new match-based AST nodes #24673 bpo-42128: Add __match_args__ to structseq-based classes #24732 bpo-42128: Add 'missing :' syntax error message to match statements #24733 bpo-42128: Update pattern matching docs to match new PEP changes #25185 bpo-42128: __match_args__ can't be a list anymore #25203 bpo-42128: Improve "Binding of names" doc section #25642 bpo-42182: stdtypes doc - update and fix links to several dunder methods  #27384

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/brandtbucher'
closed_at = <Date 2021-04-21.03:24:32.247>
created_at = <Date 2020-10-23.15:22:20.943>
labels = ['interpreter-core', 'type-feature', '3.10']
title = 'Structural Pattern Matching (PEP 634)'
updated_at = <Date 2021-07-27.04:27:58.725>
user = 'https://github.com/brandtbucher'

bugs.python.org fields:

activity = <Date 2021-07-27.04:27:58.725>
actor = 'andrei.avk'
assignee = 'brandtbucher'
closed = True
closed_date = <Date 2021-04-21.03:24:32.247>
closer = 'brandtbucher'
components = ['Interpreter Core']
creation = <Date 2020-10-23.15:22:20.943>
creator = 'brandtbucher'
dependencies = []
files = []
hgrepos = []
issue_num = 42128
keywords = ['patch']
message_count = 56.0
messages = ['379442', '379443', '379831', '379833', '379834', '386673', '386702', '386712', '386717', '386720', '386727', '386748', '387362', '387364', '387365', '387369', '387371', '387381', '387384', '387385', '387386', '387389', '387390', '387392', '387409', '387410', '387753', '387769', '387773', '387801', '387815', '387817', '387818', '387819', '387827', '387836', '387968', '388040', '388044', '388046', '388066', '388554', '388555', '388567', '388580', '388583', '388584', '388772', '388775', '388784', '388785', '388790', '388792', '388979', '390270', '391472']
nosy_count = 14.0
nosy_names = ['gvanrossum', 'rhettinger', 'willingc', 'Daniel Moisset', 'lys.nikolaou', 'pablogsal', 'xtreak', 'brandtbucher', 'BTaskaya', 'gousaiyang', 'kj', 'andrei.avk', 'laurenjl', 'freundTech']
pr_nums = ['22917', '24588', '24664', '24667', '24673', '24732', '24733', '25185', '25203', '25642', '27384']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42128'
versions = ['Python 3.10']

[brandtbucher]

PEP-634 has not yet been accepted, but we'd like to hit the ground running and get this into alphas as soon as it (hopefully) is.

Several people have volunteered to review the implementation, since it's so huge. Other reviews are very welcome, if anybody has a bit of time to pitch in. This touches tons of stuff: the parser, the compiler, the VM, the builtins, the stdlib, the tests... I'd like as many eyeballs as possible!

I'll have a draft PR up against master in a few minutes. Let's try to keep all of the review comments there.

[brandtbucher]

Sorry, just resolving some changes with master. Are you parser people finished breaking my grammar yet? Sheesh. ;)

[brandtbucher]

Thinking ahead...

A lot of work has gone into writing these PEPs... we should see how much we can easily convert into actual docs. It seems to me:

• Parts of PEP-634 and PEP-635 can be worked into the language reference.

• Guido's overview (the appendix of PEP-636) can probably just be dropped verbatim into the What's New for 3.10.

• The rest of PEP-636 can just be lifted into the docs as a proper tutorial and linked to from the What's New.

Also, we need to document the stdlib changes (ast, collections, dataclasses, keyword, ...). The dis module docs are already part of the implementation.

[gvanrossum]

If you feel up to it, you might see if you could open a new, separate
(draft) PR that updates all those docs. (But you could also wait and see if
someone volunteers. There are some good doc writers active ATM.)

[brandtbucher]

I'll wait till the SC makes a ruling, then send a message to our docs list (I think we have one)? I'm fine coordinating/reviewing that, or making PRs myself if nobody else steps up.

[gvanrossum]

The PEP has been accepted.

I propose that we use separate PRs for docs (and incremental improvements to the code) but tie them all together using the same issue number (this will require adding "skip news" to later PRs).

[Fidget-Spinner]

Guido and Brandt, may I take a stab at the docs for this? I'll probably base most of it off PEP-636's tutorial.

Relevant parts of the docs that I've identified:
reference/lexical_analysis.rst
reference/compound_stmts.rst
reference/grammar.rst

# this probably needs a summarized version for non-technical users
whatsnew/3.10.rst

If I missed anything, please point them out :).

[dmoisset]

Hey Ken, I was about to take on this myself, but I wouldn't mind a bit of help.

Your list seems ok, I would add to it a section about PM in the python tutorial (which was explicitly mentioned in the SC acceptance notice).

Regarding the grammar, I believe that one is autogenerated from the parser, so it's likely that no changes are required there. Correct me if I'm wrong here.

I'll set up a branch in github and add you as a collaborator so we can prepare a PR. I'll base the branch of Brandt's implementation PR. If you give me your github id I can add you as collaborator so we can work there.

[Fidget-Spinner]

Hi Daniel, wow thanks for the offer! My GitHub username is Fidget-Spinner.
I'm not sure about the grammar file either.
Oh, thanks for mentioning the tutorial recommendation by the SC, I'll keep that in mind. 

[lysnikolaou]

The grammar is indeed auto-generated by reading the actual Grammar file, python.gram. Here's the lexer Pablo had developed for doing the syntax-highlighting.

https://github.com/python/cpython/blob/master/Doc/tools/extensions/peg_highlight.py

[brandtbucher]

Also, see my msg379831 above. We can't entirely rely on the PEPs, of course, but I think we could still get some decent reuse out of them.

BTW, has the new docs WG started up yet? I keep hearing about it every once in a while, but I'm not sure if it's fully-formed yet. Could be good to loop them in for review and such, if so.

+nosy Carol since I'm pretty sure she's involved with that.