missing dataclass decorator in match-statement example

[18bac53a-00a3-4fa8-8300-7203320f18d9]

BPO	44109
Nosy	@rhettinger, @ericvsmith, @brandtbucher, @akulakov, @ahmetveburak

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['type-bug', '3.10', 'docs'] title = 'missing dataclass decorator in match-statement example' updated_at = user = 'https://github.com/ahmetveburak' ``` bugs.python.org fields: ```python activity = actor = 'andrei.avk' assignee = 'docs@python' closed = False closed_date = None closer = None components = ['Documentation'] creation = creator = 'ahmetveburak' dependencies = [] files = [] hgrepos = [] issue_num = 44109 keywords = [] message_count = 6.0 messages = ['393454', '393607', '393678', '393812', '396527', '396550'] nosy_count = 6.0 nosy_names = ['rhettinger', 'eric.smith', 'docs@python', 'brandtbucher', 'andrei.avk', 'ahmetveburak'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'open' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue44109' versions = ['Python 3.10'] ```

Linked PRs

• gh-120281

[18bac53a-00a3-4fa8-8300-7203320f18d9]

Using Point class as in the documentation example, raises TypeError: Point() takes no arguments https://docs.python.org/3.10/whatsnew/3.10.html#patterns-and-classes

Also there is same example in the PEP-636's latests parts, Point class used with dataclass decorator and works perfectly. https://www.python.org/dev/peps/pep-0636/

[brandtbucher]

I don't really think there is anything wrong with the documentation. I can copy-and-paste all of the code from the PEP-634 section of the 3.10 What's New into the REPL without any issues (provided that named subjects like point/points/test_variable/color are actually defined).

Note that none of the example snippets actually *create Point instances. Rather, some of the patterns *match Point instances... which is sort of the opposite of instantiation.

As the Point class is currently written, instantiation would probably look like:

>> p = Point() >> p.x = 0 >> p.y = 0

Sure, it would help to have an __init__ defined in real code, but I sort of feel like adding it to this example would just distract (especially since the example doesn't actually *need* it).

Thoughts from others?

[rhettinger]

Thoughts from others?

As it stands, the Point class is weird and atypical by only using class variables. The example would be improved by adding @dataclass or inheriting from typing.NamedTuple.

[ericvsmith]

I agree with Raymond: a more useful example would use dataclasses or typing.NamedTuple. The use of class variables is unusual, and it took me a while to understand how it would work.

[akulakov]

I agree the example is confusing for all of the stated reasons.

It seems to me it's better to use a plain class with a __init__() for two reason:

• for people who are not familiar with namedtuples or dataclasses, it would be harder to learn two fairly complex topics at the same time. Docs for both are fairly extensive.

• for those who *are* familiar with namedtuples or dataclasses (whichever is used in the example), the example may imply they should be used with pattern matching, and that plain classes may not work or not work well.

I can make a PR that adds __init__ if this makes sense..

[akulakov]

Ahmet: once there's agreement on how to fix this, would you like to work on a patch?

[Eclips4]

We have a similar example in https://docs.python.org/3.12/tutorial/controlflow.html#match-statements, which is define the __init__ method. Let's make them similar.