Preserve natural order of args in the union type

[serhiy-storchaka]

BPO	44652
Nosy	@gvanrossum, @serhiy-storchaka, @miss-islington, @Fidget-Spinner
PRs	bpo-44652: Preserve natural order of args in the union type. #27185 [3.10] bpo-44652: Preserve natural order of args in the union type. (GH-27185) #27190

^{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 = None
closed_at = <Date 2021-07-16.13:43:52.697>
created_at = <Date 2021-07-16.11:07:49.662>
labels = ['interpreter-core', 'type-bug', 'library', '3.10', '3.11']
title = 'Preserve natural order of args in the union type'
updated_at = <Date 2021-07-16.13:43:52.697>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2021-07-16.13:43:52.697>
actor = 'kj'
assignee = 'none'
closed = True
closed_date = <Date 2021-07-16.13:43:52.697>
closer = 'kj'
components = ['Interpreter Core', 'Library (Lib)']
creation = <Date 2021-07-16.11:07:49.662>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 44652
keywords = ['patch']
message_count = 3.0
messages = ['397612', '397618', '397628']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'miss-islington', 'kj']
pr_nums = ['27185', '27190']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44652'
versions = ['Python 3.10', 'Python 3.11']

[serhiy-storchaka]

There are two issues related to the order of __args__ in typing.Union and the union type.

1. Indexing typing.Union preserves the order of arguments, but it is not always true when use the | operator.

>>> A = typing.NewType('A', str)
>>> typing.Union[typing.List[int], A]
typing.Union[typing.List[int], A]
>>> typing.Union[A, typing.List[int]]
typing.Union[A, typing.List[int]]
>>> typing.List[int] | A
typing.Union[typing.List[int], A]
>>> A | typing.List[int]
typing.Union[typing.List[int], A]

The cause is errors in __ror__ implementations.

2. There is a difference between deduplication algorithms for typing.Union and the union type.

>>> typing.Union[int, str, int]
typing.Union[int, str]
>>> int | str | int
str | int

It is not particularly important, because the order of __args__ mostly affects only representation. But it is better to be consistent, and it is easy to fix these tiny issues.

Note that it does not make the order of __args__ deterministic in all cases. Due to using caching for typing.Union it can be not always preserved if we merger typing.Union objects which differs only by order of arguments.

>>> import typing
>>> typing.Union[typing.Union[int, str], list]
typing.Union[int, str, list]
>>> typing.Union[typing.Union[str, int], list]
typing.Union[int, str, list]

[serhiy-storchaka]

New changeset 0cd2d51 by Serhiy Storchaka in branch 'main':
bpo-44652: Preserve natural order of args in the union type. (GH-27185)
0cd2d51

[miss-islington]

New changeset 80844d1 by Serhiy Storchaka in branch '3.10':
[3.10] bpo-44652: Preserve natural order of args in the union type. (GH-27185) (GH-27190)
80844d1