GenericAlias does not support union type expressions

[Fidget-Spinner]

BPO	42233
Nosy	@gvanrossum, @serhiy-storchaka, @ilevkivskyi, @miss-islington, @Fidget-Spinner
PRs	bpo-42233: Add union type expression support for GenericAlias and fix de-duplicating of GenericAlias #23077 bpo-42233: Correctly repr GenericAlias when used with typing module #23081 [3.9] bpo-42233: Correctly repr GenericAlias when used with typing module (GH-23081) #23082

^{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 2020-11-09.04:11:33.989>
created_at = <Date 2020-11-01.15:32:10.464>
labels = ['interpreter-core', '3.10']
title = 'GenericAlias does not support union type expressions'
updated_at = <Date 2021-11-21.10:53:44.595>
user = 'https://github.com/Fidget-Spinner'

bugs.python.org fields:

activity = <Date 2021-11-21.10:53:44.595>
actor = 'nemeskeyd'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-09.04:11:33.989>
closer = 'kj'
components = ['Interpreter Core']
creation = <Date 2020-11-01.15:32:10.464>
creator = 'kj'
dependencies = []
files = []
hgrepos = []
issue_num = 42233
keywords = ['patch']
message_count = 9.0
messages = ['380145', '380147', '380150', '380157', '380512', '380571', '406576', '406589', '406709']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'nemeskeyd', 'levkivskyi', 'miss-islington', 'kj']
pr_nums = ['23077', '23081', '23082']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue42233'
versions = ['Python 3.10']

[Fidget-Spinner]

Union type expressions added in PEP-604 throw an error when both operands are GenericAlias objects.

Eg the following works::

int | list[str]

The following throws TypeError::

list[int] | dict[float, str]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'types.GenericAlias' and 'types.GenericAlias'

I have submitted a PR to fix this. Coincidentally, it also fixes the fact that union expressions weren't de-duplicating GenericAlias properly.

Eg::
>>> list[int] | int | list[int]
list[int] | int | list[int]

For non-GenericAlias type expressions, the new code shouldn't be much slower. Rich compare is only used for GenericAlias objects. This isn't very scientific, but

python -m timeit "int | str | float"

# original
1000000 loops, best of 5: 295 nsec per loop

# purely rich compare
1000000 loops, best of 5: 344 nsec per loop

# check for GenericAlias and rich compare only for that
1000000 loops, best of 5: 297 nsec per loop

[serhiy-storchaka]

There is also a problem with typing module.

>>> typing.List[int] | dict[float, str]
typing.Union[typing.List[int], dict]

[Fidget-Spinner]

@serhiy, wow interesting find, it seems to be typing's repr problem rather than the actual types itself:

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

>>> typing.Union[dict[int, str], list[str]].__args__
(dict[int, str], list[str])

The __args__ seem to be correct, so I'm guessing the typing repr went wrong somewhere. That should be the case for your example too:

>>> alias = typing.List[int] | dict[float, str]
>>> alias
typing.Union[typing.List[int], dict]

>>> type(alias)
<class 'typing._UnionGenericAlias'>

>>> alias.__args__
(typing.List[int], dict[float, str])

I'll work on this. If I don't reply back in a week, someone else is free to take over this issue.

[miss-islington]

New changeset 1f7dfb2 by kj in branch 'master':
bpo-42233: Correctly repr GenericAlias when used with typing module (GH-23081)
1f7dfb2

[miss-islington]

New changeset e81e09b by Miss Islington (bot) in branch '3.9':
bpo-42233: Correctly repr GenericAlias when used with typing module (GH-23081)
e81e09b

[gvanrossum]

New changeset 4eb41d0 by kj in branch 'master':
bpo-42233: Add union type expression support for GenericAlias and fix de-duplicating of GenericAlias (GH-23077)
4eb41d0

[nemeskeyd]

Guys, any chance the fix will land in 3.9? It is affected as well.

[Fidget-Spinner]

@dávid

No it won't land in 3.9. Union type expressions (PEP-604) were only officially added in 3.10. We don't backport new features, only bugfixes. Sorry!

[nemeskeyd]

@kj Sorry, for some reason, I thought the that issue affected Union as well, but I have just re-tested it and found that it works.