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It is possible to create a 1-type union type #88802

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serhiy-storchaka opened this issue Jul 14, 2021 · 7 comments
Closed

It is possible to create a 1-type union type #88802

serhiy-storchaka opened this issue Jul 14, 2021 · 7 comments
Labels
3.10 only security fixes 3.11 only security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error

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@serhiy-storchaka
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BPO 44636
Nosy @gvanrossum, @serhiy-storchaka, @Fidget-Spinner
PRs
  • bpo-44636: Collapse union of equal types #27178
  • [3.10] bpo-44636: Collapse union of equal types (GH-27178) #27181
    		•
    Dependencies
  • bpo-44632: Union with TypeVar does not work as intended
    		•

    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 2021-07-16.11:49:32.820>
created_at = <Date 2021-07-14.11:04:42.002>
labels = ['interpreter-core', 'type-bug', '3.10', '3.11']
title = 'It is possible to create a 1-type union type'
updated_at = <Date 2021-07-16.11:49:32.820>
user = 'https://github.com/serhiy-storchaka'

    bugs.python.org fields:

    activity = <Date 2021-07-16.11:49:32.820>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2021-07-16.11:49:32.820>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2021-07-14.11:04:42.002>
creator = 'serhiy.storchaka'
dependencies = ['44632']
files = []
hgrepos = []
issue_num = 44636
keywords = ['patch']
message_count = 7.0
messages = ['397476', '397527', '397559', '397564', '397577', '397608', '397615']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'kj']
pr_nums = ['27178', '27181']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44636'
versions = ['Python 3.10', 'Python 3.11']
    @serhiy-storchaka
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    typing.Union always collapsed to a simple type if it contains a single type.

    >>> import typing
>>> typing.Union[int, typing.T][int]
<class 'int'>

    But the builtin union type can contain a single type:

    >>> (int | typing.T)[int]
int
>>> type((int | typing.T)[int])
<class 'types.Union'>

    @serhiy-storchaka serhiy-storchaka added 3.10 only security fixes 3.11 only security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error labels Jul 14, 2021
    @serhiy-storchaka
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    Simple example (without indexing):

    >>> import typing
>>> typing.Union[int, int]
<class 'int'>
>>> int | int
int

    @gvanrossum
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    I feel that keeping the singleton is more consistent. This normalization seems to me an example of the typing module doing too much. Singleton tuples are different from scalar values too.

    @serhiy-storchaka
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    It is a difference with typing.Union which can cause confusion. If the union type is like a tuple and we leave a 1-type union, why do we bother with deduplication? Why int | str | int is collapsed into int | str?

    Also it complicates the comparison implementation and produces surprising exceptions:

    >>> int | str == {}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'dict'

    Also it breaks one of fundamental properties -- equal objects should have equal hashes.

    >>> (int | int) == int
True
>>> hash(int | int) == hash(int)
False

    @gvanrossum
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    Hm, you are right. Make it so, Mr. Spock! :-)

    @serhiy-storchaka
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    New changeset d9f9232 by Serhiy Storchaka in branch 'main':
    bpo-44636: Collapse union of equal types (GH-27178)
    d9f9232

    @serhiy-storchaka
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    New changeset c3007ab by Serhiy Storchaka in branch '3.10':
    [3.10] bpo-44636: Collapse union of equal types (GH-27178) (GH-27181)
    c3007ab

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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