PEP 604 Union syntax does not support forward references

[TNThung]

BPO	45857
Nosy	@gvanrossum, @ericvsmith, @Fidget-Spinner, @AlexWaygood

^{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 = None
created_at = <Date 2021-11-21.14:48:31.479>
labels = ['type-bug', '3.10', '3.11']
title = 'PEP 604 Union syntax does not support forward references'
updated_at = <Date 2021-11-22.16:46:08.109>
user = 'https://bugs.python.org/TNThung'

bugs.python.org fields:

activity = <Date 2021-11-22.16:46:08.109>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-11-21.14:48:31.479>
creator = 'TNThung'
dependencies = []
files = []
hgrepos = []
issue_num = 45857
keywords = []
message_count = 6.0
messages = ['406720', '406724', '406726', '406731', '406775', '406792']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'eric.smith', 'kj', 'AlexWaygood', 'TNThung']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45857'
versions = ['Python 3.10', 'Python 3.11']

Linked PRs

• gh-90015: Document that PEP-604 unions do not support forward references #105366
• [3.12] gh-90015: Document that PEP-604 unions do not support forward references (GH-105366) #105460
• [3.11] gh-90015: Document that PEP-604 unions do not support forward references (GH-105366) #105461

[TNThung]

The class methods have a problem compiling when the type refers to the union of itself and others.

# tmp.py
class Foo:
    def __init__(self, tmp: "Foo"|int):
        pass

# Error
Traceback (most recent call last):
  File "/Project/Mslc/Grammar/tmp.py", line 1, in <module>
    class Foo:
  File "/Project/Mslc/Grammar/tmp.py", line 2, in Foo
    def __init__(self, tmp: "Foo"|int):
TypeError: unsupported operand type(s) for |: 'str' and 'type'

[AlexWaygood]

Reproduced on 3.11. The error occurs if a type is on the left-hand-side of the operand, as well as if a type is on the right-hand-side:

>>> int | "str"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'type' and 'str'
>>> "str" | int
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'str' and 'type'

[ericvsmith]

Presumably the correct way to do this is:

def __init__(self, tmp: "Foo|int"):

That is, the entire type hint is a string.