NoReturn and Never are underspecified in the spec
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Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 25/100
- Issue type
- Documentation
- Clarity
- Needs clarification
- Activity status
- Stale
- Tech stack
- python
- Domain
- documentation
Research direction
Start with the Never example in this issue and review the linked subtyping discussion and ongoing Python typing formalization work. Done means reaching and documenting a clear specification for Never and NoReturn, including their interaction with structural subtyping and the LSP case described here.
Written by the indexing model from the issue text.
Description
Never was added without a PEP as a way to directly indicate that something is uninhabited in 3.11
This brings up something from https://github.com/CarliJoy/intersection_examples/issues/5#issuecomment-1699490563 where we now have a way to express what appears to be an LSP violation that type checks (incorrectly) properly via structural subtyping by mishandling Never as a subtype of other types. Never is not a subtype of all other types. As the bottom type, it is uninhabited and does not follow subtyping rules.
from typing import Never
class A:
def foo(self, x: int) -> None: ...
class B(A):
foo: Never # __getattr__(B, "foo") -> raise NotImplementedError
x: A = B() # type checks
reveal_type(B.foo)
https://mypy-play.net/?mypy=latest&python=3.11&flags=strict&gist=a4279b36d82c1a28d7b17be9a4bbcbdf
I believe under LSP, B is no longer a safe drop-in replacement for A. It's important to note that ongoing work to formalize the type system, including Never does not treat Never as a subtype of all other types, see the ongoing work here: http://bit.ly/python-subtyping
- Dominant language
- Python
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- Merged PRs (30d)
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