[issue46669] Add types.Self

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> Your suggested signature looks like it's trying to support
> the second invariant, but it doesn't quite: if the types 
> don't match, the type checker will just set T to the 
> common base type of the two arguments.

Is there a way to write the second invariant correctly?

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[issue46669] Add types.Self

[Jelle Zijlstra]

Jelle Zijlstra  added the comment:

The conventional way is to write

def __exit__(
self,
__typ: type[BaseException] | None,
__exc: BaseException | None,
__tb: types.TracebackType | None,
) -> None: ...

But there are two invariants about how __exit__ works in practice that this 
doesn't capture:
1. The arguments are either all None, or none of them are
2. If they are not None, then the typ argument is the type of the exception 
passed to the exc argument

I believe these invariants are always true in 3.11 thanks to Irit's work, but 
previously there could be rare circumstances where they didn't hold. You 
probably know the details better than I do.

We could support the first invariant by using two overloads, one where the 
arguments are all None and one where they aren't. I don't think that's 
generally worth doing though: it's a lot of verbose code and doesn't fix many 
real problems.

Your suggested signature looks like it's trying to support the second 
invariant, but it doesn't quite: if the types don't match, the type checker 
will just set T to the common base type of the two arguments.

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[issue46669] Add types.Self

[Raymond Hettinger]

Change by Raymond Hettinger :


--
resolution:  -> duplicate
stage:  -> resolved
status: open -> closed
superseder:  -> Implementing PEP 673 (Self type)

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[issue46669] Add types.Self

[Raymond Hettinger]

Raymond Hettinger  added the comment:

On a related note, is this the correct way to annotate __exit__?


Exc = TypeVar('Exc', bound=Exception)

def __exit__(self, exctype: Optional[Type[Exc]], excinst: Optional[Exc], exctb: 
Any) -> None:

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[issue46669] Add types.Self

[Jelle Zijlstra]

Jelle Zijlstra  added the comment:

PEP 673 (which was accepted) adds typing.Self already. bpo-46534 tracks 
implementing it.

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[issue46669] Add types.Self

[Raymond Hettinger]

New submission from Raymond Hettinger :

Typeshed now has a nice self-describing type variable to annotate context 
managers:

Self = TypeVar('Self')

def __enter__(self: Self) -> Self:
return self

It would be nice to have that in the standard library types module as well.

--
messages: 412682
nosy: Jelle Zijlstra, gvanrossum, rhettinger
priority: normal
severity: normal
status: open
title: Add types.Self
versions: Python 3.11

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