Let SupportsDunderLT return SupportsBool instead of bool.

[randolf-scholz]

Fixes #12562, alternative to #12573. Introduces a new protocol in _typeshed/__init__.pyi

class SupportsBool(Protocol):
    def __bool__(self) -> bool: ...

Changes SupportsDunderLT and variants to return SupportsBool instead of bool.

Note that an earlier comment by JelleZijlstra suggested that SupportsBool is equivalent to object in practice. This does not seem to quite be the case: Code sample in pyright playground

from typing import Protocol, runtime_checkable

@runtime_checkable
class SupportsBool(Protocol):
    def __bool__(self) -> bool: ...

x: SupportsBool = True
y: SupportsBool = object()  # mypy: ❌ pyright:  ❌

assert isinstance(x, SupportsBool)  # ✅
assert not isinstance(y, SupportsBool)  # ✅

Indeed, despite the documentation mentioning object.__bool__, this method is not defined on object. Rather, the truthiness of an object is defined roughly like:

if hasattr(obj, "__bool__"):
    return obj.__bool__()
if hasattr(obj, "__len__"):
   return bool(len(obj))
return True

[jorenham]

Even though this technically doesn't cover all cases, I like this "explicit is better than implicit" approach, and I'm a big fan of structural typing solutions like these 👌🏻.

[randolf-scholz]

self is considered positional-only by type checkers as far as I am aware of python/typing#1355

…

On Fri, 1 Nov 2024, 15:45 Joren Hammudoglu, ***@***.***> wrote: Even though this technically doesn't cover all cases, I like this "explicit is better than implicit" approach, and I'm a big fan of structural typing solutions like these 👌🏻. — Reply to this email directly, view it on GitHub <#12939 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AJO3RGFNL23MLRPRRWMDBU3Z6OHY5AVCNFSM6AAAAABRAJIIOOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDINJRHE4TKNBYGU> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[jorenham]

@randolf-scholz ahh good to know 👍🏻

[randolf-scholz]

Ibis can easily fix the issue by adding __bool__ to the BooleanValue class (via Value.__ge__, the superclass of NumericValue)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

ibis (https://github.com/ibis-project/ibis)
- ibis/expr/types/relations.py:2924: error: Value of type variable "SupportsRichComparisonT" of "sorted" cannot be "NumericValue | float"  [type-var]

jax (https://github.com/google/jax)
+ jax/_src/pallas/mosaic/pipeline.py:981: error: Unused "type: ignore" comment  [unused-ignore]