Suprising enum difference in PyQt 6 from PyQt 5

[Florian Bruhin]

Hey,

On Sun, Apr 17, 2022 at 01:01:25PM +0100, Phil Thompson wrote:
> > I also think it would be good to be explicit about this matching in
> > code: Should a PartialMatch be true or false?. Or in your case, should
> > partially checked really count as checked?
> 
> Agreed. I tend to use 'is' when testing the value of enums rather than '=='.
> Also 'in' a tuple of multiple values.

Both are fine I suppose, and equivalent - but just an "if" without
either is problematic.

> > However, this is tricky, as it's a slient breakage, and those are rather
> > hard to spot... Maybe those kind of enums shuold instead have some
> > annotation in the .sip file which overrides their __bool__ to raise a
> > TypeError instead?
> 
> Wouldn't it apply to all enums that sub-class Enum (rather than IntEnum)? It
> surprises me that this isn't the default behaviour.

Looks like enum.Enum just doesn't implement __bool__ at all, and thus
all enum values are truthy:
https://docs.python.org/3/reference/datamodel.html#object.__bool__

There's a bit of information about it here:
https://docs.python.org/3/library/enum.html#boolean-value-of-enum-classes-and-members

> I'm happy to consider adding such a __bool__ method, however doing so
> departs from standard Python behaviour and the main motivation for moving to
> using Python enums in PyQt6 was to make it more Pythonic.

Right. It's arguably questionable whether Python should really default
__bool__ to return True even for things where it makes no sense for them
to be truthy or falsey.

Raising TypeError in __bool__ seems rather untypical, but also not
totally unheard of:

    >>> bool(pandas.NA)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "pandas/_libs/missing.pyx", line 382, in pandas._libs.missing.NAType.__bool__
    TypeError: boolean value of NA is ambiguous

Some discussion: https://github.com/pandas-dev/pandas/issues/38224

This definitely would be useful now that I'm porting things from PyQt5
to PyQt6 (to turn those into loud failures), but I'm not sure if it's a
good thing to have in general, as you say...

Florian

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