Suprising enum difference in PyQt 6 from PyQt 5

[Phil Thompson]

On 16/04/2022 18:38, Florian Bruhin wrote:
> Hi,
> 
> On Tue, Apr 12, 2022 at 07:02:19PM -0700, Greg Couch wrote:
>> So I have a QTreeWidgetItem, and in PyQt 5, I was able to use:
>> 
>> > if item.checkState(0):
>> to check if the item was checked or not.  The return value is a
>> QtCore.Qt.CheckState enum.   That fails in PyQt 6 because
>> QtCore.Qt.CheckState.Unchecked evaluates as True.  If 
>> QtCore.Qt.CheckState
>> were changed to subclass from enum.IntEnum, instead of enum.Enum, this 
>> would
>> not be a porting problem.
> 
> I see similar issues in other boolean-but-not-quite enums.
> 
> With PyQt 5:
> 
>     >>> full_match = QKeySequence.fromString("a,
> b").matches(QKeySequence.fromString("a, b"))
>     >>> partial_match =
> QKeySequence.fromString("a").matches(QKeySequence.fromString("a, b"))
>     >>> no_match =
> QKeySequence.fromString("x").matches(QKeySequence.fromString("a, b"))
>     >>> full_match, partial_match, no_match
>     (2, 1, 0)
>     >>> bool(full_match), bool(partial_match), bool(no_match)
>     (True, True, False)
> 
> but PyQt 6:
> 
>     >>> full_match, partial_match, no_match
>     (<SequenceMatch.ExactMatch: 2>, <SequenceMatch.PartialMatch: 1>,
> <SequenceMatch.NoMatch: 0>)
>     >>> bool(full_match), bool(partial_match), bool(no_match)
>     (True, True, True)
> 
> so a "if seq.matches(other):" now silently is true even for
> QKeySequence.SequenceMatch.PartialMatch.
> 
> I'm not sure if "just make it an IntEnum" is a suitable fix. I like the
> idea of having a strict separation between ints and enums, and a match
> (or a check state) clearly isn't a number (that's just an 
> implementation
> detail of old-style C++ enums essentially).

Agreed.

> 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.

> 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.

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.

Phil