[PyQt] Qt/PyQt 5.9.x. Correct way to get an "enumeration flags" to return 0?

[Phil Thompson]

On 11/07/2019 16:32, J Barchan wrote:
> On Thu, 11 Jul 2019 at 16:07, Phil Thompson 
> <phil at riverbankcomputing.com>
> wrote:
> 
>> On 11/07/2019 15:59, J Barchan wrote:
>> > On Thu, 11 Jul 2019 at 15:10, Phil Thompson
>> > <phil at riverbankcomputing.com>
>> > wrote:
>> >
>> >> On 11/07/2019 07:56, J Barchan wrote:
>> >> > On Wed, 10 Jul 2019 at 21:28, Barry Scott <barry at barrys-emacs.org>
>> >> > wrote:
>> >> >
>> >> >>
>> >> >>
>> >> >> On 10 Jul 2019, at 11:55, J Barchan <jnbarchan at gmail.com> wrote:
>> >> >>
>> >> >> If I return QOrientations() that works, but I'm unsure if that
>> >> >> guarantees
>> >> >> to return zero.
>> >> >>
>> >> >>
>> >> >> I've not tried this - not ready to update Qt yet - does this print 0
>> >> >> to
>> >> >> confirm the value in the enum?
>> >> >>
>> >> >> print( int( QOrientations())
>> >> >>
>> >> >> Barry
>> >> >>
>> >> >>
>> >> > I confirm that int( QOrientations() ) returns 0.  Assuming that is
>> >> > consistent/deliberate and not random (which I imagine it is) I guess
>> >> > that
>> >> > is what I will use here.
>> >> >
>> >> > I am still a little troubled by the fact that I can convert enum to
>> int
>> >> > via
>> >> > int(QOrientations(enum_value)) but have trouble converting int to enum
>> >> > via
>> >> > QOrientations(int_value).  However the latter does seem to be a
>> PyCharm
>> >> > IDE
>> >> > warning ("Unexpected type 'int', expected 'Orientations',
>> >> > 'Orientation'")
>> >> > but works at run-time, so I guess it's OK even if I cannot find a
>> >> > formula
>> >> > which works without warning.  It would be nice to see a PyQt expert
>> >> > statement on what is the correct way to handle this not-unusual
>> >> > requirement....
>> >>
>> >> As you say the warning is coming from your IDE and I have no idea what
>> >> introspection it is doing to determine exactly what types are allowed.
>> >> As Orientation is an enum then an int is also allowed.
>> >>
>> >> Phil
>> >>
>> >
>> > Hi Phil,
>> >
>> > You wrote:
>> >
>> > As Orientation is an enum then an int is also allowed.
>> >
>> >
>> >  Yes, Orientation is an enum, so Orientation(0) is an acceptable
>> > construct
>> > for that.  But the function returns an Orientations, not an
>> > Orientation.
>> > Note the spelling (the extra "s" on the end), it's vital.  Orientations
>> > is
>> > a *flags*, not an *enum*.  The IDE accepts Orientation(0) (an enum) but
>> > not
>> > Orientations(0) (a flags).  For Orientations it only accepts one or
>> > more
>> > Orientation joined together.
>> >
>> > That is why in my latest post a while ago, which you don't seem to be
>> > replying to, I said I seemed to have cracked it: Orientations(0)
>> > offends
>> > the IDE, but Orientations(Orientation(0)) works good!
>> 
>> Yes, I know. As we both said, that warning is coming from the IDE.
>> Orientations(0) is fine as far as PyQt is concerned (because passing
>> Orientation is Ok and, therefore, passing 0 is Ok).
>> 
>> Phil
>> 
> 
> Hi Phill,
> 
> Yes, I see, thanks.
> 
> It's a touch confusing (to me).  I have an answer which works for me 
> now,
> so the following is an academic question not a show-stopper.
> 
> You made the point of saying:
> 
> because passing Orientation is Ok and, therefore, passing 0 is Ok
> 
> 
> which implies that 0 is a valid Orientation.

The type is valid, it doesn't follow that the value is valid.

> The C++ code for the function
> returning an Orientations just goes return 0.  When I tried that first 
> from
> Python/PyQt I got a *run-time* error (so nothing to do with IDE) 
> stating
> something like "0 is an int and cannot be converted/used as a
> Orientations".  I guess this is because at some level the C++ enum 
> stuff is
> just treated as an int, but in Python/PyQt Orientation/Orientations are
> implemented as some kind of *class*, and that's different enough to 
> cause
> the run-time failure, is that the reason?

Pretty much.

Phil