QGestureRecognizer.registerRecognizer() crashes using PySide2

nutrx

I'm trying to register custom gestures in a QGraphicsView using PySide2. Calling

        pan_gesture_id = QGestureRecognizer.registerRecognizer(recognizer)

results in a crash without any error. I couldn't find anything in the docs specifying the process of registering a gesture recognizer using PySide2. The following example reproduces the crash:

class View(QGraphicsView):
    def __init__(self):
        super().__init__()
        scene = QGraphicsScene(self)
        scene.setSceneRect(0, 0, self.width(), self.height())
        self.setScene(scene)

        recognizer = PanGestureRecognizer()
        pan_gesture_id = QGestureRecognizer.registerRecognizer(recognizer)
        self.grabGesture(pan_gesture_id)

am I missing anything there?

JonB

@nutrx
If you have PyQt5 installed/available, the usual is to try it there. If same code works, blame PySide2....

SGaist

Hi,

Can you provide the full script you use to trigger the crash ?

By the way:

which version of PySide2 are you using ?
which version of Python ?
on which OS ?
how did you install PySide2 ?

nutrx

@SGaist sorry, should've added that

from PySide2.QtWidgets import QMainWindow, QApplication, QGraphicsView, QGraphicsScene, QGestureRecognizer, QGesture
from PySide2.QtGui import QTouchEvent
from PySide2.QtCore import QEvent


class PanGesture(QGesture):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.pos = None
        self.press = None
        self.last = None

class PanGestureRecognizer(QGestureRecognizer):
    """Not complete, just some test"""
    def create(self, target) -> QGesture:
        return PanGesture()

    def reset(self, state: QGesture) -> None:
        pass

    def recognize(self, state: QGesture, watched, event: QEvent) -> QGestureRecognizer.Result:
        if event.type() == QEvent.TouchBegin:
            touch_event: QTouchEvent = event
            if len(touch_event.touchPoints()) == 1:
                state.press = touch_event.touchPoints()[0]
                state.pos = state.press
                return QGestureRecognizer.MayBeGesture
        elif event.type() == QEvent.TouchUpdate:
            touch_event: QTouchEvent = event
            state.last = state.pos
            state.pos = touch_event.touchPoints()[0]
            return QGestureRecognizer.TriggerGesture

class View(QGraphicsView):
    def __init__(self):
        super().__init__()

        scene = QGraphicsScene(self)
        scene.setSceneRect(0, 0, self.width(), self.height())
        self.setScene(scene)

        recognizer = PanGestureRecognizer()
        pan_gesture_id = QGestureRecognizer.registerRecognizer(recognizer)
        self.grabGesture(pan_gesture_id)

class ViewWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setCentralWidget(View())

if __name__ == '__main__':
    app = QApplication()
    widget = ViewWindow()
    widget.show()
    app.exec_()

see View's constructor.

PySide2 5.15.2, installed via pip, inside python virtual environment
Python 3.9.0
Windows 10

the GestureRecognizer probably doesn't work, I had just quickly put that together for some tests, but since I don't see any runtime error, I assume it's not the class custom GestureRecognizer what's causing the crash

jeremy_k

After converting the example to run with PyQt5:

$ python3 main.py 
TypeError: invalid result from PanGestureRecognizer.recognize(), NoneType cannot be converted to PyQt5.QtWidgets.Result in this context
Abort trap: 6

PanGestureRecognizer.recognize is falling off the end and effectively returning None for anything other than a TouchBegin or TouchUpdate.

nutrx

@jeremy_k yes absolutely, I didn't pay attention to that method, because it still fails if I ignore otherwise, or everything from the beginning

    def recognize(self, state: QGesture, watched, event: QEvent) -> QGestureRecognizer.Result:
        return QGestureRecognizer.Ignore

or in the example above:

    def recognize(self, state: QGesture, watched, event: QEvent) -> QGestureRecognizer.Result:
        if event.type() == QEvent.TouchBegin:
            touch_event: QTouchEvent = event
            if len(touch_event.touchPoints()) == 1:
                state.press = touch_event.touchPoints()[0]
                state.pos = state.press
                return QGestureRecognizer.MayBeGesture
        elif event.type() == QEvent.TouchUpdate:
            touch_event: QTouchEvent = event
            state.last = state.pos
            state.pos = touch_event.touchPoints()[0]
            return QGestureRecognizer.TriggerGesture
        return QGestureRecognizer.Ignore

Shouldn't that be sufficient?

jeremy_k

@nutrx said in QGestureRecognizer.registerRecognizer() crashes using PySide2:

@jeremy_k yes absolutely, I didn't pay attention to that method, because it still fails if I ignore otherwise, or everything from the beginning
    def recognize(self, state: QGesture, watched, event: QEvent) -> QGestureRecognizer.Result:
        return QGestureRecognizer.Ignore

...

Shouldn't that be sufficient?

I'm not sure what you mean here. With the addition of the default Ignore, the application displays a window and doesn't do anything noteworthy in my environment. Are you still experiencing a crash?

nutrx

@jeremy_k yes, exactly. I just tried to get it to run, so I could move on from there. But it still crashes the same as before. Sorry, I don't have PyQt set up rn

jeremy_k

PanGestureRecognizer doesn't have an __init__ to call QGestureRecognizer.__init__. I vaguely recall reports of misbehavior using PyQt if a Qt object's constructor isn't invoked. This may have been limited to QObjects. It might not apply to PySide. Either way, it's an inexpensive experiment to run. QGestureRecognizer's C++ constructor is empty, but there may be some bookkeeping on the Python side.

nutrx

@jeremy_k you mean, I should try just calling QGestureRecognizer's __init__ in PanGestureRecognizer.__init__? I would've guessed that's ambiguous, or does instanciation lead to a direct call of the C++ parent constructor instead of the parent's __init__ in Python? However, I just tried, still no luck :/

jeremy_k

class PanGestureRecognizer(QGestureRecognizer):
    def __init__(self):
        super().__init__()

This doesn't seem to be necessary with pure python classes. The wording of object.__init__ led me to believe that it was, but a test suggests that the base class __init__ is called if no derived __init__ is defined.

JonB

@jeremy_k
Well, in Python just like in, say, C++, if you do not define your own __init__() (constructor in C++) the language will always cause the base class's __init__()/constructor. If you do define your own, you must then explicitly call the base class __init__()~~/constructor, in either language.~~ in Python.

[EDIT As @jeremy_k pointed out below, I was wrong to say this applies in all C++ cases. Let's leave it as: in Python, if you define any derived __init__() you must (should) explicitly call the base __init__() from there; but if you do not define a derived __init__() the base one will still be called.]

I vaguely recall reports of misbehavior using PyQt if a Qt object's constructor isn't invoked

I don't know if somehow PySide2/PyQt requires you to explicitly define your own __init__() and call the base, unless you have some reference for that?

jeremy_k

@JonB said in QGestureRecognizer.registerRecognizer() crashes using PySide2:

@jeremy_k
Well, in Python just like in, say, C++, if you do not define your own __init__() (constructor in C++) the language will always cause the base class's __init__()/constructor. If you do define your own, you must then explicitly call the base class __init__()/constructor, in either language.

That's inaccurate for C++ base class default constructors. The compiler will insert a call to the base constructor if the programmer does not. Explicit invocation is only required if there are arguments to pass.

JonB

@jeremy_k
You are quite correct, and thank you for pointing this out. Partly I was unconsciously thinking of constructors with parameters, and partly I make slips as I swap between C#/C++/Python/JavaScript! I will correct my previous post, so as not to mislead.

SGaist

You may have found a bug in the PySide2 implementation. Did you check if you have the same behaviour with PySide6 ?

nutrx

@SGaist yes, same with PySide6

SGaist

Then you should go to the bug report system and see if there's already something there. If not, please consider opening a new issue providing your minimal runnable script that shows that behaviour.

nutrx

Yeah, just found a bugreport reporting this exact issue. It's open since 2015..........
I am really confused. I mean ... isn't this making the whole custom gesture recognizer system completely unsuable?

SGaist

It's an issue on Python and it has likely not been used that much with that language. You can add a comment on the issue to remind the current assigned person that this is still not solved.