Python C++ Template Equivalent

[deanhystad]

I have the following code that adds a couple of features to QWidget and QGroupBox.

class View(QtWidgets.QWidget):
    """Add upload and update loop to QWidget"""

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.update_list = UpdateList(1.0)

    def showEvent(self, event):
        """Start update loop when window is shown"""
        self.upload()
        self.update_list.start()
        event.accept()

    def closeEvent(self, event):
        """Stop update loop when window is closed"""
        self.update_list.stop()
        event.accept()

    def upload(self):
        """Forward update message to all ValueControl objects"""
        if self.isVisible():
            for child in self.children():
                if isinstance(child, ValueControl):
                    child.update_value()


class GroupView(QtWidgets.QGroupBox):
    """Add upload and update loop to QBoxGroup"""

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.update_list = UpdateList(1.0)

    def showEvent(self, event):
        """Start update loop when window is shown"""
        self.upload()
        self.update_list.start()
        event.accept()

    def closeEvent(self, event):
        """Stop update loop when window is closed"""
        self.update_list.stop()
        event.accept()

    def upload(self):
        """Forward update message to all ValueControl objects"""
        if self.isVisible():
            for child in self.children():
                if isinstance(child, ValueControl):
                    child.update_value()

The code is identical except for the superclass. I would like to make a superclass that implements the common behavior, but openEvent(), closeEvent() and self.children() are all part of the new behavior and these methods are inherited from QWidget.

In C++ I would make a template. How would you do this in Python?

[Gribouillis]

You could perhaps write a Mixin class implementing the three methods.

[deanhystad]

I tried a mixin. It works, but the mixin provides only half the additional functionality because it really needs to act like a QWidget to get the show/close events.

class BaseView():
    """Abstract base class for views"""
    def __init__(self):
       self.update_list = UpdateList(1.0)

    def upload(self):
        """Forward update message to all ValueControl objects"""
        if self.isVisible():
            for child in self.children():
                if isinstance(child, ValueControl):
                    child.update_value()


class View(QtWidgets.QWidget, BaseView):
    """Add upload and update loop to QWidget"""

    def __init__(self, *args, **kwargs):
        QtWidgets.QWidget.__init__(self, *args, **kwargs)
        BaseView.__init__(self)

    def showEvent(self, event):
        """Start update loop when window is shown"""
        self.upload()
        self.update_list.start()
        event.accept()

    def closeEvent(self, event):
        """Stop update loop when window is closed"""
        self.update_list.stop()
        event.accept()

A benefit of this approach is giving View and GroupView a common superclass makes them easier to find using methods like QtWidget.findChildren(BaseView).

[Gribouillis]

Why dont you write showEvent() and closeEvent() in the BaseView class ? You could also call it ViewMixin to clarify its role. Also there are arguments for writing the Mixin classes before the normal parent classes in the sequence of base classes.

[deanhystad]

I would swear that I tried this before and it threw up all over me. Now it works fine.

class BaseView():
    """Abstract base class for views"""
    def __init__(self):
       self.update_list = UpdateList(1.0)

    def showEvent(self, event):
        """Start update loop when window is shown"""
        self.upload()
        self.update_list.start()
        event.accept()

    def closeEvent(self, event):
        """Stop update loop when window is closed"""
        self.update_list.stop()
        event.accept()

    def upload(self):
        """Forward update message to all ValueControl objects"""
        if self.isVisible():
            for child in self.children():
                if isinstance(child, ValueControl):
                    child.update_value()


class View(QtWidgets.QWidget, BaseView):
    """Add upload and update loop to QWidget"""

    def __init__(self, *args, **kwargs):
        BaseView.__init__(self)
        QtWidgets.QWidget.__init__(self, *args, **kwargs)

[deanhystad]

Actually it doesn't work fine. Moving showEvent() and closeEvent() to the mixin class results in not capturing these events.

[Gribouillis]

The problem is that I know almost nothing in PyQt5. Do you have the same issue if you write BaseView before QWidget in the list of base classes of View ?

You could also try something like

class View...
    closeEvent = BaseView.closeEvent

[deanhystad]

This discussion has been very educational. Thanks!

This works.

class BaseView():
    """Abstract base class for views"""
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.update_list = UpdateList(1.0)

    def showEvent(self, event):
        """Start update loop when window is shown"""
        self.upload()
        self.update_list.start()
        event.accept()

    def closeEvent(self, event):
        """Stop update loop when window is closed"""
        self.update_list.stop()
        event.accept()

    def upload(self):
        """Forward update message to all ValueControl objects"""
        print(self)
        if self.isVisible():
            for child in self.children():
                if isinstance(child, ValueControl):
                    child.update_value()

class View(BaseView, QtWidgets.QWidget):
    """Add upload and update loop to QWidget"""

class GroupView(BaseView, QtWidgets.QGroupBox):
    """Add upload and update loop to QBoxGroup"""