Extending QML (advanced) - Inheritance and Coercion

This is the second of a series of 6 examples forming a tutorial using the example of a birthday party to demonstrate some of the advanced features of QML.

Right now, each attendant is being modelled as a person. This is a bit too generic and it would be nice to be able to know more about the attendees. By specializing them as boys and girls, we can already get a better idea of who’s coming.

To do this, the Boy and Girl classes are introduced, both inheriting from Person.

@QmlElement
class Boy(Person):
    def __init__(self, parent=None):

@QmlElement
class Girl(Person):
    def __init__(self, parent=None):

The Person class remains unaltered and the Boy and Girl classes are trivial extensions of it. The types and their QML name are registered with the QML engine with @QmlElement.

Notice that the host and guests properties in BirthdayParty still take instances of Person.

The implementation of the Person class itself has not been changed. However, as the Person class was repurposed as a common base for Boy and Girl, Person should no longer be instantiable from QML directly. An explicit Boy or Girl should be instantiated instead.

@QmlElement
@QmlUncreatable("Person is an abstract base class.")

While we want to disallow instantiating Person from within QML, it still needs to be registered with the QML engine so that it can be used as a property type and other types can be coerced to it. This is what @QmlUncreatable does. As all three types, Person, Boy and Girl, have been registered with the QML system, on assignment, QML automatically (and type-safely) converts the Boy and Girl objects into a Person.

With these changes in place, we can now specify the birthday party with the extra information about the attendees as follows.

BirthdayParty {
    host: Boy {
        name: "Bob Jones"
        shoe_size: 12
    }
    guests: [
        Boy { name: "Leo Hodges" },
        Boy { name: "Jack Smith" },
        Girl { name: "Anne Brown" }
    ]
}

Download this example

main.py

# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
from __future__ import annotations

"""PySide6 port of the
   qml/examples/qml/tutorials/extending-qml-advanced/advanced2-Inheritance-and-coercion example
   from Qt v6.x"""

from pathlib import Path
import sys

from PySide6.QtCore import QCoreApplication
from PySide6.QtQml import QQmlComponent, QQmlEngine

from person import Boy, Girl  # noqa: F401
from birthdayparty import BirthdayParty  # noqa: F401


app = QCoreApplication(sys.argv)
engine = QQmlEngine()
engine.addImportPath(Path(__file__).parent)
component = QQmlComponent(engine)
component.loadFromModule("People", "Main")
party = component.create()
if not party:
    print(component.errors())
    del engine
    sys.exit(-1)
host = party.host
print(f"{host.name} is having a birthday!")
if isinstance(host, Boy):
    print("He is inviting:")
else:
    print("She is inviting:")
for g in range(party.guestCount()):
    name = party.guest(g).name
    print(f"    {name}")
del engine
sys.exit(0)

birthdayparty.py

# Copyright (C) 2023 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
from __future__ import annotations

from PySide6.QtCore import QObject, Property, Signal
from PySide6.QtQml import QmlElement, ListProperty

from person import Person


# To be used on the @QmlElement decorator
# (QML_IMPORT_MINOR_VERSION is optional)
QML_IMPORT_NAME = "People"
QML_IMPORT_MAJOR_VERSION = 1


@QmlElement
class BirthdayParty(QObject):
    host_changed = Signal()
    guests_changed = Signal()

    def __init__(self, parent=None):
        super().__init__(parent)
        self._host = None
        self._guests = []

    @Property(Person, notify=host_changed, final=True)
    def host(self):
        return self._host

    @host.setter
    def host(self, h):
        if self._host != h:
            self._host = h
            self.host_changed.emit()

    def guest(self, n):
        return self._guests[n]

    def guestCount(self):
        return len(self._guests)

    def appendGuest(self, guest):
        self._guests.append(guest)
        self.guests_changed.emit()

    guests = ListProperty(Person, appendGuest, notify=guests_changed, final=True)

person.py

# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
from __future__ import annotations

from PySide6.QtCore import QObject, Property, Signal
from PySide6.QtQml import QmlElement, QmlUncreatable

# To be used on the @QmlElement decorator
# (QML_IMPORT_MINOR_VERSION is optional)
QML_IMPORT_NAME = "People"
QML_IMPORT_MAJOR_VERSION = 1


@QmlElement
@QmlUncreatable("Person is an abstract base class.")
class Person(QObject):
    name_changed = Signal()
    shoe_size_changed = Signal()

    def __init__(self, parent=None):
        super().__init__(parent)
        self._name = ''
        self._shoe_size = 0

    @Property(str, notify=name_changed, final=True)
    def name(self):
        return self._name

    @name.setter
    def name(self, n):
        if self._name != n:
            self._name = n
            self.name_changed.emit()

    @Property(int, notify=shoe_size_changed, final=True)
    def shoe_size(self):
        return self._shoe_size

    @shoe_size.setter
    def shoe_size(self, s):
        self._shoe_size = s


@QmlElement
class Boy(Person):
    def __init__(self, parent=None):
        super().__init__(parent)


@QmlElement
class Girl(Person):
    def __init__(self, parent=None):
        super().__init__(parent)

People/Main.qml

// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

import People

BirthdayParty {
    host: Boy {
        name: "Bob Jones"
        shoe_size: 12
    }
    guests: [
        Boy { name: "Leo Hodges" },
        Boy { name: "Jack Smith" },
        Girl { name: "Anne Brown" }
    ]
}

People/qmldir

module People
typeinfo coercion.qmltypes
Main 1.0 Main.qml