Comparisons between QtWidgets and Qml | Python Gui - Graphical User Interfaces in Python with PyQt

In this video, we are going to do a comparison between QtWidgets and Qml.

First, let's get to know about what is Qml and QtWidgets.
* QML (Qt Modeling Language) - is a user interface markup language. It is a declarative language (similar to CSS and JSON) for designing user interface–centric applications.

* The Qt Widgets module provides a set of UI elements to create classic desktop-style user interfaces.

Why switch to QML from QWidget?

These are the possible reasons:

1. QML uses native platform-style whereas QWidget fails to have native look and feel on some platforms (iOS)

2. QML more strongly follows the ModelViewController paradigm

3. QML is declarative (more compact and dense)

4. A QML GUI is easier to change (to provide multiple or improved GUI’s for the same business model)

A strategy for  developing (code changes) to switch to QML from QWidget

Assuming your app using QWidgets already runs and has tests, a good strategy is to keep the QWidget implementation of the View working, while you develop the QML implementation of the View.   In other words, use test-driven development: make small changes, then test the QWidget implementation AND the QML implementation.  The QWidget implementation should always remain fully functional after each change.  The QML implementation will also always work, but in a stubbed-out, minimal fashion.

Changing the existing QWidget app will largely entail:

1. splitting classes along model/view lines

2. converting procedure calls to signals, connections, and properties

Swapping in the QML implementation of the view will be just a small change in the startup code: creating QML instead of QWidgets.  (Its a little more complicated than that, you might be making connections in the QML instead of in the main app.)

PyQt and QML

PyQt fully supports QML.  You can use Python and PyQt for your model.

For the same reason you use QML for the view, you should consider Python for the model (more expressive in fewer lines of code.)

PyQt and QML uses two interpreters

A PyQt app using QML uses two interpreters: the Python interpreter and QML’s Javascript interpreter.  (And on Android, there is a third Java interpreter.)  There is much discussion about the safety of, and constraints applied by app stores, re interpreters.   But this is just a curiosity, there are already shipping apps, using interpreters, in the stores.

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