PyOtherSide Developer Guide

PyOtherSide is a Qt 5 QML Plugin that provides access to a Python 3 interpreter from QML. It was designed with mobile devices in mind, where high-framerate touch interfaces are common, and where the user usually interfaces only with one application at a time via a touchscreen. As such, it is important to never block the UI thread, so that the user can always continue to use the interface, even when the backend is processing, downloading or calculating something in the background.

At its core, PyOtherSide is basically a simple layer that converts Qt (QML) objects to Python objects and vice versa, with focus on asynchronous events and continuation-passing style function calls.

While PyOtherSide once worked with Qt 4.x, and still works with Python 2.7, its focus now lies on Python 3.x and Qt 5. Python 3 has been out for several years, and offers some nice language features and clean-ups, while Qt 5 supports most mobile platforms well, and has an improved QML engine and a faster renderer (Qt Scene Graph) compared to Qt 4.


This section describes the QML API exposed by the PyOtherSide QML Plugin.

Import Versions

The current QML API version of PyOtherSide is 1.0. When new features are introduced, the API version will be bumped and documented here.

io.thp.pyotherside 1.0

  • Initial API release.

QML Python Element

The Python element exposes a Python interpreter in a QML file. In PyOtherSide 1.0, if multiple Python elements are instantiated, they will share the same underlying Python interpreter, so Python module-global state will be shared between all Python elements.

To use the Python element in a QML file, you have to import the plugin using:

import io.thp.pyotherside 1.0


received(var data)

Default event handler for pyotherside.send() if no other event handler was set.

error(string traceback)

Error handler for errors from Python.


To configure event handlers for events from Python, you can use the setHandler() method:

setHandler(string event, callable callback)

Set the handler for events sent with pyotherside.send().

Importing modules is then done by optionally adding an import path and then importing the module asynchronously:

addImportPath(string path)

Add a local filesystem path to Python’s sys.path.

Changed in version 1.1.0: addImportPath() will automatically strip a leading file:// from the path, so you can use Qt.resolvedUrl() without having to manually strip the leading file:// in QML.

importModule(string name, callable callback)

Import a Python module.

Once modules are imported, Python function can be called on the imported modules using:

call(string func, args=[], function callback(result) {})

Call the Python function func with args asynchronously. If args is omitted, func will be called without arguments. If callback is a callable, it will be called with the Python function result as single argument when the call has succeeded.

For some of these methods, there also exist synchronous variants, but it is highly recommended to use the asynchronous variants instead to avoid blocking the QML UI thread:

evaluate(string expr) → var

Evaluate a Python expression synchronously.

importModule_sync(string name) → bool

Import a Python module. Returns true on success, false otherwise.

call_sync(string func, var args=[]) → var

Call a Python function. Returns the return value of the Python function.

The following functions allow access to the version of the running PyOtherSide plugin and Python interpreter.

pluginVersion() → string

Get the version of the PyOtherSide plugin that is currently used.

New in version 1.1.0.

pythonVersion() → string

Get the version of the Python interpreter that is currently used.

New in version 1.1.0.

Python API

PyOtherSide uses a normal Python 3.x interpreter for running your Python code.

The pyotherside module

When a module is imported in PyOtherSide, it will have access to a special module called pyotherside in addition to all Python Standard Library modules and Python modules in sys.path:

import pyotherside

The module can be used to send events asynchronously (even from different threads) to the QML layer, register a callback for doing clean-ups at application exit and integrate with other QML-specific features of PyOtherSide.


pyotherside.send(event, *args)

Send an asynchronous event with name event with optional arguments args to QML.


Register a callback to be called when the application is closing.


Set the QML image provider (image://python/).

New in version 1.1.0.


New in version 1.1.0.

These constants are used in the return value of a image provider function:

Mono pixel format (QImage::Format_Mono).
Mono pixel format, LSB alignment (QImage::Format_MonoLSB).
32-bit RGB format (QImage::Format_RGB32).
32-bit ARGB format (QImage::Format_ARGB32).
16-bit RGB format (QImage::Format_RGB16).
18bpp RGB666 format (QImage::Format_RGB666).
15bpp RGB555 format (QImage::Format_RGB555).
24-bit RGB format (QImage::Format_RGB888).
12bpp RGB format (QImage::Format_RGB444).
Encoded image file data (e.g. PNG/JPEG data).

Data Type Mapping

PyOtherSide will automatically convert Python data types to Qt data types (which in turn will be converted to QML data types by the QML engine). The following data types are supported and can be used to pass data between Python and QML (and vice versa):

Python QML Remarks
bool bool  
int int  
float double  
str string  
list JS Array  
tuple JS Array JS Arrays are converted to lists, not tuples
dict JS Object Keys must be strings

Trying to pass in other types than the ones listed here is undefined behavior and will usually result in an error.

Image Provider

New in version 1.1.0.

A QML Image Provider can be registered from Python to load image data (e.g. map tiles, diagrams, graphs or generated images) in QML Image elements without resorting to saving/loading files.

An image provider has the following argument list and return values:

def image_provider(image_id, requested_size):
    return bytearray(pixels), (width, height), format

The parameters to the image provider functions are:

The ID of the image URL (image://python/<image_id>).
The source size of the QML Image as tuple: (width, height). (-1, -1) if the source size is not set.

The image provider must return a tuple (data, size, format):

A bytearray object containing the pixel data for the given size and the given format.
A tuple (width, height) describing the size of the pixel data in pixels.
The pixel format of data (see constants), or pyotherside.format_data if data contains an encoded (PNG/JPEG) image instead of raw pixel data.

In order to register the image provider with PyOtherSide for use as provider for image://python/ URLs, the image provider function needs to be passed to PyOtherSide:

import pyotherside

def image_provider(image_id, requested_size):


Because Python modules are usually imported asynchronously, the image provider will only be registered once the module registering the image provider is successfully imported. You have to make sure that setting the source property on a QML Image element only happens after the image provider has been set (e.g. by setting the source property in the callback function passed to importModule()).


This section contains code examples and best practices for combining Python and QML.

Importing modules and calling functions asynchronously

In this example, we import the Python Standard Library module os and - when the module is imported - call the os.getcwd() function on it. The result of the os.getcwd() function is then printed to the console and os.chdir() is called with a single argument ('/') - again, after the os.chdir() function has returned, a message will be printed.

In this example, importing modules and calling functions are both done in an asynchronous way - the QML/GUI thread will not block while these functions execute. In fact, the Component.onCompleted code block will probably finish before the os module has been imported in Python.

Python {
    Component.onCompleted: {
        importModule('os', function() {
            call('os.getcwd', [], function (result) {
                console.log('Working directory: ' + result);
                call('os.chdir', ['/'], function (result) {
                    console.log('Working directory changed.');

While this continuation-passing style might look a like a little pyramid due all the nesting and indentation at first, it makes sure your application’s UI is always responsive. The user will be able to interact with the GUI (e.g. scroll and move around in the UI) while the Python code can process requests.

Evaluating Python expressions in QML

The evaluate() method on the Python object can be used to evaluate a simple Python expression and return its result as JavaScript object:

Python {
    Component.onCompleted: {
        console.log('Squares: ' + evaluate('[x for x in range(10)]'));

Evaluating expressions is done synchronously, so make sure you only use it for expressions that are not long-running calculations / operations.

Error handling in QML

If an error happens in Python while calling functions, the traceback of the error (or an error message in case the error happens in the PyOtherSide layer) will be sent with the error() signal of the Python element. During early development, it’s probably enough to just log the error to the console:

Python {
    // ...

    onError: console.log('Error: ' + traceback)

Once your application grows, it might make sense to maybe show the error to the user in a dialog box, message or notification in addition to or instead of using console.log() to print the error.

Handling asynchronous events from Python in QML

Your Python code can send asynchronous events with optional data to the QML layer using the pyotherside.send() function. You can call this function from functions called from QML, but also from anywhere else - including threads that you created in Python. The first parameter is mandatory, and must be a string that identifies the event. Additional parameters are optional and can be of any data type that PyOtherSide supports:

import pyotherside

pyotherside.send('new-entries', 100, 123)

If you do not add a special handler on the Python object, such events would be handled by the received() signal handler in QML - its data parameter contains the event name and all arguments in a list:

Python {
    // ..

    onReceived: console.log('Event: ' + data)

Usually, you want to install a handler for such events. If you have e.g. the 'new-entries' event like shown above (with two numeric parameters that we will call first and last for this example), you might want to define a simple handler function that will process this event:

Python {
    // ..

    Component.onCompleted: {
        setHandler('new-entries', function (first, last) {
            console.log('New entries from ' + first + ' to ' + last);

Once a handler for a given event is defined, the received() signal will not be emitted anymore. If you need to unset a handler for a given event, you can use setHandler('event', undefined) to do so.

In some cases, it might be useful to not install a handler function directly, but turn the pyotherside.send() call into a new signal on the Python object. As there is no easy way for PyOtherSide to determine the names of the arguments of the event, you have to define and hook up these signals manually. The upside of having to define the signals this way is that all signals will be nicely documented in your QML file for future reference:

Python {
    signal updated()
    signal newEntries(int first, int last)
    signal entryRenamed(int index, string name)

    Component.onCompleted: {
        setHandler('updated', updated);
        setHandler('new-entries', newEntries);
        setHandler('entry-renamed', entryRenamed);

With this setup, you can now emit these signals from the Python object by using pyotherside.send() in your Python code:

pyotherside.send('new-entries', 20, 30)
pyotherside.send('entry-renamed', 11, 'Hello World')

Loading ListModel data from Python

Most of the time a PyOtherSide QML application will display some data stored somewhere and retrieved or generated with Python. The easiest way to do this is to return a list-of-dicts in your Python function:

def get_data():
    return [
        {'name': 'Alpha', 'team': 'red'},
        {'name': 'Beta', 'team': 'blue'},
        {'name': 'Gamma', 'team': 'green'},
        {'name': 'Delta', 'team': 'yellow'},
        {'name': 'Epsilon', 'team': 'orange'},

Of course, the function could do other things (such as doing web requests, querying databases, etc..) - as long as it returns a list-olf-dicts, it will be fine (if you are using a generator that yields dicts, just wrap the generator with list()). Using this function from QML is straightforward:


import QtQuick 2.0
import io.thp.pyotherside 1.0

Rectangle {
    color: 'black'
    width: 400
    height: 400

    ListView {
        anchors.fill: parent

        model: ListModel {
            id: listModel

        delegate: Text {
            // Both "name" and "team" are taken from the model
            text: name
            color: team

    Python {
        id: py

        Component.onCompleted: {
            // Add the directory of this .qml file to the search path

            // Import the main module and load the data
            importModule('listmodel', function () {
      'listmodel.get_data', [], function(result) {
                    // Load the received data into the list model
                    for (var i=0; i<result.length; i++) {

Instead of passing a list-of-dicts, it is of course also possible to send new list items via pyotherside.send(), one item at a time, and append them to the list model that way.

Rendering RGBA image data in Python


This example uses the image provider feature of PyOtherSide to render RGB image data in Python and display the rendered data in QML using a normal QtQuick 2.0 Image element:

import pyotherside
import math

def render(image_id, requested_size):
    print('image_id: "{image_id}", size: {requested_size}'.format(**locals()))

    # width and height will be -1 if not set in QML
    if requested_size == (-1, -1):
        requested_size = (300, 300)

    width, height = requested_size

    # center for circle
    cx, cy = width/2, 10

    pixels = []
    for y in range(height):
        for x in range(width):
                255, # alpha
                int(10 + 10 * ((x - y * 0.5) % 20)), # red
                20 + 10 * (y % 20), # green
                int(255 * abs(math.sin(0.3*math.sqrt((cx-x)**2 + (cy-y)**2)))) # blue
    return bytearray(pixels), (width, height), pyotherside.format_argb32


This module can now be imported in QML and used as source in the QML Image element:


import QtQuick 2.0
import io.thp.pyotherside 1.0

Image {
    id: image
    width: 300
    height: 300

    Python {
        Component.onCompleted: {
            // Add the directory of this .qml file to the search path

            importModule('imageprovider', function () {
                image.source = 'image://python/image-id-passed-from-qml';

        onError: console.log('Python error: ' + traceback)

Building PyOtherSide

The following build requirements have to be satisfied to build PyOtherSide:

  • Qt 5.1.0 or newer
  • Python 3.2.0 or newer

If you have the required build-dependencies installed, building and installing the PyOtherSide plugin should be as simple as:

make install

In case your system doesn’t provide python3-config, you might have to pass a suitable python-config to qmake at configure time:

qmake PYTHON_CONFIG=python3.3-config
make install

Alternatively, you can edit python.pri manually and specify the compiler flags for compiling and linking against Python on your system.

As of version 1.1.0, PyOtherSide still builds against Python 2.x (tested with Python 2.7, use qmake PYTHON_CONFIG=python2.7-config), but future point releases of PyOtherSide might drop support for Python 2.x. However, only one version of PyOtherSide can be installed/active at one time. It is highly recommended that you do not use Python 2 support except on legacy platforms.

Building for Blackberry 10

On Blackberry 10 (tested versions: 10.1, 10.2), Python 3.2.2 is already installed on-device. Qt 5 is not installed (only Qt 4), so if you are packaging a PyOtherSide application, you need to ship Qt 5 with it.

The approach we currently use is:

  1. Build Qt 5 using the Native SDK
  2. Get a set of matching Python 3.2.2 headers
  3. Fetch the following files from the device’s filesystem:
  • /usr/lib/
  • /usr/include/python3.2m/pyconfig.h
  1. Use pyconfig.h with the Python 3.2.2 headers and link against libpython3.2m

Modify python.pri to point to the fetched library and your Python 3.2.2 headers (with pyconfig.h from the device):

QMAKE_LIBS += -lpython3.2m -L/path/to/where/the/library/is
QMAKE_CXXFLAGS += -I/path/to/where/the/headers/are/include/python3.2m

After installing PyOtherSide in the locally-build Qt 5 (cross-compiled for BB10), the QML plugins folder can be deployed with the .bar file.


Version 1.1.0 (2014-02-06)

Version 1.0.0 (2013-08-08)

  • Initial QML plugin release.

Version 0.0.1 (2013-05-17)

  • Proof-of-concept (based on a prototype from May 2011).