pqthreads 0.3.3

Expose class interfaces from the main GUI Thread in another QThread in Qt for Python

Pqthreads

Pqthreads exposes class interfaces from the main GUI Thread in another QThread in Qt for Python (PySide). In doing so, it facilitates communication between the main (GUI) thread and a dedicated QThreads as offered by Qt for Python.

Installation

Pqthreads comes as a package in the Python Package Index (PyPi) and can be installed using pip:

pip install pqthreads

Usage

In order to use pqhreads, you'll first need a GUI implementation (in Qt for Python (PySide)) whose interface you'd like to expose. Then you need to have a corresponding class that exposes (chosen) methods and attributes in another thread.

GUI implementation

The GUI implementation would be a class that derives from QWidget, e.g. QMainWindow. A very basic example class called FigureWindow can be found in window.py.

Worker class

In a class that inherits from containers.WorkerItem you then choose which methods and attributes are exposed. An examples of this is the class FigureWorker as found in worker.py.

Putting it all together

Using the GUI implementation FigureWindow and worker class FigureWorker the utilities from decorator.py can be used to create a custom decorator:

from pqthreads import decorator

DecoratorCore = decorator.DecoratorCore
DecoratorCore.add_agent('figure', window.FigureWindow, FigureWorker)
decorator_example = decorator.Decorator(DecoratorCore)

Any decorated function will run in the worker thread, while all GUI elements run in the (main) GUI thread.

To simplify access to worker class interfaces, a helper function is useful. This also illustrates how to create and access new GUI elements:

from pqthreads import refs

def figure(*args, **kwargs):
    """ Create, raise or modify FigureWorker objects """
    container = refs.worker.get('figure')
    if not args:
        return container.create(**kwargs)
    figure_worker = args[0]
    container.current = figure_worker
    return figure_worker

This can finally be used to to expose GUI implementation in an existing python program that will run in another worker thread.

from pqthreads.examples import worker

@decorator_example
def main():
    fig = worker.figure(title='Initial title')
    fig.change_title(title='Another title')
    fig.close()

Pittfalls

As illustrated in the previous section, worker class interfaces are accessed through the so-called worker references (as is provided in the module refs). All interfaces are provided as weak references. As soon as the decorated function is exited, the weak references will invalidate and can't be used anymore. Therefore, it's recommended to decorate the function that encapsulates the whole python program in question. This assures that different parts of your own program run in different threads.

The module refs also comes with an object called gui references that stores weak references to the GUI objects. These also will invalidate when the decorated function is exited.

Finally, one should not access the worker references from the (main) GUI thread and also not access gui references from the worker thread. This can lead to trace errors and all sorts of undefined behavior. You have been warned!

Design

Pqthreads separates the GUI elements from all programming elements in QThreads. Since Qt demands that the main thread is used for GUI elements, all other programming functionalities are moved into a dedicated QThread. This shall be called the worker threads.

The following schematic depicts this design.

Communication between the GUI and worker threads is solely done using Signal/Slot connections. This is facilitated by the GUIAgents and WorkerAgents.

The interface of a GUI Object is exposed by means of a Worker Object in the Worker Thread.

It is possible expose the interface of multiple types of GUI Objects (in the shown schematic FigureWindow and GraphWindow), which requires multiple Worker- and GUIAgents. These are held the GUIAgency and WorkerAgency respectively.

For each type of GUI/Worker object pair, it's also possible to instantiate multiple objects (of the same type). GUI and Worker objects are held in the GUIItemContainer and WorkerItemContainer respectively.

License

An MIT style license applies for pqthreads, see the LICENSE file for more details.