Context:
In Python a main thread spawns a 2nd process (using multiprocessing module) and then launches a GUI (using PyQt4). At this point the main thread blocks until the GUI is closed. The 2nd process is always processing and ideally should emit signal(s) to specific slot(s) in the GUI in an asynchronous manner.
Question:
Which approach/tools are available in Python and PyQt4 to achieve that and how? Preferably in a soft-interrupt manner rather than polling.
Abstractly speaking, the solution I can think of is a "tool/handler" instantiated in the main thread that grabs the available slots from the GUI instance and connects with the grabbed signals from the 2nd process, assuming I provide this tool some information of what to expect or hard coded. This could be instantiated to a 3rd process/thread.
I had the same problem in C++. From a QApplication, I spawn a Service object. The object creates the Gui Widget but it's not its parent (the parent is QApplication then). To control the GuiWidget from the service widget, I just use signals and slots as usual and it works as expected. 
Note: The thread of GuiWidget and the one of the service are different. The service is a subclass of QObject.
If you need multi process signal/slot mechanism, then try to use Apache Thrift or use a Qt-monitoring process which spawns 2 QProcess objects.
How to signal slots in a GUI from a different process?