[PyKDE] How to do *INTERACTIVE* plotting with PyQt (or PyQwt)

[Phil Thompson]

Peter Lipa wrote:

> Hi,
> 
> First, let me thank you for providing and maintaining the PyQt package!  
> As a feedback, we compiled and installed sucessfully your package and demos
> 
> for the commercial Qt3.0.4 and Qt3.0.5 versions on Win2K (the latest 
> PyQt 3.3 and sip3.3.2 versions!).
> 
>  
> 
> Now my problem, which I think is a very fundamental one, that needs to 
> be resolved to make a  PyQt  based, interactive
> 
> plotting package:
> 
> I am banging my head against the wall in trying to get PyQt (or PyQwt) 
> to allow for INTERACTIVE plotting
> 
> such as with the scipy.gplt (using gnuplot) or scipy.plt (using 
> wxWindows)  packages. I want a similar API as matlab plotting commands or
> the scipy.plt API but based on PyQt (and/or PyQwt but the problem is at 
> the PyQt eventloop and threading level).
> 
>  
> 
> All examples with PyQt (and PyQwt demos etc) assume that one has only
> 
> ONE pyqt app running and, if called as a script, block the python 
> interpreter until the PyQt window is closed -- or one runs each demo as 
> a stand alone app.
> 
> In order to do INTERACTIVE plotting one needs to issue plot commands 
> from the  python interpeter command line such as:
> 
>  >>>  import qplt
> 
>  >>> x = [0,1,2,3, 4]; y = [0,1.5,3,1.5,0]
> 
>  >>> fig1 = qplt.plot(x,y)   # creates a figure window with default axes 
> and line plot with x versus y and returns control to the interpreter
> 
>  >>> qplt.xlabel("x-axis")  # adds a x-label to the current axes in the 
> current figure
> 
>  >>> qplt.ylabel("y-axis")  # adds a y-label to the current axes in the 
> current figure
> 
>  >>> fig2 = qplt.figure()    #  creates a second figure window and all 
> subsequent commands (plot(), xlabel() etc.)  apply to this new window
> 
> etc, etc
> 
> In order to get such functionality one needs to create a qt qApp 
> singleton, a figure main-window, start the eventloop in a separate 
> thread (either python thread or QThread) and gives control back to the 
> interpeter to let the user issue new commands (e.g. another figure 
> main-window) interactively.
> 
> This seems to be a simple and natural approach to interactive plotting, 
> but I have not found any example or idea based on pyQt that would give 
> me a hint how to prevent the Qt eventloop qApp.exec_loop() blocking the 
> interpreter or how to add new QMainWindows from the interpreter after 
> the eventloop is already running in a separate python thread.
> 
>  So far I have managed to to open a pyqt figure-window from the 
> interpreter by wrapping the main()  with the eventloop qApp.exec_loop() 
> into a python thread; but then I can't open another window from the main 
> thread. Using the QThread class didn't help at all: the subclassed 
> myQThread.run() does never give control back to the interpeter if it 
> contains the qApp.exec_loop(), and myQThread.start() does never call 
> myQThread.run() but the dummy QThread.run().  It is not clear from the 
> Qt (and PyQt) docu if it is possible to run the main Qt eventloop in a 
> QThread; in my experiments it always blocked
> 
> any GUI.
> 
>  
> 
> I have tried and tried (using all possible varaints of python threads as 
> well as QThread ) and can't figure out how to do it.
> 
> Do you have any idea how to do real *INTERACTIVE* plotting with PyQt? 
> Essentially all that needs to be done is to start a qApp and qt 
> eventloop in a separate thread and launch several figure windows and  
> from the python interpreter thread.
> 
> Since this is a fundamental problem that many people in the scientific 
> area need resolved I hope that someone on this list (or maybe someone 
> you know) knows alread how to do this!
> 
> Any pointers, ideas or sample code would be GREAT!


eric is able to do pretty much what you describe. It presents you with a 
Python interpreter that you can interactively enter Python statements in 
while the same interpreter is running a PyQt application. There may be 
easier ways of doing it though.

The problem you describe with QThread sounds like you weren't using it 
properly - although I couldn't say if running the event loop from a 
thread is expected to work. (Might do, so long as you manage the Qt lock 
properly.)

Phil