[IPython-dev] [matplotlib-devel] Interactive wx/pylab with no threads (PyOS_InputHook)

[Brian Granger]

Michiel,

Thanks for jumping into the discussion.

> I wrote the code in PyGTK that uses PyOS_InputHook for interactivity, as well as the Mac OS X native backend for matplotlib that uses PyOS_InputHook in exactly the same way. PyQT and Tkinter also use PyOS_InputHook, though the code is a bit kludgy on Windows. So I definitely agree that PyOS_InputHook is the right way to go.

Great, I was wondering how the Mac OS X backend works - now I know.  I
will have a look at the code for both PyGTK and OS X.  Hopefully that
will show me more of the best way of handling this.

> Your current code should work, but there's a better way to do it. If I understand the code correctly, you rely on the fact that PyOS_InputHook is called repeatedly by readline, and you use PyOS_InputHook to process wx events that need to be processed at that time.

Yes, at least that is my understanding.  I put in some debug
statements and you could see that it was being called repeatedly.

A better way is to use PyOS_InputHook to start the wx event loop, but
have this event loop check stdin. As soon as some input is available
on stdin, you exit the event loop, which means that PyOS_InputHook
returns, and Python can proceed to handle the command that was just
entered by the user on stdin.
>
> Essentially, think of wx's event loop as sitting in a call to select(), waiting for the next wx event to arrive. You want to add fileno(stdin) to the set of file descriptors watched by select().

I have seen that this is how the PyQt4 implementation handles it.

> There are two advantages to this approach. First, it does not rely on readline calling PyOS_InputHook repeatedly. This is important, since Python may not be using readline at all, and if it is, depending on the Python version and how readline was installed it may call PyOS_InputHook only once.

OK, I was wondering about this.  But, what happens if PyOS_InputHook
is called repeatedly.  Are you not then starting the event loop
multiple times.  Can you say more about what happens in this case?

Second, this approach is more efficient (not wasting processor cycles
going back and forth between readline and PyOS_InputHook), and gives a
better response time (essentially immediate).

That would be very nice as my implementation is less responsive.

> The best place to put this code is in wxPython. Hopefully (I haven't checked this), wx exposes enough of the event loop to allow you to have it watch stdin. This may be an issue, since for example qt4 does not on Windows, which is why the event loop is kludgy with PyQT on Windows. You could have a look at the PyOS_InputHook code in PyGTK (you'll need to get the developer's version of PyGTK, since this code is not yet in an official release). It's actually quite straightforward and you may be able to modify it directly for wx.

Yes, I fully agree with this.  I might end up contacting the wx devs
to get their help on this.  I actually don't know wx at all, so I am
amazed that I got this far.  I will have a look at the PyGTK
implementation.

> It's actually unfortunate that we have to use PyOS_InputHook; all this would be a lot easier if Python itself supported event loops.

Yes, that would be nice!!!

Cheers,

Brian

> --Michiel
>
> --- On Sun, 2/8/09, Brian Granger <ellisonbg.net at gmail.com> wrote:
>
>> From: Brian Granger <ellisonbg.net at gmail.com>
>> Subject: [matplotlib-devel] Interactive wx/pylab with no threads (PyOS_InputHook)
>> To: matplotlib-devel at lists.sourceforge.net, "IPython Development list" <ipython-dev at scipy.org>
>> Date: Sunday, February 8, 2009, 7:08 PM
>> IPython and matplotlib devs,
>>
>> Over the weekend I have been playing around to see if it is
>> possible
>> to do interactive GUI work with wx from IPython *without
>> using
>> threads*.  The idea here is to use PyOS_InputHook.
>> Currently, recent
>> versions of PyQt4 and PyGTK do this and if we can get wx
>> working, we
>> can probably get rid of IPython's subtle threaded
>> shells that
>> currently allow interactive GUIs to work.
>>
>> I am attaching a Cython module that mostly works.  Here is
>> a simple
>> example that works in IPython (without the -wthread
>> option!)
>>
>> In [1]: import pyximport
>>
>> In [2]: pyximport.install()
>>
>> In [3]: import inputhook
>>
>> In [4]: inputhook.set_input_hook()
>>
>> In [5]: import wx
>>
>> In [6]: app = wx.PySimpleApp()
>>
>> In [7]: app.MainLoop()
>>
>> In [8]: f = wx.Frame(None,-1,"Look mom, no
>> threads!")
>>
>> In [9]: f.Show()
>> Out[9]: True
>>
>> The docstring of the module also has a matplotlib example.
>> This
>> really does work and I am pretty sure it will also work in
>> plain
>> vanilla python as well.  There are a few issues to work
>> out:
>>
>> * When frame are closed by clicking the red button or the
>> "X", the
>> Windows don't close.  In addition, in matplotlib, this
>> causes further
>> problems.
>>
>> * In the current matplotlib backend wx.Yield() is called in
>> a way that
>> is not safe as far as protecting against recursive calls to
>> Yield.  I
>> think it should be called in this way:
>>
>> app = wx.GetApp()
>> if app is not None:
>>   app.Yield(True)
>>
>> * I don't think that interupts work yet, but I
>> haven't tested this
>> thoroughly yet.
>>
>> I don't have any more time to work on this right now,
>> but I at least
>> wanted to share my findings with both IPython and
>> matplotlib devs.  It
>> would be great if someone familiar with wx could try to
>> figure out the
>> remaining issues.  If there are no takers here, I might
>> eventually see
>> if wxpython itself is interested in this code (that is
>> probably where
>> it really belongs anyway).
>>
>> Cheers,
>>
>> Brian
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