Help on PyQt4 QProcess

[Edgar Fuentes]

On Aug 20, 4:36 am, Phil Thompson <p... at riverbankcomputing.com> wrote:
> On Fri, 19 Aug 2011 14:32:12 -0700 (PDT), Edgar Fuentes
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> <fuente... at gmail.com> wrote:
> > On Aug 19, 4:21 pm, Carl Banks <pavlovevide... at gmail.com> wrote:
> >> On Friday, August 19, 2011 12:55:40 PM UTC-7, Edgar Fuentes wrote:
> >> > On Aug 19, 1:56 pm, Phil Thompson
> >> >  wrote:
> >> > > On Fri, 19 Aug 2011 10:15:20 -0700 (PDT), Edgar Fuentes
> >> > > <fuen... at gmail.com> wrote:
> >> > > > Dear friends,
>
> >> > > > I need execute an external program from a gui using PyQt4, to
> avoid
> >> > > > that hang the main thread, i must connect the signal
> >> > > > "finished(int)"
> >> > > > of a QProcess to work properly.
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> >> > > > for example, why this program don't work?
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> >> > > >    from PyQt4.QtCore import QProcess
> >> > > >    pro = QProcess() # create QProcess object
> >> > > >    pro.connect(pro, SIGNAL('started()'), lambda
> >> > > > x="started":print(x))        # connect
> >> > > >    pro.connect(pro, SIGNAL("finished(int)"), lambda
> >> > > > x="finished":print(x))
> >> > > >    pro.start('python',['hello.py'])        # star hello.py
> program
> >> > > > (contain print("hello world!"))
> >> > > >    timeout = -1
> >> > > >    pro.waitForFinished(timeout)
> >> > > >    print(pro.readAllStandardOutput().data())
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> >> > > > output:
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> >> > > >    started
> >> > > >    0
> >> > > >    b'hello world!\n'
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> >> > > > see that not emit the signal finished(int)
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> >> > > Yes it is, and your lambda slot is printing "0" which is the return
> >> > > code
> >> > > of the process.
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> >> > > Phil
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> >> > Ok, but the output should be:
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> >> >     started
> >> >     b'hello world!\n'
> >> >     finished
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> >> > no?.
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> >> > thanks Phil
>
> >> Two issues.  First of all, your slot for the finished function does not
> >> have the correct prototype, and it's accidentally not throwing an
> >> exception because of your unnecessary use of default arguments.
>  Anyway,
> >> to fix that, try this:
>
> >> pro.connect(pro, SIGNAL("finished(int)"), lambda v,
> >> x="finished":print(x))
>
> >> Notice that it adds an argument to the lambda (v) that accepts the int
> >> argument of the signal.  If you don't have that argument there, the int
> >> argument goes into x, which is why Python prints 0 instead of
> "finished".
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> >> Second, processess run asynchrously, and because of line-buffering, IO
> >> can output asynchronously, and so there's no guarantee what order
> output
> >> occurs.  You might try calling the python subprocess with the '-u'
> switch
> >> to force unbuffered IO, which might be enough to force synchronous
> output
> >> (depending on how signal/slot and subprocess semantics are
> implemented).
>
> >> Carl Banks
>
> > Thanks Carl, your intervention was very helpful for me, this solve my
> > semantic error. I need to study more about signal/slots and process.
>
> In which case you should look at the modern, Pythonic connection syntax
> rather than the old one...
>
>     pro.started.connect(lambda: print("started"))
>     pro.finished.connect(lambda: print("finished"))
>
> Phil

Pythonic, great!, more straightforward.

Thanks again Phil and Carl.

best regards,

Edgar Fuentes