BUG : make qApp global before using it

[tacaswell]

Make qApp global in backend_qt5.py before using it.

This silences a warning.

I don't think this change requires a new RC.

closes #3379

[ezust]

I am confused by this. "qApp" is actually something that comes from PyQt5.QtCore.
Are you defining it to be something different in matplotlib? Why not just add a
"from PyQt5.QtCore import qApp" in the places you need it?

[jenshnielsen]

Not in this case. Look a few lines below it is initialised either as QtWidgets.QApplication.instance()
or as QtWidgets.QApplication([str(" ")])

[jenshnielsen]

In addition I can't see it in QtCore

In [4]: from PyQt5.QtCore import qApp
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-4-333c82883eb3> in <module>()
----> 1 from PyQt5.QtCore import qApp

ImportError: cannot import name 'qApp'

There is a qAbs

[ezust]

Sorry, I meant

from PyQt5.Qt import qApp

On Tue, Aug 19, 2014 at 8:48 AM, Jens H Nielsen notifications@github.com
wrote:

In addition I can't see it in QtCore

In [4]: from PyQt5.QtCore import qApp---------------------------------------------------------------------------ImportError Traceback (most recent call last) in ()----> 1 from PyQt5.QtCore import qApp
ImportError: cannot import name 'qApp'

There is a qAbs

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#3381 (comment)
.

[ezust]

Or

from PyQt5.QtWidgets import qApp

Wrong module, sorry.

[efiring]

To avoid this name confusion, we should not be using qApp as a variable name.

[efiring]

@ezust, what is the right way to handle this? At the C++ level, qApp is described as a Macro; what is it in PyQt5? How is it initialized?

[jenshnielsen]

The only reference I could find in PyQT docs is this http://pyqt.sourceforge.net/Docs/PyQt4/qtgui.html#qApp-var

(In PyQT4 it was in qtgui) But it is not very informative

[ezust]

It looks like qApp is initialized by Qt in a very similar way to how you
guys are initializing it (QtWidgets.QApplication.instance()), so I might be
making a big deal out of nothing.
Before, I just was looking at the patch all by itself and it made no sense
to me that you would need to declare another global variable for it if you
can just import it.

On Tue, Aug 19, 2014 at 10:29 AM, Eric Firing notifications@github.com
wrote:

@ezust https://github.com/ezust, what is the right way to handle this?
At the C++ level, qApp is described as a Macro; what is it in PyQt5? How is
it initialized?

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#3381 (comment)
.

[ezust]

The reason it is a macro in c++ is so it can return a QCoreApplication if
you are only using QtCore, and a QGuiApplication if you are only using
QtGui and a QApplication if you are using QWidget, depending on which
header file you included.

[tacaswell]

My understanding of this was that there can only be one running QApplication in the process. QWidgets.QApplication.instance() will either return a reference to the currently running application (not sure how it is done at the c++ level, but I assume one of those macros) or None. If it's None, we start up the application so that we can safely make the gui windows (if we don't have an QApp starting Painter throws errors).

For convince we store the the reference in a module-level global.

[efiring]

@tacaswell, I think your understanding is correct, and the existing code is OK. I suspect that the same thing could be accomplished using the imported qApp, but it would take some investigation to figure out how to do it (e.g., how is it initialized upon import?). This might be worth sorting out, but if the present code works, then changing it is not high priority.

The present organization, in which backend_qt4 imports _create_qApp() and other things from backend_qt5 seems a little odd; but it all went through review before being merged.