os.environ['QT_API'] gets overwritten in event-loop. No pyside2 integration.

[bergtholdt]

Consider this IPython interactive session:

Jupyter QtConsole 4.5.0.dev0
Python 3.6.8 |Anaconda, Inc.| (default, Feb 21 2019, 18:30:04) [MSC v.1916 64 bit (AMD64)]
Type 'copyright', 'credits' or 'license' for more information
IPython 7.3.0.dev -- An enhanced Interactive Python. Type '?' for help.

In [1]: IP = get_ipython()

In [2]: import os

In [3]: os.environ['QT_API']
Out[3]: 'pyside2'

In [4]: IP.enable_gui('qt')

In [5]: os.environ['QT_API']
Out[5]: 'pyqt5'

In [6]: os.environ['QT_API'] = 'pyside2'

In [7]: os.environ['QT_API']
Out[7]: 'pyqt5'

In [8]: from IPython.lib.guisupport import get_app_qt4

In [9]: app = get_app_qt4()

In [10]: app
Out[10]: <PySide2.QtWidgets.QApplication at 0x7ff9ddd4c568>

What happened between 6 and 7?

The answer is that os.environ['QT_API'] gets overwritten in the event-loop all the time once it has been activated. And even though I initially selected pyside2 it changes the env-var to pyqt5. But apparently the desired backend (pyside2) is still used. I do however run into problems, e.g. when importing directly from PyQt5 versus PySide2 depending on the environment variable.

I will attempt a fix.

[bergtholdt]

The fix is out, though I'm not really sure if it's the desired behavior. It looks like this in ipykernel/eventloops.py:125 ff

@register_integration('qt', 'qt5')
def loop_qt5(kernel):
    """Start a kernel with PyQt5 event loop integration."""
    if os.environ['QT_API'] not in ['pyqt5', 'pyside2']:
        os.environ['QT_API'] = 'pyqt5'
    return loop_qt4(kernel)

The if os.environ ... part is new. As mentioned, this gets looped over and over again, causing many writes to the environment. In my opinion this is not necessary and a single write operation would be sufficient outside the loop (or none at all!).
Any reason for the design choice here to put the os.environ['QT_API'] = 'pyqt5' call in the event-loop or having it in the first place?

[bergtholdt]

5 and 1/2 month later and no comment from any core developer.
Apparently commercial software development is not a target for the IPython guys :(
PyQt5 is not an option for us.