Do not create a QApplication in Qt input hook · Issue #13617 · ipython/ipython · GitHub

Currently if the Qt input hook runs and there is not as exiting QApplication IPython will create one for the user and then spin it. On one hand this is convenient for doing Qt work as the Application is automatically created after the first cell runs. However, this can lead to weird behavior where running commands in one line vs serval cells leads to different results (see DeepLabCut/DeepLabCut#1730).

Although I advocated for the current state of affairs, I now think that it is more confusing this way. I propose that basically this entire block of code is removed and replaced with return

ipython/IPython/terminal/pt_inputhooks/qt.py

Lines 27 to 56 in 6ddb518

    
           if sys.platform == 'linux': 
        
               if not os.environ.get('DISPLAY') \ 
        
                       and not os.environ.get('WAYLAND_DISPLAY'): 
        
                   import warnings 
        
                   global _already_warned 
        
                   if not _already_warned: 
        
                       _already_warned = True 
        
                       warnings.warn( 
        
                           'The DISPLAY or WAYLAND_DISPLAY environment variable is ' 
        
                           'not set or empty and Qt5 requires this environment ' 
        
                           'variable. Deactivate Qt5 code.' 
        
                       ) 
        
                   return 
        
           try: 
        
               QtCore.QApplication.setAttribute(QtCore.Qt.AA_EnableHighDpiScaling) 
        
           except AttributeError:  # Only for Qt>=5.6, <6. 
        
               pass 
        
           try: 
        
               QtCore.QApplication.setHighDpiScaleFactorRoundingPolicy( 
        
                   QtCore.Qt.HighDpiScaleFactorRoundingPolicy.PassThrough 
        
               ) 
        
           except AttributeError:  # Only for Qt>=5.14. 
        
               pass 
        
           _appref = app = QtGui.QApplication([" "]) 
        
           # "reclaim" IPython sys.excepthook after event loop starts 
        
           # without this, it defaults back to BaseIPythonApplication.excepthook 
        
           # and exceptions in the Qt event loop are rendered without traceback 
        
           # formatting and look like "bug in IPython". 
        
           QtCore.QTimer.singleShot(0, _reclaim_excepthook)

A bulk of the work on this issue will be documenting the consequences and thinking through what the consequences might be.

https://gist.github.com/pag/f7d1ebbd1b14fe9058bf68d73a52bc84 is a patch that implements this a related idea that does not spin the event loop until it is needed.

Do not create a QApplication in Qt input hook · Issue #13617 · ipython/ipython · GitHub

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Do not create a QApplication in Qt input hook #13617

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	if sys.platform == 'linux':
	if not os.environ.get('DISPLAY') \
	and not os.environ.get('WAYLAND_DISPLAY'):
	import warnings
	global _already_warned
	if not _already_warned:
	_already_warned = True
	warnings.warn(
	'The DISPLAY or WAYLAND_DISPLAY environment variable is '
	'not set or empty and Qt5 requires this environment '
	'variable. Deactivate Qt5 code.'
	)
	return
	try:
	QtCore.QApplication.setAttribute(QtCore.Qt.AA_EnableHighDpiScaling)
	except AttributeError: # Only for Qt>=5.6, <6.
	pass
	try:
	QtCore.QApplication.setHighDpiScaleFactorRoundingPolicy(
	QtCore.Qt.HighDpiScaleFactorRoundingPolicy.PassThrough
	)
	except AttributeError: # Only for Qt>=5.14.
	pass
	_appref = app = QtGui.QApplication([" "])

	# "reclaim" IPython sys.excepthook after event loop starts
	# without this, it defaults back to BaseIPythonApplication.excepthook
	# and exceptions in the Qt event loop are rendered without traceback
	# formatting and look like "bug in IPython".
	QtCore.QTimer.singleShot(0, _reclaim_excepthook)

Do not create a QApplication in Qt input hook · Issue #13617 · ipython/ipython · GitHub

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Do not create a QApplication in Qt input hook #13617

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tacaswell commented on Apr 30, 2022

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