old_contents: import sys
  from IPython.external.qt_for_kernel import QtCore, QtGui
  
  # If we create a QApplication, keep a reference to it so that it doesn't get
  # garbage collected.
  _appref = None
  
  
  def inputhook(context):
      global _appref
      app = QtCore.QCoreApplication.instance()
      if not app:
          _appref = app = QtGui.QApplication([" "])
      event_loop = QtCore.QEventLoop(app)
  
      if sys.platform == 'win32':
          # The QSocketNotifier method doesn't appear to work on Windows.
          # Use polling instead.
          timer = QtCore.QTimer()
          timer.timeout.connect(event_loop.quit)
          while not context.input_is_ready():
              timer.start(50)  # 50 ms
              event_loop.exec_()
              timer.stop()
      else:
          # On POSIX platforms, we can use a file descriptor to quit the event
          # loop when there is input ready to read.
          notifier = QtCore.QSocketNotifier(context.fileno(),
                                            QtCore.QSocketNotifier.Read)
          # connect the callback we care about before we turn it on
          notifier.activated.connect(event_loop.exit)
          notifier.setEnabled(True)
          # only start the event loop we are not already flipped
          if not context.input_is_ready():
              event_loop.exec_()
  
new_contents: import sys
  import os
  from IPython.external.qt_for_kernel import QtCore, QtGui
  
  # If we create a QApplication, keep a reference to it so that it doesn't get
  # garbage collected.
  _appref = None
  _already_warned = False
  
  def inputhook(context):
      global _appref
      app = QtCore.QCoreApplication.instance()
      if not app:
          if sys.platform == 'linux':
              try:
                  os.environ['DISPLAY']  # DISPLAY is set
                  assert os.environ['DISPLAY'] != ''  # DISPLAY is not empty
              except Exception:
                  import warnings
                  global _already_warned
                  if not _already_warned:
                      _already_warned = True
                      warnings.warn(
                          'The DISPLAY enviroment variable is not set or empty '
                          'and qt requires this enviroment variable. '
                          'Deaktivate qt code.\n'
                          'Backend: {}'.format(QtGui)
                      )
                  return
          _appref = app = QtGui.QApplication([" "])
      event_loop = QtCore.QEventLoop(app)
  
      if sys.platform == 'win32':
          # The QSocketNotifier method doesn't appear to work on Windows.
          # Use polling instead.
          timer = QtCore.QTimer()
          timer.timeout.connect(event_loop.quit)
          while not context.input_is_ready():
              timer.start(50)  # 50 ms
              event_loop.exec_()
              timer.stop()
      else:
          # On POSIX platforms, we can use a file descriptor to quit the event
          # loop when there is input ready to read.
          notifier = QtCore.QSocketNotifier(context.fileno(),
                                            QtCore.QSocketNotifier.Read)
          # connect the callback we care about before we turn it on
          notifier.activated.connect(event_loop.exit)
          notifier.setEnabled(True)
          # only start the event loop we are not already flipped
          if not context.input_is_ready():
              event_loop.exec_()
  
subject: Disable qt inputhook backend when DISPLAY is not set on linux, because the qt lib does not handle this correct