Backport PR #10974: IPython breaks import matplotlib.pyplot when there is no display

IPython/terminal/pt_inputhooks/qt.py

@@ -1,35 +1,49 b''
 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':
+            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 enviroment variable is '
+                        'not set or empty and Qt5 requires this enviroment '
+                        'variable. Deactivate Qt5 code.'
+                    )
+                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_()