FIX: re-order qt eventloop hook a bit

try to reduce race conditions by:

 - connect the exit callback before enabling the notifier (this might
   not matter)
 - only bother starting the event loop if the input is not ready.

closes  #10201

IPython/terminal/pt_inputhooks/qt.py

@@ -1,30 +1,35 b''
 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)
-        notifier.setEnabled(True)
+        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)
-        event_loop.exec_()
+        notifier.setEnabled(True)
+        # only start the event loop we are not already flipped
+        if not context.input_is_ready():
+            event_loop.exec_()