Initial notes

IPython/external/qt_for_kernel.py

@@ -23,6 +23,9 b' if QT_API not set:'
 else:
     use what QT_API says
 
+    Note that %gui's implementation will always set a `QT_API`, see
+    `IPython.terminal.pt_inputhooks.get_inputhook_name_and_func`
+
 """
 # NOTE: This is no longer an external, third-party module, and should be
 # considered part of IPython. For compatibility however, it is being kept in
@@ -99,6 +102,7 b' def get_options():'
         return [loaded]
 
     mpl = sys.modules.get('matplotlib', None)
+    print(f'{mpl=}') # will be None of matplotlib has not yet been imported
 
     if mpl is not None and tuple(mpl.__version__.split(".")) < ("1", "0", "2"):
         # 1.0.1 only supports PyQt4 v1
@@ -120,6 +124,7 b' def get_options():'
         raise RuntimeError("Invalid Qt API %r, valid values are: %r" %
                            (qt_api, ', '.join(_qt_apis)))
     else:
+        print(f'{qt_api=}')
         return [qt_api]
 
 

IPython/terminal/interactiveshell.py

@@ -712,6 +712,7 b' class TerminalInteractiveShell(InteractiveShell):'
     active_eventloop = None
     def enable_gui(self, gui=None):
         if gui and (gui not in {"inline", "webagg"}):
+            # This hook runs with each cycle of the `prompt_toolkit`'s event loop.
             self.active_eventloop, self._inputhook = get_inputhook_name_and_func(gui)
         else:
             self.active_eventloop = self._inputhook = None

IPython/terminal/pt_inputhooks/__init__.py

@@ -55,8 +55,13 b' def get_inputhook_name_and_func(gui):'
         os.environ["QT_API"] = "pyqt5"
         gui_mod = "qt"
     elif gui == "qt6":
+        # XXX: this locks us into pyqt6 even if pyside6 is installed.
         os.environ["QT_API"] = "pyqt6"
         gui_mod = "qt"
 
+    print(f'{gui_mod=}')
+    # Note: `IPython.terminal.pt_inputhooks.qt` imports `IPython.external.qt_for_kernel` and that's
+    # where the environment variable locks us into `pyqt6`, despite the fact that it seems `PySide6`
+    # is supported.
     mod = importlib.import_module('IPython.terminal.pt_inputhooks.'+gui_mod)
     return gui, mod.inputhook

IPython/terminal/pt_inputhooks/qt.py

@@ -1,5 +1,6 b''
 import sys
 import os
+#`qt_for_kernel` will import the "best" qt version
 from IPython.external.qt_for_kernel import QtCore, QtGui, enum_helper
 from IPython import get_ipython
 
@@ -63,6 +64,7 b' def inputhook(context):'
         timer = QtCore.QTimer()
         timer.timeout.connect(event_loop.quit)
         while not context.input_is_ready():
+            # NOTE: run the event loop, and after 50 ms, call `quit` to exit it.
             timer.start(50)  # 50 ms
             _exec(event_loop)
             timer.stop()