Keep track of Qt version since it can't change

IPython/terminal/pt_inputhooks/__init__.py

@@ -1,137 +1,141 b''
 import importlib
 import os
 
 aliases = {
     'qt4': 'qt',
     'gtk2': 'gtk',
 }
 
 backends = [
     "qt",
     "qt4",
     "qt5",
     "qt6",
     "gtk",
     "gtk2",
     "gtk3",
     "gtk4",
     "tk",
     "wx",
     "pyglet",
     "glut",
     "osx",
     "asyncio",
 ]
 
 registered = {}
 
 def register(name, inputhook):
     """Register the function *inputhook* as an event loop integration."""
     registered[name] = inputhook
 
 
 class UnknownBackend(KeyError):
     def __init__(self, name):
         self.name = name
 
     def __str__(self):
         return ("No event loop integration for {!r}. "
                 "Supported event loops are: {}").format(self.name,
                                     ', '.join(backends + sorted(registered)))
 
 
+last_qt_version = None  # stores which version (i.e. `gui`) was requested the first time.
+
 def set_qt_api(gui):
     """Sets the `QT_API` environment variable if it isn't already set."""
-    # TODO: how do we do this here?
-    # if hasattr(kernel, "app"):
-    #     raise RuntimeError("Kernel already running a Qt event loop.")
 
-    # if gui != "qt" and hasattr(kernel, "last_qt_version"):
-    #     if kernel.last_qt_version != gui:
-    #         raise ValueError(
-    #             "Cannot switch Qt versions for this session; "
-    #             f"must use {kernel.last_qt_version}."
-    #         )
+    global last_qt_version
+
+    if gui != "qt" and last_qt_version is not None:
+        if last_qt_version != gui:
+            raise ValueError(
+                "Cannot switch Qt versions for this session; "
+                f"must use {last_qt_version}."
+            )
 
     qt_api = os.environ.get("QT_API", None)
     if qt_api is not None and gui != "qt":
         env2gui = {
             "pyside": "qt4",
             "pyqt": "qt4",
             "pyside2": "qt5",
             "pyqt5": "qt5",
             "pyside6": "qt6",
             "pyqt6": "qt6",
         }
         if env2gui[qt_api] != gui:
             print(
                 f'Request for "{gui}" will be ignored because `QT_API` '
                 f'environment variable is set to "{qt_api}"'
             )
     else:
         if gui == "qt4":
             try:
                 import PyQt  # noqa
 
                 os.environ["QT_API"] = "pyqt"
             except ImportError:
                 try:
                     import PySide  # noqa
 
                     os.environ["QT_API"] = "pyside"
                 except ImportError:
                     # Neither implementation installed; set it to something so IPython gives an error
                     os.environ["QT_API"] = "pyqt"
         elif gui == "qt5":
             try:
                 import PyQt5  # noqa
 
                 os.environ["QT_API"] = "pyqt5"
             except ImportError:
                 try:
                     import PySide2  # noqa
 
                     os.environ["QT_API"] = "pyside2"
                 except ImportError:
                     os.environ["QT_API"] = "pyqt5"
         elif gui == "qt6":
             try:
                 import PyQt6  # noqa
 
                 os.environ["QT_API"] = "pyqt6"
             except ImportError:
                 try:
                     import PySide6  # noqa
 
                     os.environ["QT_API"] = "pyside6"
                 except ImportError:
                     os.environ["QT_API"] = "pyqt6"
         elif gui == "qt":
             # Don't set QT_API; let IPython logic choose the version.
             if "QT_API" in os.environ.keys():
                 del os.environ["QT_API"]
         else:
             raise ValueError(
                 f'Unrecognized Qt version: {gui}. Should be "qt4", "qt5", "qt6", or "qt".'
             )
 
+    # Due to the import mechanism, we can't change Qt versions once we've chosen one. So we tag the
+    # version so we can check for this and give an error.
+    last_qt_version = gui
 
 def get_inputhook_name_and_func(gui):
     print(f'`get_inputhook_name_and_func` called with {gui=}')
     if gui in registered:
         return gui, registered[gui]
 
     if gui not in backends:
         raise UnknownBackend(gui)
 
     if gui in aliases:
         print('gui has an alias')
         return get_inputhook_name_and_func(aliases[gui])
 
     gui_mod = gui
     if gui.startswith("qt"):
         set_qt_api(gui)
         gui_mod = "qt"
 
     mod = importlib.import_module("IPython.terminal.pt_inputhooks." + gui_mod)
     return gui, mod.inputhook