Shaperilio/qtgui fixes (#13957)

I started using the released version of my `PySide6`-enabling changes
and noted some problems. In this PR, I fix those, and also overall
improve the feedback to the user when a GUI event loop is hooked in:

- Report which event loop is running when using `%gui <some GUI>`; e.g.
`%gui qt` will show `Installed qt6 event loop hook.`
- Report when the event loop is disabled; i.e. `%gui` will show `GUI
event loop hook disabled.` if an event loop hook was installed, or `No
event loop hook running.` if nothing was installed.
- Requesting a second event loop will give the message `Shell is already
running a gui event loop for <some GUI>. Call with no arguments to
disable current loop.`
- Requesting a different version of Qt, i.e. `%gui qt6` followed by
`%gui` followed by `%gui qt5` will show `Cannot switch Qt versions for
this session; will use qt6.` followed by `Installed qt6 event loop
hook.`

(Fixes / improves  #13864)

IPython/external/qt_loaders.py

@@ -10,6 +10,7 b' be accessed directly from the outside'
 """
 import importlib.abc
 import sys
+import os
 import types
 from functools import partial, lru_cache
 import operator
@@ -368,6 +369,10 b' def load_qt(api_options):'
         commit_api(api)
         return result
     else:
+        # Clear the environment variable since it doesn't work.
+        if "QT_API" in os.environ:
+            del os.environ["QT_API"]
+
         raise ImportError(
             """
     Could not load requested Qt binding. Please ensure that

IPython/terminal/interactiveshell.py

@@ -913,10 +913,19 b' class TerminalInteractiveShell(InteractiveShell):'
 
     active_eventloop = None
     def enable_gui(self, gui=None):
+        if self._inputhook is None and gui is None:
+            print("No event loop hook running.")
+            return
+
         if self._inputhook is not None and gui is not None:
-            warn(
-                f"Shell was already running a gui event loop for {self.active_eventloop}; switching to {gui}."
+            print(
+                f"Shell is already running a gui event loop for {self.active_eventloop}. "
+                "Call with no arguments to disable the current loop."
             )
+            return
+        if self._inputhook is not None and gui is None:
+            self.active_eventloop = self._inputhook = 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)
@@ -934,15 +943,18 b' class TerminalInteractiveShell(InteractiveShell):'
                 # same event loop as the rest of the code. don't use an actual
                 # input hook. (Asyncio is not made for nesting event loops.)
                 self.pt_loop = get_asyncio_loop()
+                print("Installed asyncio event loop hook.")
 
             elif self._inputhook:
                 # If an inputhook was set, create a new asyncio event loop with
                 # this inputhook for the prompt.
                 self.pt_loop = new_eventloop_with_inputhook(self._inputhook)
+                print(f"Installed {self.active_eventloop} event loop hook.")
             else:
                 # When there's no inputhook, run the prompt in a separate
                 # asyncio event loop.
                 self.pt_loop = asyncio.new_event_loop()
+                print("GUI event loop hook disabled.")
 
     # Run !system commands directly, not through pipes, so terminal programs
     # work correctly.

IPython/terminal/pt_inputhooks/__init__.py

@@ -69,9 +69,9 b' def set_qt_api(gui):'
     if loaded is not None and gui != "qt":
         if qt_env2gui[loaded] != gui:
             print(
-                f"Cannot switch Qt versions for this session; must use {qt_env2gui[loaded]}."
+                f"Cannot switch Qt versions for this session; will use {qt_env2gui[loaded]}."
             )
-            return
+            return qt_env2gui[loaded]
 
     if qt_api is not None and gui != "qt":
         if qt_env2gui[qt_api] != gui:
@@ -79,6 +79,7 b' def set_qt_api(gui):'
                 f'Request for "{gui}" will be ignored because `QT_API` '
                 f'environment variable is set to "{qt_api}"'
             )
+            return qt_env2gui[qt_api]
     else:
         if gui == "qt5":
             try:
@@ -112,6 +113,11 b' def set_qt_api(gui):'
             print(f'Unrecognized Qt version: {gui}. Should be "qt5", "qt6", or "qt".')
             return
 
+        # Import it now so we can figure out which version it is.
+        from IPython.external.qt_for_kernel import QT_API
+
+        return qt_env2gui[QT_API]
+
 
 def get_inputhook_name_and_func(gui):
     if gui in registered:
@@ -125,7 +131,7 b' def get_inputhook_name_and_func(gui):'
 
     gui_mod = gui
     if gui.startswith("qt"):
-        set_qt_api(gui)
+        gui = set_qt_api(gui)
         gui_mod = "qt"
 
     mod = importlib.import_module("IPython.terminal.pt_inputhooks." + gui_mod)

IPython/terminal/tests/test_pt_inputhooks.py

@@ -33,18 +33,18 b' _get_qt_vers()'
     len(guis_avail) == 0, reason="No viable version of PyQt or PySide installed."
 )
 def test_inputhook_qt():
-    gui = guis_avail[0]
-
-    # Choose a qt version and get the input hook function. This will import Qt...
-    get_inputhook_name_and_func(gui)
-
-    # ...and now we're stuck with this version of Qt for good; can't switch.
-    for not_gui in ["qt6", "qt5"]:
-        if not_gui not in guis_avail:
-            break
-
-    with pytest.raises(ImportError):
-        get_inputhook_name_and_func(not_gui)
-
-    # A gui of 'qt' means "best available", or in this case, the last one that was used.
-    get_inputhook_name_and_func("qt")
+    # Choose the "best" Qt version.
+    gui_ret, _ = get_inputhook_name_and_func("qt")
+
+    assert gui_ret != "qt"  # you get back the specific version that was loaded.
+    assert gui_ret in guis_avail
+
+    if len(guis_avail) > 2:
+        # ...and now we're stuck with this version of Qt for good; can't switch.
+        for not_gui in ["qt6", "qt5"]:
+            if not_gui != gui_ret:
+                break
+        # Try to import the other gui; it won't work.
+        gui_ret2, _ = get_inputhook_name_and_func(not_gui)
+        assert gui_ret2 == gui_ret
+        assert gui_ret2 != not_gui