ENH: support for `PySide6` in `%gui` (#13864)

Addresses  #13859

Changes here parallel the changes in
[ipykernel](https://github.com/ipython/ipykernel/pull/1054), i.e. prefer
`PyQt` over `PySide` and allow requesting explicit versions (e.g. 'qt5')
with one difference (see below).

I believe that, eventually, the Qt importing logic should be all here,
and `ipykernel` should defer to`ipython`. I chose not to do that at this
time since it would mean the latest `ipykernel` would require the latest
`IPython`; I'm happy to discuss further.

The only difference between `IPython` and `ipykernel`, after these two
pull requests, is that, in `IPython`, it's not possible to explicitly
request an event loop for Qt4. This is because an alias exists
[here](https://github.com/ipython/ipython/blob/5409de68d87ddd073a35111aca0cb8360ff63ca8/IPython/terminal/pt_inputhooks/__init__.py#L5)
which effectively makes "qt4" be "the latest Qt available". I did not
remove the alias because I don't know the history behind it.

IPython/terminal/tests/test_pt_inputhooks.py

@@ -0,0 +1,50 b''
+import os
+import importlib
+
+import pytest
+
+from IPython.terminal.pt_inputhooks import set_qt_api, get_inputhook_name_and_func
+
+
+guis_avail = []
+
+
+def _get_qt_vers():
+    """If any version of Qt is available, this will populate `guis_avail` with 'qt' and 'qtx'. Due
+    to the import mechanism, we can't import multiple versions of Qt in one session."""
+    for gui in ["qt", "qt6", "qt5"]:
+        print(f"Trying {gui}")
+        try:
+            set_qt_api(gui)
+            importlib.import_module("IPython.terminal.pt_inputhooks.qt")
+            guis_avail.append(gui)
+            if "QT_API" in os.environ.keys():
+                del os.environ["QT_API"]
+        except ImportError:
+            pass  # that version of Qt isn't available.
+        except RuntimeError:
+            pass  # the version of IPython doesn't know what to do with this Qt version.
+
+
+_get_qt_vers()
+
+
[e-mail](
+    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")

IPython/core/magics/basic.py

@@ -493,8 +493,10 b' Currently the magic system has the following functions:""",'
         are supported:  wxPython, PyQt4, PyGTK, Tk and Cocoa (OSX)::
 
             %gui wx      # enable wxPython event loop integration
-            %gui qt4|qt  # enable PyQt4 event loop integration
-            %gui qt5     # enable PyQt5 event loop integration
+            %gui qt      # enable PyQt/PySide event loop integration
+                         # with the latest version available.
+            %gui qt6     # enable PyQt6/PySide6 event loop integration
+            %gui qt5     # enable PyQt5/PySide2 event loop integration
             %gui gtk     # enable PyGTK event loop integration
             %gui gtk3    # enable Gtk3 event loop integration
             %gui gtk4    # enable Gtk4 event loop integration

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
@@ -42,7 +45,6 b' from IPython.external.qt_loaders import ('
     QT_API_PYQT5,
     QT_API_PYSIDE2,
     # QT4
-    QT_API_PYQTv1,
     QT_API_PYQT,
     QT_API_PYSIDE,
     # default
@@ -56,10 +58,6 b' _qt_apis = ('
     # QT5
     QT_API_PYQT5,
     QT_API_PYSIDE2,
-    # QT4
-    QT_API_PYQTv1,
-    QT_API_PYQT,
-    QT_API_PYSIDE,
     # default
     QT_API_PYQT_DEFAULT,
 )
@@ -98,7 +96,7 b' def get_options():'
     if loaded is not None:
         return [loaded]
 
-    mpl = sys.modules.get('matplotlib', None)
+    mpl = sys.modules.get("matplotlib", None)
 
     if mpl is not None and tuple(mpl.__version__.split(".")) < ("1", "0", "2"):
         # 1.0.1 only supports PyQt4 v1
@@ -113,8 +111,6 b' def get_options():'
             QT_API_PYSIDE6,
             QT_API_PYQT5,
             QT_API_PYSIDE2,
-            QT_API_PYQT,
-            QT_API_PYSIDE,
         ]
     elif qt_api not in _qt_apis:
         raise RuntimeError("Invalid Qt API %r, valid values are: %r" %

IPython/external/qt_loaders.py

@@ -24,6 +24,7 b" QT_API_PYQT5 = 'pyqt5'"
 QT_API_PYSIDE2 = 'pyside2'
 
 # Qt4
+# NOTE: Here for legacy matplotlib compatibility, but not really supported on the IPython side.
 QT_API_PYQT = "pyqt"  # Force version 2
 QT_API_PYQTv1 = "pyqtv1"  # Force version 2
 QT_API_PYSIDE = "pyside"
@@ -367,23 +368,28 b' def load_qt(api_options):'
         commit_api(api)
         return result
     else:
-        raise ImportError("""
+        raise ImportError(
+            """
     Could not load requested Qt binding. Please ensure that
-    PyQt4 >= 4.7, PyQt5, PySide >= 1.0.3 or PySide2 is available,
-    and only one is imported per session.
+    PyQt4 >= 4.7, PyQt5, PyQt6, PySide >= 1.0.3, PySide2, or
+    PySide6 is available, and only one is imported per session.
 
     Currently-imported Qt library:                              %r
-    PyQt4 available (requires QtCore, QtGui, QtSvg):            %s
     PyQt5 available (requires QtCore, QtGui, QtSvg, QtWidgets): %s
-    PySide >= 1.0.3 installed:                                  %s
+    PyQt6 available (requires QtCore, QtGui, QtSvg, QtWidgets): %s
     PySide2 installed:                                          %s
+    PySide6 installed:                                          %s
     Tried to load:                                              %r
-    """ % (loaded_api(),
-           has_binding(QT_API_PYQT),
-           has_binding(QT_API_PYQT5),
-           has_binding(QT_API_PYSIDE),
-           has_binding(QT_API_PYSIDE2),
-           api_options))
+    """
+            % (
+                loaded_api(),
+                has_binding(QT_API_PYQT5),
+                has_binding(QT_API_PYQT6),
+                has_binding(QT_API_PYSIDE2),
+                has_binding(QT_API_PYSIDE6),
+                api_options,
+            )
+        )
 
 
 def enum_factory(QT_API, QtCore):

IPython/lib/guisupport.py

@@ -106,21 +106,21 b' def start_event_loop_wx(app=None):'
         app._in_event_loop = True
 
 #-----------------------------------------------------------------------------
-# qt4
+# Qt
 #-----------------------------------------------------------------------------
 
 def get_app_qt4(*args, **kwargs):
-    """Create a new qt4 app or return an existing one."""
+    """Create a new Qt app or return an existing one."""
     from IPython.external.qt_for_kernel import QtGui
     app = QtGui.QApplication.instance()
     if app is None:
         if not args:
-            args = ([''],)
+            args = ([""],)
         app = QtGui.QApplication(*args, **kwargs)
     return app
 
 def is_event_loop_running_qt4(app=None):
-    """Is the qt4 event loop running."""
+    """Is the qt event loop running."""
     # New way: check attribute on shell instance
     ip = get_ipython()
     if ip is not None:
@@ -128,17 +128,17 b' def is_event_loop_running_qt4(app=None):'
 
     # Old way: check attribute on QApplication singleton
     if app is None:
-        app = get_app_qt4([''])
+        app = get_app_qt4([""])
     if hasattr(app, '_in_event_loop'):
         return app._in_event_loop
     else:
-        # Does qt4 provide a other way to detect this?
+        # Does qt provide a other way to detect this?
         return False
 
 def start_event_loop_qt4(app=None):
-    """Start the qt4 event loop in a consistent manner."""
+    """Start the qt event loop in a consistent manner."""
     if app is None:
-        app = get_app_qt4([''])
+        app = get_app_qt4([""])
     if not is_event_loop_running_qt4(app):
         app._in_event_loop = True
         app.exec_()

IPython/terminal/interactiveshell.py

@@ -911,7 +911,12 b' class TerminalInteractiveShell(InteractiveShell):'
 
     active_eventloop = None
     def enable_gui(self, gui=None):
+        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}."
+            )
         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

@@ -8,7 +8,6 b' aliases = {'
 
 backends = [
     "qt",
-    "qt4",
     "qt5",
     "qt6",
     "gtk",
@@ -40,6 +39,80 b' class UnknownBackend(KeyError):'
                                     ', '.join(backends + sorted(registered)))
 
 
+def set_qt_api(gui):
+    """Sets the `QT_API` environment variable if it isn't already set."""
+
+    qt_api = os.environ.get("QT_API", None)
+
+    from IPython.external.qt_loaders import (
+        QT_API_PYQT,
+        QT_API_PYQT5,
+        QT_API_PYQT6,
+        QT_API_PYSIDE,
+        QT_API_PYSIDE2,
+        QT_API_PYSIDE6,
+        QT_API_PYQTv1,
+        loaded_api,
+    )
+
+    loaded = loaded_api()
+
+    qt_env2gui = {
+        QT_API_PYSIDE: "qt4",
+        QT_API_PYQTv1: "qt4",
+        QT_API_PYQT: "qt4",
+        QT_API_PYSIDE2: "qt5",
+        QT_API_PYQT5: "qt5",
+        QT_API_PYSIDE6: "qt6",
+        QT_API_PYQT6: "qt6",
+    }
+    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]}."
+            )
+            return
+
+    if qt_api is not None and gui != "qt":
+        if qt_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 == "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:
+            print(f'Unrecognized Qt version: {gui}. Should be "qt5", "qt6", or "qt".')
+            return
+
+
 def get_inputhook_name_and_func(gui):
     if gui in registered:
         return gui, registered[gui]
@@ -51,12 +124,9 b' def get_inputhook_name_and_func(gui):'
         return get_inputhook_name_and_func(aliases[gui])
 
     gui_mod = gui
-    if gui == "qt5":
-        os.environ["QT_API"] = "pyqt5"
-        gui_mod = "qt"
-    elif gui == "qt6":
-        os.environ["QT_API"] = "pyqt6"
+    if gui.startswith("qt"):
+        set_qt_api(gui)
         gui_mod = "qt"
 
-    mod = importlib.import_module('IPython.terminal.pt_inputhooks.'+gui_mod)
+    mod = importlib.import_module("IPython.terminal.pt_inputhooks." + gui_mod)
     return gui, mod.inputhook

IPython/terminal/pt_inputhooks/qt.py

@@ -63,6 +63,7 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()

docs/source/config/eventloops.rst

@@ -7,7 +7,7 b' loop, so you can use both a GUI and an interactive prompt together. IPython'
 supports a number of common GUI toolkits, but from IPython 3.0, it is possible
 to integrate other event loops without modifying IPython itself.
 
-Supported event loops include ``qt4``, ``qt5``, ``gtk2``, ``gtk3``, ``gtk4``,
+Supported event loops include ``qt5``, ``qt6``, ``gtk2``, ``gtk3``, ``gtk4``,
 ``wx``, ``osx`` and ``tk``. Make sure the event loop you specify matches the
 GUI toolkit used by your own code.
 

docs/source/interactive/reference.rst

@@ -44,7 +44,7 b' the command-line by passing the full class name and a corresponding value; type'
     <...snip...>
     --matplotlib=<CaselessStrEnum> (InteractiveShellApp.matplotlib)
         Default: None
-        Choices: ['auto', 'gtk', 'gtk3', 'gtk4', 'inline', 'nbagg', 'notebook', 'osx', 'qt', 'qt4', 'qt5', 'tk', 'wx']
+        Choices: ['auto', 'gtk', 'gtk3', 'gtk4', 'inline', 'nbagg', 'notebook', 'osx', 'qt', 'qt5', 'qt6', 'tk', 'wx']
         Configure matplotlib for interactive use with the default matplotlib
         backend.
     <...snip...>
@@ -892,7 +892,7 b' GUI event loop support'
 ======================
 
 IPython has excellent support for working interactively with Graphical User
-Interface (GUI) toolkits, such as wxPython, PyQt4/PySide, PyGTK and Tk. This is
+Interface (GUI) toolkits, such as wxPython, PyQt/PySide, PyGTK and Tk. This is
 implemented by running the toolkit's event loop while IPython is waiting for
 input.
 
@@ -902,7 +902,7 b' For users, enabling GUI event loop integration is simple. You simple use the'
     %gui [GUINAME]
 
 With no arguments, ``%gui`` removes all GUI support.  Valid ``GUINAME``
-arguments include ``wx``, ``qt``, ``qt5``, ``gtk``, ``gtk3`` ``gtk4``, and
+arguments include ``wx``, ``qt``, ``qt5``, ``qt6``, ``gtk``, ``gtk3`` ``gtk4``, and
 ``tk``.
 
 Thus, to use wxPython interactively and create a running :class:`wx.App`
@@ -936,16 +936,9 b' PyQt and PySide'
 .. attempt at explanation of the complete mess that is Qt support
 
 When you use ``--gui=qt`` or ``--matplotlib=qt``, IPython can work with either
-PyQt4 or PySide.  There are three options for configuration here, because
-PyQt4 has two APIs for QString and QVariant: v1, which is the default on
-Python 2, and the more natural v2, which is the only API supported by PySide.
-v2 is also the default for PyQt4 on Python 3.  IPython's code for the QtConsole
-uses v2, but you can still use any interface in your code, since the
-Qt frontend is in a different process.
-
-The default will be to import PyQt4 without configuration of the APIs, thus
-matching what most applications would expect. It will fall back to PySide if
-PyQt4 is unavailable.
+PyQt or PySide.  ``qt`` implies "use the latest version available", and it favors
+PyQt over PySide. To request a specific version, use ``qt5`` or ``qt6``. Note that
+Qt4 is not supported with the ``--gui`` switch (and has not been for some time now).
 
 If specified, IPython will respect the environment variable ``QT_API`` used
 by ETS.  ETS 4.0 also works with both PyQt4 and PySide, but it requires