MNT: add warning if crossing Qt major versions

Author: tacaswell

lib/matplotlib/backends/backend_qt.py

@@ -114,6 +114,28 @@ def _create_qApp():
                     QtCore.Qt.AA_EnableHighDpiScaling)
             except AttributeError:  # Only for Qt>=5.6, <6.
                 pass
+
+            # Check to make sure a QApplication from a different major version
+            # of Qt is not instantiated in the process
+            if QT_API in {'PyQt6', 'PySide6'}:
+                other_bindings = ('PyQt5', 'PySide2')
+            elif QT_API in {'PyQt5', 'PySide2'}:
+                other_bindings = ('PyQt6', 'PySide6')
+            else:
+                raise RuntimeError("Should never be here")
+
+            for binding in other_bindings:
+                mod = sys.modules.get(f'{binding}.QtWidgets')
+                if mod is not None and mod.QApplication.instance() is not None:
+                    other_core = sys.modules.get(f'{binding}.QtCore')
+                    _api.warn_external(
+                        f'Matplotlib is using {QT_API} which wraps '
+                        f'{QtCore.qVersion()} however an instantiated '
+                        f'QApplication from {binding} which wraps '
+                        f'{other_core.qVersion()} exists.  Mixing Qt major '
+                        'versions may not work as expected.'
+                    )
+                    break
             try:
                 QtWidgets.QApplication.setHighDpiScaleFactorRoundingPolicy(
                     QtCore.Qt.HighDpiScaleFactorRoundingPolicy.PassThrough)

lib/matplotlib/tests/test_backends_interactive.py

@@ -23,6 +23,7 @@ def _run_function_in_subprocess(func):
     return f"{func_source}\n{func.__name__}()"
 
 
+
 # Minimal smoke-testing of the backends for which the dependencies are
 # PyPI-installable on CI.  They are not available for all tested Python
 # versions so we don't fail on missing backends.
@@ -344,6 +345,50 @@ def _implcore():
         stdout=subprocess.PIPE, universal_newlines=True)
 
 
+def _impl_test_cross_Qt_imports():
+    import sys
+    import importlib
+    import pytest
+
+    _, host_binding, mpl_binding = sys.argv
+    # import the mpl binding.  This will force us to use that binding
+    importlib.import_module(f'{mpl_binding}.QtCore')
+    mpl_binding_qwidgets = importlib.import_module(f'{mpl_binding}.QtWidgets')
+    import matplotlib.backends.backend_qt
+    host_qwidgets = importlib.import_module(f'{host_binding}.QtWidgets')
+
+    host_app = host_qwidgets.QApplication(["mpl testing"])
+    with pytest.warns(UserWarning, match="Mixing Qt major"):
+        matplotlib.backends.backend_qt._create_qApp()
+
+
+def test_cross_Qt_imports():
+    qt5_bindings = [
+        dep for dep in ['PyQt5', 'PySide2']
+        if importlib.util.find_spec(dep) is not None
+    ]
+    qt6_bindings = [
+        dep for dep in ['PyQt6', 'PySide6']
+        if importlib.util.find_spec(dep) is not None
+    ]
+    if len(qt5_bindings) == 0 or len(qt6_bindings) == 0:
+        pytest.skip('need both QT6 and QT5 bindings')
+
+    for qt5 in qt5_bindings:
+        for qt6 in qt6_bindings:
+            for pair in ([qt5, qt6], [qt6, qt5]):
+                try:
+                    _run_helper(__name__, _impl_test_cross_Qt_imports,
+                                *pair,
+                                timeout=_test_timeout)
+                except subprocess.CalledProcessError as ex:
+                    # if segfauldt, carry on.  We do try to warn the user they
+                    # are doing something that we do not expect to work
+                    if ex.returncode == -11:
+                        continue
+                    raise
+
+
 @pytest.mark.skipif('TF_BUILD' in os.environ,
                     reason="this test fails an azure for unknown reasons")
 @pytest.mark.skipif(os.name == "nt", reason="Cannot send SIGINT on Windows.")