Drop support for Qt<5.10.

... and update GUI minver policy.

Author: anntzer

doc/api/next_api_changes/development/24710-AL.rst

@@ -0,0 +1,4 @@
+Support for Qt<5.10 has been dropped
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... as there are no wheels or conda packages that support both Qt 5.9 (or
+older) and Python 3.8 (or newer).

doc/devel/min_dep_policy.rst

@@ -71,6 +71,10 @@ Ghostscript, FFmpeg) support as old as practical.  These can be difficult to
 install for end-users and we want to be usable on as many systems as
 possible.  We will bump these on a case-by-case basis.
 
+In the case of GUI frameworks for which we rely on Python bindings being
+available, we will also drop support for bindings so old that they don't
+support any Python version that we support.
+
 .. _list-of-dependency-min-versions:
 
 List of dependency versions

lib/matplotlib/backends/backend_qt.py

@@ -14,9 +14,7 @@
 from . import qt_compat
 from .qt_compat import (
     QtCore, QtGui, QtWidgets, __version__, QT_API,
-    _enum, _to_int,
-    _devicePixelRatioF, _isdeleted, _setDevicePixelRatio,
-    _maybe_allow_interrupt
+    _enum, _to_int, _isdeleted, _maybe_allow_interrupt
 )
 
 
@@ -220,7 +218,8 @@ def __init__(self, figure=None):
         self.setPalette(palette)
 
     def _update_pixel_ratio(self):
-        if self._set_device_pixel_ratio(_devicePixelRatioF(self)):
+        if self._set_device_pixel_ratio(
+                self.devicePixelRatioF() or 1):  # rarely, devicePixelRatioF=0
             # The easiest way to resize the canvas is to emit a resizeEvent
             # since we implement all the logic for resizing the canvas for
             # that event.
@@ -677,7 +676,8 @@ def _icon(self, name):
         filename = str(path_large if path_large.exists() else path_regular)
 
         pm = QtGui.QPixmap(filename)
-        _setDevicePixelRatio(pm, _devicePixelRatioF(self))
+        pm.setDevicePixelRatio(
+            self.devicePixelRatioF() or 1)  # rarely, devicePixelRatioF=0
         if self.palette().color(self.backgroundRole()).value() < 128:
             icon_color = self.palette().color(self.foregroundRole())
             mask = pm.createMaskFromColor(

lib/matplotlib/backends/backend_qtagg.py

@@ -6,8 +6,7 @@
 
 from matplotlib.transforms import Bbox
 
-from .qt_compat import QT_API, _enum, _setDevicePixelRatio
-from .. import cbook
+from .qt_compat import QT_API, _enum
 from .backend_agg import FigureCanvasAgg
 from .backend_qt import QtCore, QtGui, _BackendQT, FigureCanvasQT
 from .backend_qt import (  # noqa: F401 # pylint: disable=W0611
@@ -50,22 +49,16 @@ def paintEvent(self, event):
             bbox = Bbox([[left, bottom], [right, top]])
             buf = memoryview(self.copy_from_bbox(bbox))
 
-            fmts = _enum("QtGui.QImage.Format")
-            if hasattr(fmts, "Format_RGBA8888"):
-                fmt = fmts.Format_RGBA8888
-            else:  # Qt<=5.1 support.
-                fmt = fmts.Format_ARGB32_Premultiplied
-                buf = cbook._unmultiplied_rgba8888_to_premultiplied_argb32(buf)
-
             if QT_API == "PyQt6":
                 from PyQt6 import sip
                 ptr = int(sip.voidptr(buf))
             else:
                 ptr = buf
 
             painter.eraseRect(rect)  # clear the widget canvas
-            qimage = QtGui.QImage(ptr, buf.shape[1], buf.shape[0], fmt)
-            _setDevicePixelRatio(qimage, self.device_pixel_ratio)
+            qimage = QtGui.QImage(ptr, buf.shape[1], buf.shape[0],
+                                  _enum("QtGui.QImage.Format").Format_RGBA8888)
+            qimage.setDevicePixelRatio(self.device_pixel_ratio)
             # set origin using original QT coordinates
             origin = QtCore.QPoint(rect.left(), rect.top())
             painter.drawImage(origin, qimage)

lib/matplotlib/backends/backend_qtcairo.py

@@ -2,7 +2,7 @@
 
 from .backend_cairo import cairo, FigureCanvasCairo
 from .backend_qt import QtCore, QtGui, _BackendQT, FigureCanvasQT
-from .qt_compat import QT_API, _enum, _setDevicePixelRatio
+from .qt_compat import QT_API, _enum
 
 
 class FigureCanvasQTCairo(FigureCanvasCairo, FigureCanvasQT):
@@ -31,10 +31,9 @@ def paintEvent(self, event):
             _enum("QtGui.QImage.Format").Format_ARGB32_Premultiplied)
         # Adjust the buf reference count to work around a memory leak bug in
         # QImage under PySide.
-        if QT_API in ('PySide', 'PySide2'):
-            if QtCore.__version_info__ < (5, 12):
-                ctypes.c_long.from_address(id(buf)).value = 1
-        _setDevicePixelRatio(qimage, self.device_pixel_ratio)
+        if QT_API == "PySide2" and QtCore.__version_info__ < (5, 12):
+            ctypes.c_long.from_address(id(buf)).value = 1
+        qimage.setDevicePixelRatio(self.device_pixel_ratio)
         painter = QtGui.QPainter(self)
         painter.eraseRect(event.rect())
         painter.drawImage(0, 0, qimage)

lib/matplotlib/backends/qt_compat.py

@@ -125,7 +125,6 @@ def _isdeleted(obj):
             (_setup_pyqt5plus, QT_API_PYQT5),
             (_setup_pyqt5plus, QT_API_PYSIDE2),
         ]
-
     for _setup, QT_API in _candidates:
         try:
             _setup()
@@ -138,13 +137,21 @@ def _isdeleted(obj):
             .format(", ".join(_ETS.values())))
 else:  # We should not get there.
     raise AssertionError(f"Unexpected QT_API: {QT_API}")
+_version_info = tuple(QtCore.QLibraryInfo.version().segments())
+
+
+if _version_info < (5, 10):
+    raise ImportError(
+        f"The Qt version imported is "
+        f"{QtCore.QLibraryInfo.version().toString()} but Matplotlib requires "
+        f"Qt>=5.10")
 
 
 # Fixes issues with Big Sur
 # https://bugreports.qt.io/browse/QTBUG-87014, fixed in qt 5.15.2
 if (sys.platform == 'darwin' and
         parse_version(platform.mac_ver()[0]) >= parse_version("10.16") and
-        QtCore.QLibraryInfo.version().segments() <= [5, 15, 2]):
+        _version_info < (5, 15, 2)):
     os.environ.setdefault("QT_MAC_WANTS_LAYER", "1")
 
 
@@ -167,36 +174,6 @@ def _exec(obj):
     obj.exec() if hasattr(obj, "exec") else obj.exec_()
 
 
-def _devicePixelRatioF(obj):
-    """
-    Return obj.devicePixelRatioF() with graceful fallback for older Qt.
-
-    This can be replaced by the direct call when we require Qt>=5.6.
-    """
-    try:
-        # Not available on Qt<5.6
-        return obj.devicePixelRatioF() or 1
-    except AttributeError:
-        pass
-    try:
-        # Not available on older Qt5.
-        # self.devicePixelRatio() returns 0 in rare cases
-        return obj.devicePixelRatio() or 1
-    except AttributeError:
-        return 1
-
-
-def _setDevicePixelRatio(obj, val):
-    """
-    Call obj.setDevicePixelRatio(val) with graceful fallback for older Qt.
-
-    This can be replaced by the direct call when we require Qt>=5.6.
-    """
-    if hasattr(obj, 'setDevicePixelRatio'):
-        # Not available on older Qt5.
-        obj.setDevicePixelRatio(val)
-
-
 @contextlib.contextmanager
 def _maybe_allow_interrupt(qapp):
     """