matplotlib · dstansby · Jul 31, 2017 · Jul 24, 2017 · Jul 24, 2017 · Jul 25, 2017
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -674,7 +674,7 @@ def save_figure(self, *args):
 
         fname, filter = _getSaveFileName(self.parent,
                                          "Choose a filename to save to",
-                                 start, filters, selectedFilter)
+                                         start, filters, selectedFilter)
         if fname:
             if startpath == '':
                 # explicitly missing key or empty str signals to use cwd

diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
@@ -38,6 +38,15 @@ def __init__(self, figure):
         self._bbox_queue = []
         self._drawRect = None
 
+        # In cases with mixed resolution displays, we need to be careful if the
+        # dpi_ratio changes - in this case we need to resize the canvas
+        # accordingly. We could watch for screenChanged events from Qt, but
+        # the issue is that we can't guarantee this will be emitted *before*
+        # the first paintEvent for the canvas, so instead we keep track of the
+        # dpi_ratio value here and in paintEvent we resize the canvas if
+        # needed.
+        self._dpi_ratio_prev = None
+
     def drawRectangle(self, rect):
         if rect is not None:
             self._drawRect = [pt / self._dpi_ratio for pt in rect]
@@ -56,11 +65,29 @@ def paintEvent(self, e):
         In Qt, all drawing should be done inside of here when a widget is
         shown onscreen.
         """
+
         # if the canvas does not have a renderer, then give up and wait for
         # FigureCanvasAgg.draw(self) to be called
         if not hasattr(self, 'renderer'):
             return
 
+        # As described in __init__ above, we need to be careful in cases with
+        # mixed resolution displays if dpi_ratio is changing between painting
+        # events.
+        if (self._dpi_ratio_prev is None or
+            self._dpi_ratio != self._dpi_ratio_prev):
+            # We need to update the figure DPI
+            self._update_figure_dpi()
+            # 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.
+            event = QtGui.QResizeEvent(self.size(), self.size())
+            # We use self.resizeEvent here instead of QApplication.postEvent
+            # since the latter doesn't guarantee that the event will be emitted
+            # straight away, and this causes visual delays in the changes.
+            self.resizeEvent(event)
+            self._dpi_ratio_prev = self._dpi_ratio
+
         painter = QtGui.QPainter(self)
 
         if self._bbox_queue:
@@ -167,9 +194,12 @@ def __init__(self, figure):
         super(FigureCanvasQTAgg, self).__init__(figure=figure)
         # We don't want to scale up the figure DPI more than once.
         # Note, we don't handle a signal for changing DPI yet.
-        if not hasattr(self.figure, '_original_dpi'):
-            self.figure._original_dpi = self.figure.dpi
-        self.figure.dpi = self._dpi_ratio * self.figure._original_dpi
+        self.figure._original_dpi = self.figure.dpi
+        self._update_figure_dpi()
+
+    def _update_figure_dpi(self):
+        dpi = self._dpi_ratio * self.figure._original_dpi
+        self.figure._set_dpi(dpi, forward=False)
 
 
 @_BackendQT5.export

diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -418,11 +418,16 @@ def _get_axes(self):
     def _get_dpi(self):
         return self._dpi
 
-    def _set_dpi(self, dpi):
+    def _set_dpi(self, dpi, forward=True):
+        """
+        The forward kwarg is passed on to set_size_inches
+        """
         self._dpi = dpi
         self.dpi_scale_trans.clear().scale(dpi, dpi)
-        self.set_size_inches(*self.get_size_inches())
+        w, h = self.get_size_inches()
+        self.set_size_inches(w, h, forward=forward)
         self.callbacks.process('dpi_changed', self)
+
     dpi = property(_get_dpi, _set_dpi)
 
     def get_tight_layout(self):

diff --git a/lib/matplotlib/tests/test_backend_qt5.py b/lib/matplotlib/tests/test_backend_qt5.py
@@ -1,10 +1,13 @@
 from __future__ import (absolute_import, division, print_function,
                         unicode_literals)
 
+import copy
+
+import matplotlib
 from matplotlib import pyplot as plt
 from matplotlib._pylab_helpers import Gcf
-import matplotlib
-import copy
+
+from numpy.testing import assert_equal
 
 import pytest
 try:
@@ -95,3 +98,60 @@ def receive(event):
 
     qt_canvas.mpl_connect('key_press_event', receive)
     qt_canvas.keyPressEvent(event)
+
+
+@pytest.mark.backend('Qt5Agg')
+def test_dpi_ratio_change():
+    """
+    Make sure that if _dpi_ratio changes, the figure dpi changes but the
+    widget remains the same physical size.
+    """
+
+    prop = 'matplotlib.backends.backend_qt5.FigureCanvasQT._dpi_ratio'
+
+    with mock.patch(prop, new_callable=mock.PropertyMock) as p:
+
+        p.return_value = 3
+
+        fig = plt.figure(figsize=(5, 2), dpi=120)
+        qt_canvas = fig.canvas
+        qt_canvas.show()
+
+        from matplotlib.backends.backend_qt5 import qApp
+
+        # Make sure the mocking worked
+        assert qt_canvas._dpi_ratio == 3
+
+        size = qt_canvas.size()
+
+        qt_canvas.manager.show()
+        qApp.processEvents()
+
+        # The DPI and the renderer width/height change
+        assert fig.dpi == 360
+        assert qt_canvas.renderer.width == 1800
+        assert qt_canvas.renderer.height == 720
+
+        # The actual widget size and figure physical size don't change
+        assert size.width() == 200
+        assert size.height() == 80
+        assert_equal(qt_canvas.get_width_height(), (600, 240))
+        assert_equal(fig.get_size_inches(), (5, 2))
+
+        p.return_value = 2
+
+        assert qt_canvas._dpi_ratio == 2
+
+        qt_canvas.draw()
+        qApp.processEvents()
+
+        # The DPI and the renderer width/height change
+        assert fig.dpi == 240
+        assert qt_canvas.renderer.width == 1200
+        assert qt_canvas.renderer.height == 480
+
+        # The actual widget size and figure physical size don't change
+        assert size.width() == 200
+        assert size.height() == 80
+        assert_equal(qt_canvas.get_width_height(), (600, 240))
+        assert_equal(fig.get_size_inches(), (5, 2))