From 3ba761f325f4c6ecc4f12ef324fb1d74d7aa5f6d Mon Sep 17 00:00:00 2001
From: David Stansby <dstansby@gmail.com>
Date: Fri, 6 Sep 2019 23:25:22 +0100
Subject: [PATCH] Backport PR #15206: FIX: be more forgiving about expecting
 internal state in draw_idle

Merge pull request #15206 from tacaswell/fix_python_ion_second_qtcanvas_init

FIX: be more forgiving about expecting internal state in draw_idle
Conflicts:
	lib/matplotlib/tests/test_backend_qt.py
          - two tests were added on master, only kept the relevant one
---
 lib/matplotlib/backends/backend_qt5.py  |  3 ++-
 lib/matplotlib/tests/test_backend_qt.py | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
index 1e60ec14dab9..d7a4fd0f0ca0 100644
--- a/lib/matplotlib/backends/backend_qt5.py
+++ b/lib/matplotlib/backends/backend_qt5.py
@@ -492,7 +492,8 @@ def draw_idle(self):
         # current event loop in order to ensure thread affinity and to
         # accumulate multiple draw requests from event handling.
         # TODO: queued signal connection might be safer than singleShot
-        if not (self._draw_pending or self._is_drawing):
+        if not (getattr(self, '_draw_pending', False) or
+                getattr(self, '._is_drawing', False)):
             self._draw_pending = True
             QtCore.QTimer.singleShot(0, self._draw_idle)
 
diff --git a/lib/matplotlib/tests/test_backend_qt.py b/lib/matplotlib/tests/test_backend_qt.py
index f6493cfb9c38..0d4f1bec56a6 100644
--- a/lib/matplotlib/tests/test_backend_qt.py
+++ b/lib/matplotlib/tests/test_backend_qt.py
@@ -307,3 +307,23 @@ def test_figureoptions():
             "matplotlib.backends.qt_editor._formlayout.FormDialog.exec_",
             lambda self: None):
         fig.canvas.manager.toolbar.edit_parameters()
+
+
+@pytest.mark.backend("Qt5Agg")
+def test_canvas_reinit():
+    import matplotlib.pyplot as plt
+    from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg
+    from functools import partial
+
+    called = False
+
+    def crashing_callback(fig, stale):
+        nonlocal called
+        fig.canvas.draw_idle()
+        called = True
+
+    fig, ax = plt.subplots()
+    fig.stale_callback = crashing_callback
+    # this should not raise
+    canvas = FigureCanvasQTAgg(fig)
+    assert called