FIX: be more forgiving about expecting internal state in draw_idle

Avoid race conditions with re-initing a new Qt5Agg canvas when:
 - in interactive mode
 - in plain python prompt
 - creating a second Qt5Agg canvas for an existing figure

closes matplotlib#15205

Author: tacaswell

lib/matplotlib/backends/backend_qt5.py

@@ -476,7 +476,8 @@ 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)
 

lib/matplotlib/tests/test_backend_qt.py

@@ -293,3 +293,23 @@ def test_double_resize():
     fig.set_size_inches(w, h)
     assert window.width() == old_width
     assert window.height() == old_height
+
+
+@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