Closing a matplotlib figure with event handling occasionally causes “TypeError: isinstance()”

[mapfiable]

Bug report

Bug summary

Sometimes, when closing a window containing a matplotlib figure with active event handling, a “TypeError: isinstance()” is raised (confirmed when using the 'X'-button to close). I was able to raise the error about every other time by moving the cursor over the figure or clicking on it and quickly closing the window afterwards. So far, I found that this only happens if at least any two of the event handling connections are active.

Code for reproduction

import sys
import matplotlib.pyplot as plt
from matplotlib.backends.backend_qt5agg import \
    FigureCanvasQTAgg as FigureCanvas

from PyQt5.QtWidgets import QDialog, QApplication, QGridLayout


class Figure:
    def __init__(self):
        self.fig, self.ax = plt.subplots()
        self.canvas = FigureCanvas(self.fig)
        self.canvas.setMaximumHeight(100)

        self.fig.canvas.mpl_connect('pick_event', self.on_pick_event)
        self.fig.canvas.mpl_connect(
            'button_release_event', self.on_release_event
        )
        self.fig.canvas.mpl_connect(
            'button_press_event', self.on_button_press_event
        )
        self.fig.canvas.mpl_connect(
            'motion_notify_event', self.on_motion_event
        )

        self.canvas.draw()

    def on_pick_event(self, _):
        # print('picked')
        pass

    def on_button_press_event(self, _):
        # print('pressed')
        pass

    def on_release_event(self, _):
        # print('released')
        pass

    def on_motion_event(self, _):
        # print('moved')
        pass


class MainWindow(QDialog):
    def __init__(self):
        super().__init__()
        self.layout = QGridLayout(self)
        self.image = Figure()
        self.layout.addWidget(self.image.canvas, 1, 0)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    GUI = MainWindow()
    GUI.show()
    sys.exit(app.exec_())

Actual outcome

Exception ignored in: <function WeakMethod.__new__.<locals>._cb at 0x00000193A3D7C7B8>
Traceback (most recent call last):
  File "C:\Users\mapf\Anaconda3\lib\weakref.py", line 58, in _cb
  File "C:\Users\mapf\Anaconda3\lib\site-packages\matplotlib\cbook\__init__.py", line 182, in _remove_proxy
  File "C:\Users\mapf\Anaconda3\lib\weakref.py", line 74, in __eq__
TypeError: isinstance() arg 2 must be a type or tuple of types 

Expected outcome

No errors.

Matplotlib version

• Operating system: Windows 10 Home 64bit
• Matplotlib version: 3.0.0 (via anaconda default installation)
• Matplotlib backend: Qt5Agg
• Python version: 3.5.6

[anntzer]

Does adding if sys.is_finalizing(): return at the very top of matplotlib.cbook.CallbackRegistry._remove_proxy fix the issue?

[mapfiable]

In a way I guess it does. However now I get the error message:

Exception ignored in: <function WeakMethod.__new__.<locals>._cb at 0x0000019262058B70> Traceback (most recent call last): File "C:\Users\mapfu\Anaconda3\lib\weakref.py", line 58, in _cb File "C:\Users\mapfu\Anaconda3\lib\site-packages\matplotlib\cbook\__init__.py", line 180, in _remove_proxy AttributeError: 'NoneType' object has no attribute 'is_finalizing'

I guess this is what is happening when it would otherwise close normally?

[anntzer]

ok, let's try something slightly better?

diff --git i/lib/matplotlib/cbook/__init__.py w/lib/matplotlib/cbook/__init__.py
index 125dbe912..496bd63eb 100644
--- i/lib/matplotlib/cbook/__init__.py
+++ w/lib/matplotlib/cbook/__init__.py
@@ -152,7 +152,12 @@ class CallbackRegistry:
         self.callbacks[s][cid] = proxy
         return cid
 
-    def _remove_proxy(self, proxy):
+    # Keep a reference to sys.is_finalizing, as sys may have been cleared out
+    # at that point.
+    def _remove_proxy(self, proxy, *, _is_finalizing=sys.is_finalizing):
+        if _is_finalizing():
+            # Weakrefs can't be properly torn down at that point anymore.
+            return
         for signal, proxies in list(self._func_cid_map.items()):
             try:
                 del self.callbacks[signal][proxies[proxy]]

[mapfiable]

I'm not 100% sure what I am supposed to do with the code. Should I remove the line with the minus sign and add the lines with the plus signs?

[anntzer]

Yes. Better with colors? (edited my message above) :)

[mapfiable]

Great! Thank you so much. It seems to work as I am unable to produce an error message and the process finishes with exit code 0. Out of curiosity, what do the first couple of lines of your code refer to? Are they relevant for me?

diff --git i/lib/matplotlib/cbook/__init__.py w/lib/matplotlib/cbook/__init__.py index 125dbe912..496bd63eb 100644 --- i/lib/matplotlib/cbook/__init__.py +++ w/lib/matplotlib/cbook/__init__.py @@ -152,7 +152,12 @@ class CallbackRegistry: self.callbacks[s][cid] = proxy return cid

[anntzer]

This is known as a patch file, see e.g. https://stackoverflow.com/questions/987372/what-is-the-format-of-a-patch-file.

[mapfiable]

I see, thanks!