Backport PR #16311: don't override non-Python signal handlers

tacaswell · meeseeksmachine · commit 444838eda0ff · 2020-01-25T05:23:30.000Z
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -1042,9 +1042,11 @@ def trigger_manager_draw(manager):
     def mainloop():
         old_signal = signal.getsignal(signal.SIGINT)
         # allow SIGINT exceptions to close the plot window.
-        signal.signal(signal.SIGINT, signal.SIG_DFL)
+        if old_signal:
+            signal.signal(signal.SIGINT, signal.SIG_DFL)
         try:
             qApp.exec_()
         finally:
             # reset the SIGINT exception handler
-            signal.signal(signal.SIGINT, old_signal)
+            if old_signal:
+                signal.signal(signal.SIGINT, old_signal)
