[Bug]: "Bad file descriptor" raised repeatedly when plt.pause() interrupted in IPython #29688

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1. tacaswell

   added this to the v3.10.2 milestone on Feb 27, 2025
2. tacaswell

   added

   status: confirmed bug

   on Feb 27, 2025
3. mehakkhosa commented on Mar 5, 2025

   

   mehakkhosa

   on Mar 5, 2025

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   Hi, I’d be happy to take it on!
4. tacaswell commented on Mar 5, 2025

   

   tacaswell

   on Mar 5, 2025

   Member

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   I had started to look at this but did not get very far yet, here are some notes to get you started @mehakkhosa :

   • where the Qt setup gets done:

   matplotlib/lib/matplotlib/backends/backend_qt.py

   Lines 150 to 179 in 52f659f

   	def _allow_interrupt_qt(qapp_or_eventloop):
   	"""A context manager that allows terminating a plot by sending a SIGINT."""
   	# Use QSocketNotifier to read the socketpair while the Qt event loop runs.
   	def prepare_notifier(rsock):
   	sn = QtCore.QSocketNotifier(rsock.fileno(), QtCore.QSocketNotifier.Type.Read)
   	@sn.activated.connect
   	def _may_clear_sock():
   	# Running a Python function on socket activation gives the interpreter a
   	# chance to handle the signal in Python land. We also need to drain the
   	# socket with recv() to re-arm it, because it will be written to as part of
   	# the wakeup. (We need this in case set_wakeup_fd catches a signal other
   	# than SIGINT and we shall continue waiting.)
   	try:
   	rsock.recv(1)
   	except BlockingIOError:
   	# This may occasionally fire too soon or more than once on Windows, so
   	# be forgiving about reading an empty socket.
   	pass
   	return sn # Actually keep the notifier alive.
   	def handle_sigint():
   	if hasattr(qapp_or_eventloop, 'closeAllWindows'):
   	qapp_or_eventloop.closeAllWindows()
   	qapp_or_eventloop.quit()
   	return _allow_interrupt(prepare_notifier, handle_sigint)

   • where we use that function:

   matplotlib/lib/matplotlib/backend_bases.py

   Lines 1606 to 1661 in 52f659f

   	@contextmanager
   	def _allow_interrupt(prepare_notifier, handle_sigint):
   	"""
   	A context manager that allows terminating a plot by sending a SIGINT. It
   	is necessary because the running backend prevents the Python interpreter
   	from running and processing signals (i.e., to raise a KeyboardInterrupt).
   	To solve this, one needs to somehow wake up the interpreter and make it
   	close the plot window. We do this by using the signal.set_wakeup_fd()
   	function which organizes a write of the signal number into a socketpair.
   	A backend-specific function, *prepare_notifier*, arranges to listen to
   	the pair's read socket while the event loop is running. (If it returns a
   	notifier object, that object is kept alive while the context manager runs.)
   	If SIGINT was indeed caught, after exiting the on_signal() function the
   	interpreter reacts to the signal according to the handler function which
   	had been set up by a signal.signal() call; here, we arrange to call the
   	backend-specific *handle_sigint* function. Finally, we call the old SIGINT
   	handler with the same arguments that were given to our custom handler.
   	We do this only if the old handler for SIGINT was not None, which means
   	that a non-python handler was installed, i.e. in Julia, and not SIG_IGN
   	which means we should ignore the interrupts.
   	Parameters
   	----------
   	prepare_notifier : Callable[[socket.socket], object]
   	handle_sigint : Callable[[], object]
   	"""
   	old_sigint_handler = signal.getsignal(signal.SIGINT)
   	if old_sigint_handler in (None, signal.SIG_IGN, signal.SIG_DFL):
   	yield
   	return
   	handler_args = None
   	wsock, rsock = socket.socketpair()
   	wsock.setblocking(False)
   	rsock.setblocking(False)
   	old_wakeup_fd = signal.set_wakeup_fd(wsock.fileno())
   	notifier = prepare_notifier(rsock)
   	def save_args_and_handle_sigint(*args):
   	nonlocal handler_args
   	handler_args = args
   	handle_sigint()
   	signal.signal(signal.SIGINT, save_args_and_handle_sigint)
   	try:
   	yield
   	finally:
   	wsock.close()
   	rsock.close()
   	signal.set_wakeup_fd(old_wakeup_fd)
   	signal.signal(signal.SIGINT, old_sigint_handler)
   	if handler_args is not None:
   	old_sigint_handler(*handler_args)

   Something is clearly going wrong in the cleanup where when we get SIGINT we are not fully cleaning up after ourselves. A quick fix would be to just exit early in the callback, but I think the correct fix is to sort out how to cleanly disconnect the Qt Signal.

   👍React with 👍1Reacted by Mehakpreet Kaur Khosa
5. ksunden

   modified the milestones: v3.10.2, v3.10.3 on May 2, 2025
6. chrisjbillington commented on Jun 23, 2025

   

   chrisjbillington

   on Jun 23, 2025

   Contributor

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   I'm seeing this without IPython, when exiting the python REPL after pressing ctrl-C whilst a plot was visible. This only occurs when running a script with python -i, not when executing the matplotlib lines from the REPL itself.

   Here's what triggers it for me on Arch Linux, Python 3.13.3, matplotlib 3.10.3:

   Test script (for me called 2060.py):

   import matplotlib.pyplot as plt
   plt.plot([1, 2, 3], [4, 5, 6])
   plt.show()

   Run the script with the -i flag:

   python -i 2060.py

   When the plot appears, go back to the terminal and hit ctrl-C, that yields this traceback, no problem so far:

   ^CTraceback (most recent call last):
     File "/home/bilbo/Documents/Scratch/2060.py", line 3, in <module>
       plt.show()
       ~~~~~~~~^^
     File "/usr/lib/python3.13/site-packages/matplotlib/pyplot.py", line 614, in show
       return _get_backend_mod().show(*args, **kwargs)
              ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
     File "/usr/lib/python3.13/site-packages/matplotlib/backend_bases.py", line 3548, in show
       cls.mainloop()
       ~~~~~~~~~~~~^^
     File "/usr/lib/python3.13/site-packages/matplotlib/backends/backend_qt.py", line 642, in start_main_loop
       with _allow_interrupt_qt(qapp):
            ~~~~~~~~~~~~~~~~~~~^^^^^^
     File "/usr/lib/python3.13/contextlib.py", line 148, in __exit__
       next(self.gen)
       ~~~~^^^^^^^^^^
     File "/usr/lib/python3.13/site-packages/matplotlib/backend_bases.py", line 1658, in _allow_interrupt
       old_sigint_handler(*handler_args)
       ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
   KeyboardInterrupt
   

   But then, press ctrl-D to exit the Python interactive interpreter. At that point I get this (sorry, screengrab from a poor-quality screen recording because the window vanishes soon after)

   

   and then weirdly the terminal emulator exits, even though it should return to bash. I get this no matter which terminal emulator I use (tried gnome-terminal, gnome console, alacritty). In gnome-console the main window is still visible but there's no prompt or anything, it's non-functional.

   Notably it doesn't happen if you execute the same lines from within a REPL - only when running Python on a script with -i.
7. chrisjbillington

   added 8 commits that reference this issue on Jun 24, 2025

   Fix issue matplotlib#29688 by cleaning up after ourselves

   ...

   8d2c1c4

   Clean up Qt socket notifier to avoid spurious interrupt handler calls

   ...

   5685b8d

   Clean up Qt socket notifier to avoid spurious interrupt handler calls

   ...

   10ab66c

   Clean up Qt socket notifier to avoid spurious interrupt handler calls

   ...

   14b531d

   Clean up Qt socket notifier to avoid spurious interrupt handler calls

   ...

   3068e7e

   Clean up Qt socket notifier to avoid spurious interrupt handler calls

   ...

   a257e36

   Clean up Qt socket notifier to avoid spurious interrupt handler calls

   ...

   037025b

   Clean up Qt socket notifier to avoid spurious interrupt handler calls

   ...

   b06d1f4
8. anntzer

   closed this as completedin #30209on Jul 1, 2025