Fix three issues with mo.mpl.interactive in marimo run mode (#8747):#8760
Merged
Fix three issues with mo.mpl.interactive in marimo run mode (#8747):#8760
mo.mpl.interactive in marimo run mode (#8747):#8760Conversation
1. **Suppress startup warning**: "Starting a Matplotlib GUI outside of the main thread will likely fail" — this is a false positive since WebAgg only uses software (Agg) rendering and communicates over the network, no GUI toolkit is involved. 2. **Reduce log noise**: Downgrade "Received message for unknown comm" from WARNING to DEBUG — this is an expected race condition when comms are closed during cell re-execution or server shutdown. 3. **Fix shutdown crash**: Properly clean up the figure manager (remove web socket, close canvas) when a cell is re-run or deleted, preventing dangling timer callbacks that cause "QObject::killTimer: Timers cannot be stopped from another thread" errors and segfaults.
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Pull request overview
This PR improves mo.mpl.interactive behavior in marimo run mode by reducing noisy/incorrect warnings, lowering expected comm-race log severity, and adding cleanup to prevent backend shutdown crashes.
Changes:
- Suppress Matplotlib’s “GUI outside of main thread” warning when creating WebAgg figure managers.
- Downgrade “unknown comm” messages from WARNING to DEBUG due to expected comm lifecycle races.
- Add figure-manager + websocket cleanup during cell lifecycle disposal, with accompanying tests.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
marimo/_plugins/stateless/mpl/_mpl.py |
Suppresses a known-false-positive Matplotlib thread warning when creating WebAgg managers. |
marimo/_plugins/ui/_impl/comm.py |
Reduces log noise by demoting “unknown comm” to DEBUG with explanatory comment. |
marimo/_plugins/ui/_impl/from_mpl_interactive.py |
Extends cell lifecycle cleanup to detach websocket and close canvas before closing the comm. |
tests/_plugins/ui/_impl/test_from_mpl_interactive.py |
Adds tests asserting figure-manager cleanup behavior and error tolerance. |
tests/_plugins/stateless/test_mpl.py |
Adds regression test ensuring no thread warning is emitted during manager creation. |
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| class _MplCleanupHandle(CellLifecycleItem): | ||
| """Closes the MarimoComm when the cell is re-run or deleted.""" | ||
| """Cleans up the matplotlib figure manager and MarimoComm on cell re-run.""" |
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| def dispose(self, context: RuntimeContext, deletion: bool) -> bool: | ||
| del context, deletion | ||
| # Disconnect the web socket from the figure manager first, | ||
| # then destroy the canvas to prevent stale timer callbacks | ||
| # on thread teardown (avoids "QObject::killTimer" errors). | ||
| if self._figure_manager is not None and self._sync_ws is not None: | ||
| try: | ||
| self._figure_manager.remove_web_socket(self._sync_ws) | ||
| except Exception: | ||
| pass | ||
| if self._figure_manager is not None: | ||
| try: | ||
| self._figure_manager.canvas.close() | ||
| except Exception: | ||
| pass | ||
| self._comm.close() | ||
| return True |
tests/_plugins/stateless/test_mpl.py
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| new_figure_manager_given_figure(id(fig), fig) | ||
| captured.extend(w) |
dmadisetti
reviewed
Mar 18, 2026
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dmadisetti
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dmadisetti
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LGTM, the test is just brittle
tests/_plugins/ui/_impl/test_comm.py::test_comm_manager_receive_unknown_message
- Update test_comm_manager_receive_unknown_message to assert debug (not warning) - Update _MplCleanupHandle docstring to mention both re-run and deletion - Clean up figure manager in thread warning test to prevent resource leaks
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Suppress startup warning: "Starting a Matplotlib GUI outside of the main thread will likely fail" — this is a false positive since WebAgg only uses software (Agg) rendering and communicates over the network, no GUI toolkit is involved.
Reduce log noise: Downgrade "Received message for unknown comm" from WARNING to DEBUG — this is an expected race condition when comms are closed during cell re-execution or server shutdown.
Fix shutdown crash: Properly clean up the figure manager (remove web socket, close canvas) when a cell is re-run or deleted, preventing dangling timer callbacks that cause "QObject::killTimer: Timers cannot be stopped from another thread" errors and segfaults.