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BUG: raise when creating a MacOS FigureManager outside the main thread #30697

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Merged
greglucas merged 2 commits into from
Oct 31, 2025
Merged

BUG: raise when creating a MacOS FigureManager outside the main thread #30697

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060fa00
BUG: raise when creating a MacOS FigureManager outside the main thread
ngoldbaum Oct 28, 2025
3c503df
TST: run test that might crash in a subprocess
ngoldbaum Oct 30, 2025
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23 changes: 23 additions & 0 deletions lib/matplotlib/tests/test_backend_macosx.py
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
import os
import threading
from pathlib import Path

import pytest
Expand Down Expand Up @@ -84,3 +85,25 @@ def _test_save_figure_return():
def test_save_figure_return():
subprocess_run_helper(_test_save_figure_return, timeout=_test_timeout,
extra_env={"MPLBACKEND": "macosx"})


def _test_create_figure_on_worker_thread_fails():
def create_figure():
warn_msg = "Matplotlib GUI outside of the main thread will likely fail."
err_msg = "Cannot create a GUI FigureManager outside the main thread"
with pytest.warns(UserWarning, match=warn_msg):
with pytest.raises(RuntimeError, match=err_msg):
plt.gcf()

worker = threading.Thread(target=create_figure)
worker.start()
worker.join()


@pytest.mark.backend('macosx', skip_on_importerror=True)
def test_create_figure_on_worker_thread_fails():
subprocess_run_helper(
_test_create_figure_on_worker_thread_fails,
timeout=_test_timeout,
extra_env={"MPLBACKEND": "macosx"}
)
10 changes: 10 additions & 0 deletions src/_macosx.m
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Original file line number Diff line number Diff line change
Expand Up @@ -582,6 +582,16 @@ bool mpl_check_modifier(bool present, PyObject* list, char const* name)
static PyObject*
FigureManager_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
if (![NSThread isMainThread]) {
PyErr_SetString(
PyExc_RuntimeError,
"Cannot create a GUI FigureManager outside the main thread "
"using the MacOS backend. Use a non-interactive "
"backend like 'agg' to make plots on worker threads."
);
return NULL;
}

lazy_init();
Window* window = [Window alloc];
if (!window) { return NULL; }
Expand Down