fix(OSX): Matplotlib backend leaks in App Switcher (#1232)

skore/src/skore/persistence/item/matplotlib_figure_item.py

@@ -5,6 +5,7 @@
 
 from __future__ import annotations
 
+from contextlib import contextmanager
 from io import BytesIO
 from typing import TYPE_CHECKING, Optional
 
@@ -18,6 +19,19 @@
     from matplotlib.figure import Figure
 
 
+@contextmanager
+def mpl_backend(backend="agg"):
+    """Context manager for switching matplotlib backend."""
+    import matplotlib
+
+    original_backend = matplotlib.get_backend()
+    matplotlib.use(backend)
+    try:
+        yield
+    finally:
+        matplotlib.use(original_backend)
+
+
 class MatplotlibFigureItem(Item):
     """A class used to persist a Matplotlib figure."""
 
@@ -75,9 +89,13 @@ def factory(cls, figure: Figure, /, **kwargs) -> MatplotlibFigureItem:
     @property
     def figure(self) -> Figure:
         """The figure from the persistence."""
+        # switch mpl backend to avoid opening windows in a background thread
         figure_bytes = b64_str_to_bytes(self.figure_b64_str)
 
-        with BytesIO(figure_bytes) as stream:
+        with (
+            BytesIO(figure_bytes) as stream,
+            mpl_backend(backend="agg"),
+        ):
             return joblib.load(stream)
 
     def as_serializable_dict(self) -> dict:

skore/tests/unit/item/test_matplotlib_figure_item.py

@@ -4,10 +4,12 @@
 
 import joblib
 import pytest
+from matplotlib import get_backend
 from matplotlib.figure import Figure
 from matplotlib.pyplot import subplots
 from matplotlib.testing.compare import compare_images
 from skore.persistence.item import ItemTypeError, MatplotlibFigureItem
+from skore.persistence.item.matplotlib_figure_item import mpl_backend
 from skore.utils import b64_str_to_bytes, bytes_to_b64_str
 
 
@@ -93,3 +95,30 @@ def test_as_serializable_dict(self, mock_nowstr):
             "media_type": "image/svg+xml;base64",
             "value": figure_b64_str,
         }
+
+    def test_backend_switch(self):
+        backend = get_backend()
+        # hoppefuly PostScript is never the default in tests ^^
+        with mpl_backend(backend="ps"):
+            assert get_backend() == "ps"
+        assert get_backend() == backend
+
+    def test_backend_switch_in_thread(self):
+        # as default osx backend could crash the interpreter
+        # if it tries to open a window in a background thread
+        # test that the figure property is usable in a thread
+        backend = get_backend()
+
+        def foo():
+            figure, ax = subplots()
+            ax.plot([1, 2, 3, 4], [1, 4, 2, 3])
+            φ = MatplotlibFigureItem.factory(figure)
+            assert isinstance(φ.figure, Figure)
+
+        import threading
+
+        t = threading.Thread(target=foo)
+        t.start()
+        t.join()
+
+        assert get_backend() == backend