From 153449ebd6da6601bb5062c30a708743c5d565de Mon Sep 17 00:00:00 2001
From: Eric Larson <larson.eric.d@gmail.com>
Date: Mon, 31 Aug 2026 07:47:30 -0400
Subject: [PATCH 1/3] Add set_data for ContourSet

---
 .../next_whats_new/contourset_set_data.rst    |  12 +++
 galleries/examples/animation/simple_anim.py   |  29 +++++
 lib/matplotlib/cbook.py                       |  20 ++++
 lib/matplotlib/cbook.pyi                      |   3 +
 lib/matplotlib/contour.py                     |  73 ++++++++++++-
 lib/matplotlib/contour.pyi                    |   1 +
 lib/matplotlib/tests/test_contour.py          | 101 +++++++++++++++++-
 7 files changed, 235 insertions(+), 4 deletions(-)
 create mode 100644 doc/release/next_whats_new/contourset_set_data.rst

diff --git a/doc/release/next_whats_new/contourset_set_data.rst b/doc/release/next_whats_new/contourset_set_data.rst
new file mode 100644
index 000000000000..bd6d3b56a1f4
--- /dev/null
+++ b/doc/release/next_whats_new/contourset_set_data.rst
@@ -0,0 +1,12 @@
+Contour sets can be updated with new data
+-----------------------------------------
+`.ContourSet.set_data` recontours new data using the existing artist, instead of
+requiring the contour set to be removed and recreated::
+
+    cs = ax.contour(X, Y, Z)
+    cs.set_data(X, Y, Z2)
+
+It works for `~.Axes.contour`, `~.Axes.contourf`, `~.Axes.tricontour` and
+`~.Axes.tricontourf`. The contour levels are kept, so the colors, the colorbar
+and the legend entries of the contour set remain valid. This is useful for
+animations and other interactive updates.
diff --git a/galleries/examples/animation/simple_anim.py b/galleries/examples/animation/simple_anim.py
index 2b65b5935b40..c1c37527ee5d 100644
--- a/galleries/examples/animation/simple_anim.py
+++ b/galleries/examples/animation/simple_anim.py
@@ -37,6 +37,35 @@ def animate(i):
 
 plt.show()
 
+# %%
+# Contours are updated the same way, with `.ContourSet.set_data`. Recontouring
+# the existing artist is faster than removing the contour set and making a new
+# one, and it keeps the contours in the same place in the draw order, which
+# matters when blitting. The levels are not recomputed, so the colors mean
+# the same thing in every frame.
+
+fig, ax = plt.subplots()
+
+X, Y = np.meshgrid(np.linspace(-3, 3, 100), np.linspace(-3, 3, 100))
+
+
+def f(t):
+    return np.sin(X + t) * np.cos(Y - t)
+
+
+cs = ax.contour(X, Y, f(0), levels=np.linspace(-0.9, 0.9, 7))
+
+
+def animate_contour(i):
+    cs.set_data(X, Y, f(i / 25))  # update the data.
+    return cs,
+
+
+ani_contour = animation.FuncAnimation(
+    fig, animate_contour, interval=20, blit=True, save_count=50)
+
+plt.show()
+
 # %%
 #
 # .. tags::
diff --git a/lib/matplotlib/cbook.py b/lib/matplotlib/cbook.py
index 7aaf900d4e61..9c05273bdb35 100644
--- a/lib/matplotlib/cbook.py
+++ b/lib/matplotlib/cbook.py
@@ -2175,6 +2175,26 @@ def _setattr_cm(obj, **kwargs):
                 setattr(obj, attr, orig)
 
 
+# TODO: Could be used in Line2D.set_data and other setters that mutate an artist
+# more than once, and so can leave it half-updated when they raise.
+@contextlib.contextmanager
+def _safe_state_update(obj):
+    """
+    Context manager to make an in-place update of *obj* all-or-nothing.
+
+    Yields a snapshot of ``obj.__dict__``, which is restored if the body raises, so
+    that an update mutating *obj* incrementally -- and able to fail partway through
+    -- leaves it as it was rather than half-updated.
+    """
+    state = obj.__dict__.copy()
+    try:
+        yield state
+    except Exception:
+        obj.__dict__.clear()
+        obj.__dict__.update(state)
+        raise
+
+
 class _OrderedSet(collections.abc.MutableSet):
     def __init__(self):
         self._od = collections.OrderedDict()
diff --git a/lib/matplotlib/cbook.pyi b/lib/matplotlib/cbook.pyi
index 653e2219c5b7..5769107f4b60 100644
--- a/lib/matplotlib/cbook.pyi
+++ b/lib/matplotlib/cbook.pyi
@@ -158,6 +158,9 @@ def _array_perimeter(arr: np.ndarray) -> np.ndarray: ...
 def _unfold(arr: np.ndarray, axis: int, size: int, step: int) -> np.ndarray: ...
 def _array_patch_perimeters(x: np.ndarray, rstride: int, cstride: int) -> np.ndarray: ...
 def _setattr_cm(obj: Any, **kwargs) -> contextlib.AbstractContextManager[None]: ...
+def _safe_state_update(
+    obj: Any,
+) -> contextlib.AbstractContextManager[dict[str, Any]]: ...
 
 class _OrderedSet(collections.abc.MutableSet):
     def __init__(self) -> None: ...
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
index 26a25c00dc09..3656d2a97687 100644
--- a/lib/matplotlib/contour.py
+++ b/lib/matplotlib/contour.py
@@ -891,6 +891,66 @@ def legend_elements(self, variable_name='x', str_format=str):
 
         return artists, labels
 
+    def set_data(self, *args, **kwargs):
+        """
+        Set new data and recompute the contours.
+
+        Call signatures::
+
+            set_data(Z)
+            set_data(X, Y, Z)
+
+        This reuses the existing artist, which is faster than removing the
+        contour set and creating a new one, and keeps its styling and its
+        place in the draw tree.
+
+        .. versionadded:: 3.12
+
+        Parameters
+        ----------
+        *args
+            The new data, interpreted as by the function that created this
+            contour set, i.e. `~.Axes.contour`, `~.Axes.contourf`,
+            `~.Axes.tricontour` or `~.Axes.tricontourf`.
+
+        **kwargs
+            Only the keywords of that function that control *how the contours are
+            computed* are accepted, and they keep their current values if not
+            given: *corner_mask*, *algorithm* and the *xunits*/*yunits* unit
+            keywords for `~.Axes.contour`. Keywords that control how the contours
+            *look* (*levels*, *colors*, *cmap*, *linewidths*, ...) raise
+            `TypeError`; set those with the corresponding `.Collection` setters, or
+            create a new contour set.
+
+        Notes
+        -----
+        The *levels* are not recomputed; the new data is contoured at the
+        levels already in use, so that the colors and the colorbar stay
+        valid. Create a new contour set if you need different levels.
+
+        Existing contour labels are not moved. The Axes limits are not
+        rescaled, as for other artists' data setters.
+        """
+        # _process_args() both validates and mutates: by the time it can fail it has
+        # already rebound the levels, zmin/zmax and the contour generator, so a
+        # rejected call would otherwise leave the contour set half-updated.
+        with cbook._safe_state_update(self) as state:
+            kwargs = self._process_args(*args, **kwargs)
+            if kwargs:
+                raise TypeError(
+                    f"set_data() got unexpected keyword arguments {[*kwargs]}")
+            if not np.array_equal(state['levels'], self.levels):
+                # Only reachable through the ContourSet(ax, levels, allsegs)
+                # signature. Keeping the levels fixed is what lets us skip
+                # reprocessing the layers and the colors here, so enforce it rather
+                # than silently mismatching them.
+                raise ValueError("set_data() cannot change the contour levels")
+            if self._paths is state['_paths']:  # Not set by _process_args.
+                self._paths = self._make_paths_from_contour_generator()
+        self.sticky_edges.x[:] = [self._mins[0], self._maxs[0]]
+        self.sticky_edges.y[:] = [self._mins[1], self._maxs[1]]
+        self.stale = True
+
     def _process_args(self, *args, **kwargs):
         """
         Process *args* and *kwargs*; override in derived classes.
@@ -937,8 +997,6 @@ def _process_args(self, *args, **kwargs):
 
     def _make_paths_from_contour_generator(self):
         """Compute ``paths`` using C extension."""
-        if self._paths is not None:
-            return self._paths
         cg = self._contour_generator
         empty_path = Path(np.empty((0, 2)))
         vertices_and_codes = (
@@ -1315,6 +1373,11 @@ class QuadContourSet(ContourSet):
     %(contour_set_attributes)s
     """
 
+    # Set on the first _process_args; a later call from set_data keeps these rather
+    # than falling back to the rcParams.
+    _algorithm = None
+    _corner_mask = None
+
     def _process_args(self, *args, corner_mask=None, algorithm=None, **kwargs):
         """
         Process args and kwargs.
@@ -1332,10 +1395,13 @@ def _process_args(self, *args, corner_mask=None, algorithm=None, **kwargs):
         else:
             import contourpy
 
-            algorithm = mpl._val_or_rc(algorithm, 'contour.algorithm')
+            algorithm = mpl._val_or_rc(
+                algorithm or self._algorithm, 'contour.algorithm')
             mpl.rcParams.validate["contour.algorithm"](algorithm)
             self._algorithm = algorithm
 
+            if corner_mask is None:
+                corner_mask = self._corner_mask
             if corner_mask is None:
                 if self._algorithm == "mpl2005":
                     # mpl2005 does not support corner_mask=True so if not
@@ -1530,6 +1596,7 @@ def _initialize_x_y(self, z):
 Returns
 -------
 `~.contour.QuadContourSet`
+    Use `.ContourSet.set_data` to recontour new data with the same artist.
 
 Other Parameters
 ----------------
diff --git a/lib/matplotlib/contour.pyi b/lib/matplotlib/contour.pyi
index 26b3a43c75ab..aac26b1e8d04 100644
--- a/lib/matplotlib/contour.pyi
+++ b/lib/matplotlib/contour.pyi
@@ -131,6 +131,7 @@ class ContourSet(ContourLabeler, Collection):
     def legend_elements(
         self, variable_name: str = ..., str_format: Callable[[float], str] = ...
     ) -> tuple[list[Artist], list[str]]: ...
+    def set_data(self, *args, **kwargs) -> None: ...
     def find_nearest_contour(
         self, x: float, y: float, indices: Iterable[int] | None = ..., pixel: bool = ...
     ) -> tuple[int, int, int, float, float, float]: ...
diff --git a/lib/matplotlib/tests/test_contour.py b/lib/matplotlib/tests/test_contour.py
index 8b13caa15e67..094c2bdc7978 100644
--- a/lib/matplotlib/tests/test_contour.py
+++ b/lib/matplotlib/tests/test_contour.py
@@ -5,7 +5,9 @@
 
 import contourpy
 import numpy as np
-from numpy.testing import assert_array_almost_equal, assert_array_almost_equal_nulp
+from numpy.testing import (
+    assert_array_almost_equal, assert_array_almost_equal_nulp,
+    assert_array_equal)
 import matplotlib as mpl
 from matplotlib import pyplot as plt, rc_context, ticker
 from matplotlib.colors import LogNorm, same_color
@@ -896,3 +898,100 @@ def test_clabel_manual_subset():
     cs = ax.contour([[1, 2], [3, 4]], levels=[1.5, 2.5, 3.5])
     # Attempt to label only one specific level manually
     ax.clabel(cs, levels=[2.5], manual=[(0.5, 0.5)])
+
+
+@pytest.mark.parametrize("levels", [None, [-0.5, 0, 0.5]])
+@pytest.mark.parametrize("plotter", ["contour", "contourf"])
+@check_figures_equal()
+def test_contour_set_data(fig_test, fig_ref, plotter, levels):
+    x, y = np.meshgrid(np.linspace(-3, 3, 20), np.linspace(-3, 3, 20))
+    z1 = np.sin(x) * np.cos(y)
+    z2 = np.cos(x) * np.sin(y)
+
+    cs = getattr(fig_test.subplots(), plotter)(x, y, z1, levels=levels)
+    orig_levels = cs.levels.copy()
+    cs.set_data(x, y, z2)
+    # The levels are kept, also where they were autoscaled from the old data.
+    assert_array_equal(cs.levels, orig_levels)
+    getattr(fig_ref.subplots(), plotter)(x, y, z2, levels=orig_levels)
+
+
+@check_figures_equal()
+def test_contour_set_data_z_only(fig_test, fig_ref):
+    z1 = np.arange(25).reshape((5, 5)) % 7
+    z2 = np.arange(25).reshape((5, 5)) % 5
+    fig_test.subplots().contour(z1, levels=[1, 2, 3]).set_data(z2)
+    fig_ref.subplots().contour(z2, levels=[1, 2, 3])
+
+
+def test_contour_set_data_updates_artist_state():
+    x, y = np.meshgrid(np.linspace(0, 1, 5), np.linspace(0, 1, 5))
+    fig = plt.figure()
+    cs = fig.add_subplot().contour(x, y, x + y, levels=[0.5, 1, 1.5])
+    fig.canvas.draw()
+    assert not cs.stale
+    cs.set_data(2 * x - 1, 2 * y - 1, x + y)
+    assert cs.stale
+    assert cs.sticky_edges.x == [-1, 1]
+    assert cs.sticky_edges.y == [-1, 1]
+
+
+def test_contour_set_data_changing_levels():
+    # The (levels, allsegs) signature is the only way set_data could change the
+    # levels, which would leave the colors and the colorbar mismatched.
+    segs = [[np.array([[0.0, 0.0], [1.0, 1.0]])]]
+    cs = mpl.contour.ContourSet(plt.figure().add_subplot(), [0.5], segs)
+    cs.set_data([0.5], segs)  # Same levels, new segments: fine.
+    with pytest.raises(ValueError, match="cannot change the contour levels"):
+        cs.set_data([0.25, 0.75], segs * 2)
+    assert_array_equal(cs.levels, [0.5])  # Rolled back, not half-applied.
+
+
+def test_contour_set_data_keeps_algorithm_and_corner_mask():
+    z = np.array([[1.0, 2.0], [3.0, 4.0]])
+    cs = plt.figure().add_subplot().contour(
+        z, algorithm='mpl2005', corner_mask=False)
+    cs.set_data(z * 2)
+    assert cs._algorithm == 'mpl2005'
+    assert cs._corner_mask is False
+    # They describe how the contours are computed, so they can also be re-set.
+    cs.set_data(z * 2, algorithm='serial', corner_mask=True)
+    assert cs._algorithm == 'serial'
+    assert cs._corner_mask is True
+
+
+def _assert_set_data_rejects(exc, match, args=(), **kwargs):
+    # A rejected set_data() must raise and leave the contour set exactly as it was,
+    # rather than with the levels or the contour generator already swapped out.
+    z = np.arange(9).reshape((3, 3))
+    fig = plt.figure()
+    cs = fig.add_subplot().contour(z)
+    before = (cs.levels.copy(), [p.vertices.copy() for p in cs.get_paths()],
+              cs.zmin, cs.zmax, cs._contour_generator)
+    with pytest.raises(exc, match=match):
+        cs.set_data(*(args or (z,)), **kwargs)
+    assert_array_equal(cs.levels, before[0])
+    for path, vertices in zip(cs.get_paths(), before[1], strict=True):
+        assert_array_equal(path.vertices, vertices)
+    assert (cs.zmin, cs.zmax, cs._contour_generator) == before[2:]
+    fig.canvas.draw()  # Must not raise.
+
+
+@pytest.mark.parametrize("kwargs", [
+    {"levels": [1, 2]}, {"colors": "red"}, {"cmap": "plasma"},
+    {"linewidths": 2}, {"linestyles": "dashed"}, {"extend": "both"},
+    {"alpha": 0.5}, {"hatches": ["/"]}, {"zorder": 5},
+])
+def test_contour_set_data_rejects_style_kwargs(kwargs):
+    # Only the keywords that feed the contour generator are accepted; anything
+    # affecting the appearance would leave the levels and the colors mismatched.
+    _assert_set_data_rejects(TypeError, "unexpected keyword arguments", **kwargs)
+
+
+@pytest.mark.parametrize("args", [
+    (np.arange(5), np.arange(5), np.empty((3, 4))),  # mismatched shapes
+    (np.arange(7),),                                 # z is not 2D
+    (np.empty((3, 3)),) * 5,                         # too many arguments
+])
+def test_contour_set_data_rejects_bad_data(args):
+    _assert_set_data_rejects((TypeError, ValueError), None, args)

From dd16a6753ce250b4095a4a792deb42cc7338dab1 Mon Sep 17 00:00:00 2001
From: Eric Larson <larson.eric.d@gmail.com>
Date: Mon, 31 Aug 2026 08:18:06 -0400
Subject: [PATCH 2/3] Fix bug with tricontour triangles

---
 lib/matplotlib/contour.py                  |  9 +++----
 lib/matplotlib/tests/test_triangulation.py | 28 ++++++++++++++++++++++
 lib/matplotlib/tri/_tricontour.py          |  6 +++--
 3 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
index 3656d2a97687..e6ace0b6b3aa 100644
--- a/lib/matplotlib/contour.py
+++ b/lib/matplotlib/contour.py
@@ -917,10 +917,11 @@ def set_data(self, *args, **kwargs):
             Only the keywords of that function that control *how the contours are
             computed* are accepted, and they keep their current values if not
             given: *corner_mask*, *algorithm* and the *xunits*/*yunits* unit
-            keywords for `~.Axes.contour`. Keywords that control how the contours
-            *look* (*levels*, *colors*, *cmap*, *linewidths*, ...) raise
-            `TypeError`; set those with the corresponding `.Collection` setters, or
-            create a new contour set.
+            keywords for `~.Axes.contour`, or *triangles* and *mask* for
+            `~.Axes.tricontour`. Keywords that control how the contours *look*
+            (*levels*, *colors*, *cmap*, *linewidths*, ...) raise `TypeError`; set
+            those with the corresponding `.Collection` setters, or create a new
+            contour set.
 
         Notes
         -----
diff --git a/lib/matplotlib/tests/test_triangulation.py b/lib/matplotlib/tests/test_triangulation.py
index 0293423e06b0..db04dae680e7 100644
--- a/lib/matplotlib/tests/test_triangulation.py
+++ b/lib/matplotlib/tests/test_triangulation.py
@@ -1323,6 +1323,34 @@ def test_tricontourset_reuse():
     assert tcs3._contour_generator == tcs1._contour_generator
 
 
+@pytest.mark.parametrize("plotter", ["tricontour", "tricontourf"])
+@check_figures_equal()
+def test_tricontour_triangles_kwarg(fig_test, fig_ref, plotter):
+    # Passing triangles by keyword used to fall through to Collection.set().
+    x = [0.0, 1.0, 2.0, 0.0, 1.0, 0.0]
+    y = [0.0, 0.0, 0.0, 1.0, 1.0, 2.0]
+    z = [0.0, 1.0, 2.0, 1.0, 2.0, 3.0]
+    triangles = [[0, 1, 3], [1, 4, 3], [1, 2, 4], [3, 4, 5]]
+    levels = [0.5, 1.5, 2.5]
+    getattr(fig_test.subplots(), plotter)(x, y, z, triangles=triangles,
+                                          levels=levels)
+    getattr(fig_ref.subplots(), plotter)(x, y, triangles, z, levels=levels)
+
+
+@check_figures_equal()
+def test_tricontour_set_data(fig_test, fig_ref):
+    x = [0.0, 0.5, 1.0, 0.0, 0.5, 0.0]
+    y = [0.0, 0.0, 0.0, 0.5, 0.5, 1.0]
+    z1 = [0.0, 1.0, 2.0, 1.0, 2.0, 3.0]
+    z2 = [3.0, 2.0, 1.0, 2.0, 1.0, 0.0]
+    levels = [0.5, 1.5, 2.5]
+    triangles = [[0, 1, 3], [1, 4, 3], [1, 2, 4], [3, 4, 5]]
+    cs = fig_test.subplots().tricontour(x, y, z1, triangles=triangles,
+                                        levels=levels)
+    cs.set_data(x, y, z2, triangles=triangles)
+    fig_ref.subplots().tricontour(x, y, z2, triangles=triangles, levels=levels)
+
+
 @check_figures_equal()
 def test_triplot_with_ls(fig_test, fig_ref):
     x = [0, 2, 1]
diff --git a/lib/matplotlib/tri/_tricontour.py b/lib/matplotlib/tri/_tricontour.py
index 8250515f3ef8..54767da8783c 100644
--- a/lib/matplotlib/tri/_tricontour.py
+++ b/lib/matplotlib/tri/_tricontour.py
@@ -42,7 +42,7 @@ def _process_args(self, *args, **kwargs):
             self._maxs = args[0]._maxs
         else:
             from matplotlib import _tri
-            tri, z = self._contour_args(args, kwargs)
+            tri, z, kwargs = self._contour_args(args, kwargs)
             C = _tri.TriContourGenerator(tri.get_cpp_triangulation(), z)
             self._mins = [tri.x.min(), tri.y.min()]
             self._maxs = [tri.x.max(), tri.y.max()]
@@ -76,7 +76,9 @@ def _contour_args(self, args, kwargs):
             func = 'contourf' if self.filled else 'contour'
             raise ValueError(f'Cannot {func} log of negative values.')
         self._process_contour_level_args(args, z.dtype)
-        return (tri, z)
+        # Return kwargs with the triangulation parameters removed; the caller must
+        # not see e.g. *triangles* again, or it ends up in Collection.set().
+        return (tri, z, kwargs)
 
 
 _docstring.interpd.register(_tricontour_doc="""

From d608076b92d7d34a5c0f36ffb6cd872466e32d2d Mon Sep 17 00:00:00 2001
From: Eric Larson <larson.eric.d@gmail.com>
Date: Thu, 10 Sep 2026 13:49:12 -0400
Subject: [PATCH 3/3] Review comments

---
 .../next_whats_new/contourset_set_data.rst       |  8 ++++----
 galleries/examples/animation/simple_anim.py      |  4 ++--
 lib/matplotlib/cbook.py                          |  6 ++++--
 lib/matplotlib/contour.py                        | 16 +++++++++-------
 lib/matplotlib/tests/test_contour.py             |  5 +++++
 5 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/doc/release/next_whats_new/contourset_set_data.rst b/doc/release/next_whats_new/contourset_set_data.rst
index bd6d3b56a1f4..1e5773f1b664 100644
--- a/doc/release/next_whats_new/contourset_set_data.rst
+++ b/doc/release/next_whats_new/contourset_set_data.rst
@@ -1,12 +1,12 @@
-Contour sets can be updated with new data
------------------------------------------
+A ``ContourSet`` can be updated with new data
+---------------------------------------------
 `.ContourSet.set_data` recontours new data using the existing artist, instead of
-requiring the contour set to be removed and recreated::
+requiring the ``ContourSet`` to be removed and recreated::
 
     cs = ax.contour(X, Y, Z)
     cs.set_data(X, Y, Z2)
 
 It works for `~.Axes.contour`, `~.Axes.contourf`, `~.Axes.tricontour` and
 `~.Axes.tricontourf`. The contour levels are kept, so the colors, the colorbar
-and the legend entries of the contour set remain valid. This is useful for
+and the legend entries of the ``ContourSet`` remain valid. This is useful for
 animations and other interactive updates.
diff --git a/galleries/examples/animation/simple_anim.py b/galleries/examples/animation/simple_anim.py
index c1c37527ee5d..d253bf8e1c58 100644
--- a/galleries/examples/animation/simple_anim.py
+++ b/galleries/examples/animation/simple_anim.py
@@ -39,8 +39,8 @@ def animate(i):
 
 # %%
 # Contours are updated the same way, with `.ContourSet.set_data`. Recontouring
-# the existing artist is faster than removing the contour set and making a new
-# one, and it keeps the contours in the same place in the draw order, which
+# the existing artist is faster than removing the ``ContourSet`` and making a
+# new one, and it keeps the contours in the same place in the draw order, which
 # matters when blitting. The levels are not recomputed, so the colors mean
 # the same thing in every frame.
 
diff --git a/lib/matplotlib/cbook.py b/lib/matplotlib/cbook.py
index 9c05273bdb35..46e8257f2f2f 100644
--- a/lib/matplotlib/cbook.py
+++ b/lib/matplotlib/cbook.py
@@ -2185,13 +2185,15 @@ def _safe_state_update(obj):
     Yields a snapshot of ``obj.__dict__``, which is restored if the body raises, so
     that an update mutating *obj* incrementally -- and able to fail partway through
     -- leaves it as it was rather than half-updated.
+
+    The snapshot is shallow: it undoes attributes being *rebound*, not an object that
+    one of them refers to being mutated in place, nor changes to other objects.
     """
     state = obj.__dict__.copy()
     try:
         yield state
     except Exception:
-        obj.__dict__.clear()
-        obj.__dict__.update(state)
+        obj.__dict__ = state
         raise
 
 
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
index e6ace0b6b3aa..a9a82d2cca80 100644
--- a/lib/matplotlib/contour.py
+++ b/lib/matplotlib/contour.py
@@ -901,7 +901,7 @@ def set_data(self, *args, **kwargs):
             set_data(X, Y, Z)
 
         This reuses the existing artist, which is faster than removing the
-        contour set and creating a new one, and keeps its styling and its
+        ``ContourSet`` and creating a new one, and keeps its styling and its
         place in the draw tree.
 
         .. versionadded:: 3.12
@@ -910,7 +910,7 @@ def set_data(self, *args, **kwargs):
         ----------
         *args
             The new data, interpreted as by the function that created this
-            contour set, i.e. `~.Axes.contour`, `~.Axes.contourf`,
+            ``ContourSet``, i.e. `~.Axes.contour`, `~.Axes.contourf`,
             `~.Axes.tricontour` or `~.Axes.tricontourf`.
 
         **kwargs
@@ -921,13 +921,13 @@ def set_data(self, *args, **kwargs):
             `~.Axes.tricontour`. Keywords that control how the contours *look*
             (*levels*, *colors*, *cmap*, *linewidths*, ...) raise `TypeError`; set
             those with the corresponding `.Collection` setters, or create a new
-            contour set.
+            ``ContourSet``.
 
         Notes
         -----
         The *levels* are not recomputed; the new data is contoured at the
         levels already in use, so that the colors and the colorbar stay
-        valid. Create a new contour set if you need different levels.
+        valid. Create a new ``ContourSet`` if you need different levels.
 
         Existing contour labels are not moved. The Axes limits are not
         rescaled, as for other artists' data setters.
@@ -1401,15 +1401,17 @@ def _process_args(self, *args, corner_mask=None, algorithm=None, **kwargs):
             mpl.rcParams.validate["contour.algorithm"](algorithm)
             self._algorithm = algorithm
 
-            if corner_mask is None:
-                corner_mask = self._corner_mask
             if corner_mask is None:
                 if self._algorithm == "mpl2005":
                     # mpl2005 does not support corner_mask=True so if not
-                    # specifically requested then disable it.
+                    # specifically requested then disable it, even when set_data
+                    # switches to it from an algorithm that had it enabled.
                     corner_mask = False
+                elif self._corner_mask is not None:
+                    corner_mask = self._corner_mask
                 else:
                     corner_mask = mpl.rcParams['contour.corner_mask']
+
             self._corner_mask = corner_mask
 
             x, y, z = self._contour_args(args, kwargs)
diff --git a/lib/matplotlib/tests/test_contour.py b/lib/matplotlib/tests/test_contour.py
index 094c2bdc7978..5546d8370871 100644
--- a/lib/matplotlib/tests/test_contour.py
+++ b/lib/matplotlib/tests/test_contour.py
@@ -958,6 +958,11 @@ def test_contour_set_data_keeps_algorithm_and_corner_mask():
     cs.set_data(z * 2, algorithm='serial', corner_mask=True)
     assert cs._algorithm == 'serial'
     assert cs._corner_mask is True
+    # Switching to mpl2005 turns corner_mask off unless requested, as contour() does,
+    # rather than inheriting a corner_mask=True that mpl2005 does not support.
+    cs = plt.figure().add_subplot().contour(z)
+    cs.set_data(z * 2, algorithm='mpl2005')
+    assert cs._corner_mask is False
 
 
 def _assert_set_data_rejects(exc, match, args=(), **kwargs):