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Fix Axes.clear() crash for custom spine types #31688

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timhoffm merged 1 commit into from
May 17, 2026
+25 −4
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Fix Axes.clear() crash for custom spine types #31688

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10 changes: 7 additions & 3 deletions lib/matplotlib/spines.py
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Original file line number Diff line number Diff line change
Expand Up @@ -208,9 +208,13 @@ def _ensure_position_is_set(self):

def _ensure_transform_is_set(self):
# Install the default blended transform if the spine still carries
# the placeholder from Spine.__init__. No-op for spines whose
# transform was set explicitly (e.g. via set_patch_arc/_circle).
if self._position is None and self._transform is self.axes.transData:
# the placeholder from Spine.__init__. Restricted to the standard
# cartesian spines: set_position/get_spine_transform only support
# those, and other spines (polar, cartopy's GeoSpine) manage their
# own transform.
if (self.spine_type in ('left', 'right', 'top', 'bottom')
and self._position is None
and self._transform is self.axes.transData):
self.set_position(('outward', 0.0))

@greglucas greglucas May 17, 2026

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I suppose an alternative question would be whether there is a way to do this same idea through a different call mechanism rather than adding these cutouts for the spine types? i.e. using some different function from set_position() to ensure the transform is set.

@timhoffm timhoffm May 17, 2026

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The semantics is encapsulated in the method _ensure_transform_is_set(). Therefore special-casing in there is ok. Possibly one could require thagt derived spines must implement set_position or override _ensure_transform_is_set. But it's easy to do this on our side. If you want to improve further, there should be documentation on subclassing spines.


def register_axis(self, axis):
Expand Down
19 changes: 18 additions & 1 deletion lib/matplotlib/tests/test_spines.py
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Original file line number Diff line number Diff line change
@@ -1,8 +1,9 @@
import numpy as np
import pytest

import matplotlib.path as mpath
import matplotlib.pyplot as plt
from matplotlib.spines import Spines
from matplotlib.spines import Spine, Spines
from matplotlib.testing.decorators import check_figures_equal, image_comparison


Expand Down Expand Up @@ -197,3 +198,19 @@ def test_spine_set_bounds_with_none():
"left bound should be numeric"
assert np.isclose(left_bound[0], ylim[0]), "Lower bound should match original value"
assert np.isclose(left_bound[1], ylim[1]), "Upper bound should match original value"


def test_clear_with_custom_spine_type():
# Spines with a non-cartesian spine_type (e.g. cartopy's GeoSpine) manage
# their own transform and may reject set_position(); Axes.clear() must not
# call _ensure_transform_is_set() on them. See SciTools/cartopy#2674.
class NoPositionSpine(Spine):
def __init__(self, axes, **kwargs):
super().__init__(axes, 'geo', mpath.Path(np.empty((0, 2))), **kwargs)

def set_position(self, position):
raise NotImplementedError('spine does not support set_position')

fig, ax = plt.subplots()
ax.spines['geo'] = NoPositionSpine(ax)
ax.clear() # must not raise
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