BUG: Skip removed colorbar axes in constrained layout (fixes #31330)

tinezivic · tinezivic · commit 4abd6e27b565 · 2026-04-22T23:33:45.000+02:00
When cbar.ax.remove() is called directly (bypassing Colorbar.remove()),
the parent axes' _colorbars list still holds a reference to the removed
colorbar Axes. On the next draw(), constrained layout crashes in
make_layout_margins() and reposition_axes() with:

    AttributeError: 'NoneType' object has no attribute 'transSubfigure'

because get_pos_and_bbox() calls ax.get_figure(root=False) which
returns None for a removed Axes.

Fix: guard both _colorbars iteration loops against removed axes by
checking get_figure(root=False) is None and continuing.

Adds regression test.
diff --git a/lib/matplotlib/_constrained_layout.py b/lib/matplotlib/_constrained_layout.py
@@ -398,6 +398,10 @@ def make_layout_margins(layoutgrids, fig, renderer, *, w_pad=0, h_pad=0,
         # make margin for colorbars.  These margins go in the
         # padding margin, versus the margin for Axes decorators.
         for cbax in ax._colorbars:
+            # Skip colorbar axes that have been removed from the figure
+            # (e.g. via ``cbar.ax.remove()`` instead of ``cbar.remove()``).
+            if cbax.get_figure(root=False) is None:
+                continue
             # note pad is a fraction of the parent width...
             pad = colorbar_get_pad(layoutgrids, cbax)
             # colorbars can be child of more than one subplot spec:
@@ -688,6 +692,8 @@ def reposition_axes(layoutgrids, fig, renderer, *,
         # one colorbar:
         offset = {'left': 0, 'right': 0, 'bottom': 0, 'top': 0}
         for nn, cbax in enumerate(ax._colorbars[::-1]):
+            if cbax.get_figure(root=False) is None:
+                continue  # colorbar axes was removed from the figure
             if ax == cbax._colorbar_info['parents'][0]:
                 reposition_colorbar(layoutgrids, cbax, renderer,
                                     offset=offset, compress=compress)
diff --git a/lib/matplotlib/tests/test_constrainedlayout.py b/lib/matplotlib/tests/test_constrainedlayout.py
@@ -809,3 +809,24 @@ def test_submerged_subfig():
     for ax in axs[1:]:
         assert np.allclose(ax.get_position().bounds[-1],
                            axs[0].get_position().bounds[-1], atol=1e-6)
+
+
+def test_colorbar_remove_axes_no_crash():
+    """
+    Regression test for GH #31330.
+
+    Directly removing the colorbar Axes (``cbar.ax.remove()``) should not
+    crash when the figure is subsequently drawn with constrained layout.
+    """
+    import numpy as np
+    x = y = np.linspace(-1, 1, 20)
+    X, Y = np.meshgrid(x, y)
+    Z = X**2 - Y**2
+
+    # cbar.ax.remove() (direct axes removal) should not raise
+    fig, ax = plt.subplots(layout='constrained')
+    CS = ax.contourf(X, Y, Z, cmap='bone')
+    cbar = fig.colorbar(CS)
+    cbar.ax.remove()
+    # Drawing must not crash with AttributeError on transSubfigure
+    fig.draw_without_rendering()
