Backport PR matplotlib#26223: Fix: pcolormesh writing to read-only input mask

Author: oscargus

lib/matplotlib/axes/_axes.py

@@ -5699,7 +5699,7 @@ def _pcolorargs(self, funcname, *args, shading='auto', **kwargs):
             else:
                 X, Y = np.meshgrid(np.arange(ncols + 1), np.arange(nrows + 1))
                 shading = 'flat'
-            C = cbook.safe_masked_invalid(C)
+            C = cbook.safe_masked_invalid(C, copy=True)
             return X, Y, C, shading
 
         if len(args) == 3:
@@ -5788,7 +5788,7 @@ def _interp_grid(X):
                     Y = _interp_grid(Y.T).T
                 shading = 'flat'
 
-        C = cbook.safe_masked_invalid(C)
+        C = cbook.safe_masked_invalid(C, copy=True)
         return X, Y, C, shading
 
     @_preprocess_data()

lib/matplotlib/tests/test_axes.py

@@ -1418,6 +1418,23 @@ def test_pcolorargs():
         ax.pcolormesh(X, Y, Z, shading='auto')
 
 
+def test_pcolorargs_with_read_only():
+    x = np.arange(6).reshape(2, 3)
+    xmask = np.broadcast_to([False, True, False], x.shape)  # read-only array
+    assert xmask.flags.writeable is False
+    masked_x = np.ma.array(x, mask=xmask)
+    plt.pcolormesh(masked_x)
+
+    x = np.linspace(0, 1, 10)
+    y = np.linspace(0, 1, 10)
+    X, Y = np.meshgrid(x, y)
+    Z = np.sin(2 * np.pi * X) * np.cos(2 * np.pi * Y)
+    Zmask = np.broadcast_to([True, False] * 5, Z.shape)
+    assert Zmask.flags.writeable is False
+    masked_Z = np.ma.array(Z, mask=Zmask)
+    plt.pcolormesh(X, Y, masked_Z)
+
+
 @check_figures_equal(extensions=["png"])
 def test_pcolornearest(fig_test, fig_ref):
     ax = fig_test.subplots()