matplotlib · joshbarrass · Oct 16, 2022 · Oct 16, 2022 · Oct 16, 2022 · Oct 16, 2022
diff --git a/doc/api/next_api_changes/behavior/24188-JB.rst b/doc/api/next_api_changes/behavior/24188-JB.rst
@@ -0,0 +1,9 @@
+``fig.subplot_mosaic`` no longer passes the ``width_ratios`` or ``height_ratios`` args to the nested gridspecs.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Previously, the entirety of the ``gridspec_kw`` dictionary was passed
+to the nested gridspecs. This caused an error if the inner layout was
+incompatible with one of these ratios. A new dictionary is now made,
+``nested_gs_kw``, where the ``"width_ratios"`` and ``"height_ratios"``
+keys have been removed, which is passed to any nested gridspecs. This
+prevents such errors.
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -1838,21 +1838,27 @@ def subplot_mosaic(self, mosaic, *, sharex=False, sharey=False,
             Defines the relative widths of the columns. Each column gets a
             relative width of ``width_ratios[i] / sum(width_ratios)``.
             If not given, all columns will have the same width.  Equivalent
-            to ``gridspec_kw={'width_ratios': [...]}``.
+            to ``gridspec_kw={'width_ratios': [...]}``. This argument applies
+            only to the outer layout and will not be passed to the inner
+            layout.
 
         height_ratios : array-like of length *nrows*, optional
             Defines the relative heights of the rows. Each row gets a
             relative height of ``height_ratios[i] / sum(height_ratios)``.
             If not given, all rows will have the same height. Equivalent
-            to ``gridspec_kw={'height_ratios': [...]}``.
+            to ``gridspec_kw={'height_ratios': [...]}``. This argument
+            applies only to the outer layout and will not be passed to the
+            inner layout.
 
         subplot_kw : dict, optional
             Dictionary with keywords passed to the `.Figure.add_subplot` call
             used to create each subplot.
 
         gridspec_kw : dict, optional
             Dictionary with keywords passed to the `.GridSpec` constructor used
-            to create the grid the subplots are placed on.
+            to create the grid the subplots are placed on. The 'width_ratios'
+            and 'height_ratios' keys, if present, will be removed from this
+            dictionary before it is passed to any inner layouts.
 
         empty_sentinel : object, optional
             Entry in the layout to mean "leave this space empty".  Defaults
@@ -1999,6 +2005,13 @@ def _do_layout(gs, mosaic, unique_ids, nested):
             for (j, k), nested_mosaic in nested.items():
                 this_level[(j, k)] = (None, nested_mosaic, 'nested')
 
+            # must remove these two options from the kwargs passed to the
+            # nested items, otherwise an error occurs if *any* of the inner
+            # layouts are not compatible with these ratios
+            nested_gs_kw = dict([(k, v) for k, v in gridspec_kw.items()
+                                 if k != "width_ratios"
+                                 and k != "height_ratios"])
+
             # now go through the things in this level and add them
             # in order left-to-right top-to-bottom
             for key in sorted(this_level):
@@ -2022,7 +2035,7 @@ def _do_layout(gs, mosaic, unique_ids, nested):
                     # recursively add the nested mosaic
                     rows, cols = nested_mosaic.shape
                     nested_output = _do_layout(
-                        gs[j, k].subgridspec(rows, cols, **gridspec_kw),
+                        gs[j, k].subgridspec(rows, cols, **nested_gs_kw),
                         nested_mosaic,
                         *_identify_keys_and_nested(nested_mosaic)
                     )

diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
@@ -922,6 +922,26 @@ def test_nested_tuple(self, fig_test, fig_ref):
         fig_ref.subplot_mosaic([["F"], [x]])
         fig_test.subplot_mosaic([["F"], [xt]])
 
+    def test_nested_width_ratios(self):
+        x = [["A", [["B"],
+                    ["C"]]]]
+        width_ratios = [2, 1]
+
+        fig, axd = plt.subplot_mosaic(x, width_ratios=width_ratios)
+
+        assert axd["A"].get_gridspec().get_width_ratios() == width_ratios
+        assert axd["B"].get_gridspec().get_width_ratios() != width_ratios
+
+    def test_nested_height_ratios(self):
+        x = [["A", [["B"],
+                    ["C"]]], ["D", "D"]]
+        height_ratios = [1, 2]
+
+        fig, axd = plt.subplot_mosaic(x, height_ratios=height_ratios)
+
+        assert axd["D"].get_gridspec().get_height_ratios() == height_ratios
+        assert axd["B"].get_gridspec().get_height_ratios() != height_ratios
+
     @check_figures_equal(extensions=["png"])
     @pytest.mark.parametrize(
         "x, empty_sentinel",