FIX: do not mutate dictionaries passed in by user

lib/matplotlib/figure.py

@@ -878,8 +878,7 @@ def subplots(self, nrows=1, ncols=1, *, sharex=False, sharey=False,
             # Note that this is the same as
             fig.subplots(2, 2, sharex=True, sharey=True)
         """
-        if gridspec_kw is None:
-            gridspec_kw = {}
+        gridspec_kw = dict(gridspec_kw or {})
         if height_ratios is not None:
             if 'height_ratios' in gridspec_kw:
                 raise ValueError("'height_ratios' must not be defined both as "
@@ -1869,7 +1868,7 @@ def subplot_mosaic(self, mosaic, *, sharex=False, sharey=False,
 
         """
         subplot_kw = subplot_kw or {}
-        gridspec_kw = gridspec_kw or {}
+        gridspec_kw = dict(gridspec_kw or {})
         if height_ratios is not None:
             if 'height_ratios' in gridspec_kw:
                 raise ValueError("'height_ratios' must not be defined both as "

lib/matplotlib/tests/test_figure.py

@@ -1412,3 +1412,11 @@ def test_unpickle_with_device_pixel_ratio():
     assert fig.dpi == 42*7
     fig2 = pickle.loads(pickle.dumps(fig))
     assert fig2.dpi == 42
+
+
+def test_gridspec_no_mutate_input():
+    gs = {'left': .1}
+    gs_orig = dict(gs)
+    plt.subplots(1, 2, width_ratios=[1, 2], gridspec_kw=gs)
+    assert gs == gs_orig
+    plt.subplot_mosaic('AB', width_ratios=[1, 2], gridspec_kw=gs)