Merge pull request matplotlib#20403 from tacaswell/noop_subplot_class_factory

FIX: if we have already subclassed mixin class just return

Author: tacaswell

lib/matplotlib/cbook/__init__.py

@@ -2211,6 +2211,10 @@ def _make_class_factory(mixin_class, fmt, attr_name=None):
 
     @functools.lru_cache(None)
     def class_factory(axes_class):
+        # if we have already wrapped this class, declare victory!
+        if issubclass(axes_class, mixin_class):
+            return axes_class
+
         # The parameter is named "axes_class" for backcompat but is really just
         # a base class; no axes semantics are used.
         base_class = axes_class

lib/matplotlib/tests/test_subplots.py

@@ -5,6 +5,7 @@
 
 import matplotlib.pyplot as plt
 from matplotlib.testing.decorators import image_comparison
+import matplotlib.axes as maxes
 
 
 def check_shared(axs, x_shared, y_shared):
@@ -219,3 +220,8 @@ def test_dont_mutate_kwargs():
                            gridspec_kw=gridspec_kw)
     assert subplot_kw == {'sharex': 'all'}
     assert gridspec_kw == {'width_ratios': [1, 2]}
+
+
+def test_subplot_factory_reapplication():
+    assert maxes.subplot_class_factory(maxes.Axes) is maxes.Subplot
+    assert maxes.subplot_class_factory(maxes.Subplot) is maxes.Subplot