matplotlib
diff --git a/‎doc/api/next_api_changes/deprecations/17737-AL.rst
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+6Lines changed: 6 additions & 0 deletions b/‎doc/api/next_api_changes/deprecations/17737-AL.rst
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diff --git a/‎examples/userdemo/custom_boxstyle01.py
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+7-21Lines changed: 7 additions & 21 deletions b/‎examples/userdemo/custom_boxstyle01.py
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+7-21Lines changed: 7 additions & 21 deletions
diff --git a/‎lib/matplotlib/cbook/deprecation.py
Copy file name to clipboardExpand all lines: lib/matplotlib/cbook/deprecation.py
+10-5Lines changed: 10 additions & 5 deletions b/‎lib/matplotlib/cbook/deprecation.py
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+10-5Lines changed: 10 additions & 5 deletions
@@ -0,0 +1,6 @@
+BoxStyles are now called without passing the *mutation_aspect* parameter
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Mutation aspect is now handled by the artist itself.  Hence the
+*mutation_aspect* parameter of ``BoxStyle._Base.__call__`` is deprecated, and
+custom boxstyles should be implemented to not require this parameter (it can be
+left as a parameter defaulting to 1 for back-compatibility).
@@ -25,7 +25,7 @@
 # ``bbox=dict(boxstyle=custom_box_style, ...)`` to `.Axes.text`.
 
 
-def custom_box_style(x0, y0, width, height, mutation_size, mutation_aspect=1):
+def custom_box_style(x0, y0, width, height, mutation_size):
     """
     Given the location and size of the box, return the path of the box around
     it.
@@ -38,11 +38,7 @@ def custom_box_style(x0, y0, width, height, mutation_size, mutation_aspect=1):
         Box location and size.
     mutation_size : float
         Mutation reference scale, typically the text font size.
-    mutation_aspect
-        Mutation aspect ratio.
     """
-    # We ignore mutation_aspect. This is okay in general.
-
     # padding
     mypad = 0.3
     pad = mutation_size * mypad
@@ -66,19 +62,17 @@ def custom_box_style(x0, y0, width, height, mutation_size, mutation_aspect=1):
 
 
 ###############################################################################
-# Alternatively, custom box styles can be implemented as subclasses of
-# ``matplotlib.patches.BoxStyle._Base``, by overriding the ``transmute``
-# method, as demonstrated below.
+# Likewise, custom box styles can be implemented as classes that implement
+# ``__call__``.
 #
-# The subclass can then be registered into the ``BoxStyle._style_list`` dict,
+# The classes can then be registered into the ``BoxStyle._style_list`` dict,
 # which allows specifying the box style as a string,
 # ``bbox=dict(boxstyle="registered_name,param=value,...", ...)``.
-#
-# Note that this approach relies on internal APIs and is therefore not
+# Note that this registration relies on internal APIs and is therefore not
 # officially supported.
 
 
-class MyStyle(BoxStyle._Base):
+class MyStyle:
     """A simple box."""
 
     def __init__(self, pad=0.3):
@@ -93,7 +87,7 @@ def __init__(self, pad=0.3):
         self.pad = pad
         super().__init__()
 
-    def transmute(self, x0, y0, width, height, mutation_size):
+    def __call__(self, x0, y0, width, height, mutation_size):
         """
         Given the location and size of the box, return the path of the box
         around it.
@@ -106,14 +100,6 @@ def transmute(self, x0, y0, width, height, mutation_size):
             Box location and size.
         mutation_size : float
             Reference scale for the mutation, typically the text font size.
-
-        Notes
-        -----
-        Unlike when defining the box style as a function (as in
-        `custom_box_style`), here there is no *mutation_aspect* parameter.
-        Matplotlib will first squeeze the box's y-axis by *mutation_aspect*
-        before calling the `transmute` method, and then later reexpand the
-        y-axis by the same amount.
         """
         # padding
         pad = mutation_size * self.pad
 
@@ -471,7 +471,8 @@ def _deprecate_method_override(method, obj, *, allow_empty=False, **kwargs):
         can always use ``__class__`` to refer to the class that is currently
         being defined.
     obj
-        An object of the class where *method* is defined.
+        Either an object of the class where *method* is defined, or a subclass
+        of that class.
     allow_empty : bool, default: False
         Whether to allow overrides by "empty" methods without emitting a
         warning.
@@ -484,15 +485,19 @@ def empty(): pass
     def empty_with_docstring(): """doc"""
 
     name = method.__name__
-    bound_method = getattr(obj, name)
-    if (bound_method != method.__get__(obj)
+    bound_child = getattr(obj, name)
+    bound_base = (
+        method  # If obj is a class, then we need to use unbound methods.
+        if isinstance(bound_child, type(empty)) and isinstance(obj, type)
+        else method.__get__(obj))
+    if (bound_child != bound_base
             and (not allow_empty
-                 or (getattr(getattr(bound_method, "__code__", None),
+                 or (getattr(getattr(bound_child, "__code__", None),
                              "co_code", None)
                      not in [empty.__code__.co_code,
                              empty_with_docstring.__code__.co_code]))):
         warn_deprecated(**{"name": name, "obj_type": "method", **kwargs})
-        return bound_method
+        return bound_child
     return None