matplotlib · vagnermcj · Apr 29, 2025 · May 13, 2025 · May 13, 2025 · May 13, 2025
diff --git a/lib/matplotlib/scale.py b/lib/matplotlib/scale.py
@@ -31,6 +31,7 @@
 
 import inspect
 import textwrap
+from functools import wraps
 
 import numpy as np
 
@@ -103,13 +104,61 @@ def limit_range_for_scale(self, vmin, vmax, minpos):
         return vmin, vmax
 
 
+def _make_axis_parameter_optional(init_func):
+    """
+    Decorator to allow leaving out the *axis* parameter in scale constructors.
+
+    This decorator ensures backward compatibility for scale classes that
+    previously required an *axis* parameter. It allows constructors to be
+    callerd with or without the *axis* parameter.
+
+    For simplicity, this does not handle the case when *axis*
+    is passed as a keyword. However,
+    scanning GitHub, there's no evidence that that is used anywhere.
+
+    Parameters
+    ----------
+    init_func : callable
+        The original __init__ method of a scale class.
+
+    Returns
+    -------
+    callable
+        A wrapped version of *init_func* that handles the optional *axis*.
+
+    Notes
+    -----
+    If the wrapped constructor defines *axis* as its first argument, the
+    parameter is preserved when present. Otherwise, the value `None` is injected
+    as the first argument.
+
+    Examples
+    --------
+    >>> from matplotlib.scale import ScaleBase
+    >>> class CustomScale(ScaleBase):
+    ...     @_make_axis_parameter_optional
+    ...     def __init__(self, axis, custom_param=1):
+    ...         self.custom_param = custom_param
+    """
+    @wraps(init_func)
+    def wrapper(self, *args, **kwargs):
+        if args and isinstance(args[0], mpl.axis.Axis):
+            return init_func(self, *args, **kwargs)
+        else:
+            # Remove 'axis' from kwargs to avoid double assignment
+            kwargs.pop('axis', None)
+            return init_func(self, None, *args, **kwargs)
+    return wrapper
+
+
 class LinearScale(ScaleBase):
     """
     The default linear scale.
     """
 
     name = 'linear'
 
+    @_make_axis_parameter_optional
     def __init__(self, axis):
         # This method is present only to prevent inheritance of the base class'
         # constructor docstring, which would otherwise end up interpolated into
@@ -180,6 +229,7 @@ class FuncScale(ScaleBase):
 
     name = 'function'
 
+    @_make_axis_parameter_optional
     def __init__(self, axis, functions):
         """
         Parameters
@@ -279,7 +329,8 @@ class LogScale(ScaleBase):
     """
     name = 'log'
 
-    def __init__(self, axis, *, base=10, subs=None, nonpositive="clip"):
+    @_make_axis_parameter_optional
+    def __init__(self, axis=None, *, base=10, subs=None, nonpositive="clip"):
         """
         Parameters
         ----------
@@ -330,6 +381,7 @@ class FuncScaleLog(LogScale):
 
     name = 'functionlog'
 
+    @_make_axis_parameter_optional
     def __init__(self, axis, functions, base=10):
         """
         Parameters
@@ -455,7 +507,8 @@ class SymmetricalLogScale(ScaleBase):
     """
     name = 'symlog'
 
-    def __init__(self, axis, *, base=10, linthresh=2, subs=None, linscale=1):
+    @_make_axis_parameter_optional
+    def __init__(self, axis=None, *, base=10, linthresh=2, subs=None, linscale=1):
         self._transform = SymmetricalLogTransform(base, linthresh, linscale)
         self.subs = subs
 
@@ -547,7 +600,8 @@ class AsinhScale(ScaleBase):
         1024: (256, 512)
     }
 
-    def __init__(self, axis, *, linear_width=1.0,
+    @_make_axis_parameter_optional
+    def __init__(self, axis=None, *, linear_width=1.0,
                  base=10, subs='auto', **kwargs):
         """
         Parameters
@@ -645,7 +699,8 @@ class LogitScale(ScaleBase):
     """
     name = 'logit'
 
-    def __init__(self, axis, nonpositive='mask', *,
+    @_make_axis_parameter_optional
+    def __init__(self, axis=None, nonpositive='mask', *,
                  one_half=r"\frac{1}{2}", use_overline=False):
         r"""
         Parameters

diff --git a/lib/matplotlib/scale.pyi b/lib/matplotlib/scale.pyi
@@ -15,6 +15,10 @@ class ScaleBase:
 
 class LinearScale(ScaleBase):
     name: str
+    def __init__(
+            self,
+            axis:  Axis | None,
+        ) -> None: ...
 
 class FuncTransform(Transform):
     input_dims: int
@@ -57,7 +61,7 @@ class LogScale(ScaleBase):
     subs: Iterable[int] | None
     def __init__(
         self,
-        axis: Axis | None,
+        axis: Axis | None = ...,
         *,
         base: float = ...,
         subs: Iterable[int] | None = ...,
@@ -104,7 +108,7 @@ class SymmetricalLogScale(ScaleBase):
     subs: Iterable[int] | None
     def __init__(
         self,
-        axis: Axis | None,
+        axis: Axis | None = ...,
         *,
         base: float = ...,
         linthresh: float = ...,
@@ -138,7 +142,7 @@ class AsinhScale(ScaleBase):
     auto_tick_multipliers: dict[int, tuple[int, ...]]
     def __init__(
         self,
-        axis: Axis | None,
+        axis: Axis | None = ...,
         *,
         linear_width: float = ...,
         base: float = ...,
@@ -165,7 +169,7 @@ class LogitScale(ScaleBase):
     name: str
     def __init__(
         self,
-        axis: Axis | None,
+        axis: Axis | None = ...,
         nonpositive: Literal["mask", "clip"] = ...,
         *,
         one_half: str = ...,
@@ -176,3 +180,4 @@ class LogitScale(ScaleBase):
 def get_scale_names() -> list[str]: ...
 def scale_factory(scale: str, axis: Axis, **kwargs) -> ScaleBase: ...
 def register_scale(scale_class: type[ScaleBase]) -> None: ...
+def _make_axis_parameter_optional(init_func: Callable[..., None]) -> Callable[..., None]: ...