matplotlib · timhoffm · Aug 5, 2019 · Jul 29, 2019
diff --git a/examples/scales/custom_scale.py b/examples/scales/custom_scale.py
@@ -114,10 +114,7 @@ class MercatorLatitudeTransform(mtransforms.Transform):
         # being connected together.  When defining transforms for a
         # scale, which are, by definition, separable and have only one
         # dimension, these members should always be set to 1.
-        input_dims = 1
-        output_dims = 1
-        is_separable = True
-        has_inverse = True
+        input_dims = output_dims = 1
 
         def __init__(self, thresh):
             mtransforms.Transform.__init__(self)
@@ -150,10 +147,7 @@ def inverted(self):
                 self.thresh)
 
     class InvertedMercatorLatitudeTransform(mtransforms.Transform):
-        input_dims = 1
-        output_dims = 1
-        is_separable = True
-        has_inverse = True
+        input_dims = output_dims = 1
 
         def __init__(self, thresh):
             mtransforms.Transform.__init__(self)
@@ -165,6 +159,7 @@ def transform_non_affine(self, a):
         def inverted(self):
             return MercatorLatitudeScale.MercatorLatitudeTransform(self.thresh)
 
+
 # Now that the Scale class has been defined, it must be registered so
 # that ``matplotlib`` can find it.
 mscale.register_scale(MercatorLatitudeScale)

diff --git a/lib/matplotlib/scale.py b/lib/matplotlib/scale.py
@@ -128,10 +128,7 @@ class FuncTransform(Transform):
     forward and inverse transform.
     """
 
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def __init__(self, forward, inverse):
         """
@@ -205,10 +202,7 @@ def set_default_locators_and_formatters(self, axis):
 
 @cbook.deprecated("3.1", alternative="LogTransform")
 class LogTransformBase(Transform):
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def __init__(self, nonpos='clip'):
         Transform.__init__(self)
@@ -224,10 +218,7 @@ def __str__(self):
 
 @cbook.deprecated("3.1", alternative="InvertedLogTransform")
 class InvertedLogTransformBase(Transform):
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def transform_non_affine(self, a):
         return ma.power(self.base, a)
@@ -285,10 +276,7 @@ def inverted(self):
 
 
 class LogTransform(Transform):
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def __init__(self, base, nonpos='clip'):
         Transform.__init__(self)
@@ -326,10 +314,7 @@ def inverted(self):
 
 
 class InvertedLogTransform(InvertedLogTransformBase):
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def __init__(self, base):
         Transform.__init__(self)
@@ -466,10 +451,7 @@ def get_transform(self):
 
 
 class SymmetricalLogTransform(Transform):
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def __init__(self, base, linthresh, linscale):
         Transform.__init__(self)
@@ -495,10 +477,7 @@ def inverted(self):
 
 
 class InvertedSymmetricalLogTransform(Transform):
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def __init__(self, base, linthresh, linscale):
         Transform.__init__(self)
@@ -610,10 +589,7 @@ def get_transform(self):
 
 
 class LogitTransform(Transform):
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def __init__(self, nonpos='mask'):
         Transform.__init__(self)
@@ -638,10 +614,7 @@ def __str__(self):
 
 
 class LogisticTransform(Transform):
-    input_dims = 1
-    output_dims = 1
-    is_separable = True
-    has_inverse = True
+    input_dims = output_dims = 1
 
     def __init__(self, nonpos='mask'):
         Transform.__init__(self)

diff --git a/lib/matplotlib/transforms.py b/lib/matplotlib/transforms.py
@@ -1191,16 +1191,22 @@ class Transform(TransformNode):
     - :attr:`input_dims`
     - :attr:`output_dims`
     - :meth:`transform`
-    - :attr:`is_separable`
-    - :attr:`has_inverse`
-    - :meth:`inverted` (if :attr:`has_inverse` is True)
+    - :meth:`inverted` (if an inverse exists)
+
+    The following attributes may be overridden if the default is unsuitable:
+
+    - :attr:`is_separable` (defaults to True for 1d -> 1d transforms, False
+      otherwise)
+    - :attr:`has_inverse` (defaults to True if :meth:`inverted` is overridden,
+      False otherwise)
 
     If the transform needs to do something non-standard with
     :class:`matplotlib.path.Path` objects, such as adding curves
     where there were once line segments, it should override:
 
     - :meth:`transform_path`
     """
+
     input_dims = None
     """
     The number of input dimensions of this transform.
@@ -1213,11 +1219,25 @@ class Transform(TransformNode):
     Must be overridden (with integers) in the subclass.
     """
 
+    is_separable = False
+    """True if this transform is separable in the x- and y- dimensions."""
+
     has_inverse = False
     """True if this transform has a corresponding inverse transform."""
 
-    is_separable = False
-    """True if this transform is separable in the x- and y- dimensions."""
+    def __init_subclass__(cls):
+        # 1d transforms are always separable; we assume higher-dimensional ones
+        # are not but subclasses can also directly set is_separable.
+        if ("is_separable" not in vars(cls)  # Was it overridden explicitly?
+                and cls.input_dims == cls.output_dims == 1):
+            cls.is_separable = True
+        # Transform.inverted raises NotImplementedError; we assume that if this
+        # is overridden then the transform is invertible but subclass can also
+        # directly set has_inverse.
+        if ("has_inverse" not in vars(cls)  # Was it overridden explicitly?
+                and hasattr(cls, "inverted")
+                and cls.inverted is not Transform.inverted):
+            cls.has_inverse = True
 
     def __add__(self, other):
         """
@@ -1733,8 +1753,6 @@ class Affine2DBase(AffineBase):
     Subclasses of this class will generally only need to override a
     constructor and :meth:`get_matrix` that generates a custom 3x3 matrix.
     """
-    has_inverse = True
-
     input_dims = 2
     output_dims = 2