Add classproperty for spec_property like behavior for classmethods.

Author: matthewwardrop

docsite/docs/usage/special_types.md

@@ -304,6 +304,7 @@ class MyClass:
         overridable=True,  # Whether to allow overriding by default.
         cache=False,  # Whether to cache the result after first evaluation.
         invalidated_by=None,  # An iterable of attributes which when mutated invalidate the cache (only supported when used with spec-classes)
+        allow_attribute_error=True,  # Whether to allow properties to raise `AttributeErrors` which are often masked during attribute lookup.
     )
     def method(self):
         ...
@@ -316,6 +317,33 @@ class MyClass:
 
 As always you can refer to the inline `help(spec_property)` for more details.
 
+
+### `classproperty`
+
+`classproperty` is implemented in `spec_classes.types.classproperty`, and
+behaves exactly like `spec_property` except that it acts on classmethods, and
+does not offer inbuilt access to spec-class state. You can use `classproperty`
+as follows:
+
+```python
+class MyClass:
+    @classproperty(
+        overridable=True,  # Whether to allow overriding by default.
+        cache=False,  # Whether to cache the result after first evaluation.
+        cache_per_subclass=False,  # Whether cache should be stored per subclass
+            # rather than once for all classes.
+        allow_attribute_error=True,  # Whether to allow properties to raise `AttributeErrors` which are often masked during attribute lookup.
+    )
+    def method(cls):
+        ...
+
+    # And of course you can override the setters as you would with regular properties
+    @method.setter
+    def method(cls, value):
+        ...
+```
+
+
 ### `AttrProxy`
 
 `AttrProxy` is implemented in `spec_classes.types.attr_proxy`, and allows one

spec_classes/__init__.py

@@ -3,6 +3,7 @@
 from .spec_class import spec_class
 from .types import (
     spec_property,
+    classproperty,
     Alias,
     DeprecatedAlias,
     Attr,
@@ -22,6 +23,7 @@
     "__author_email__",
     "spec_class",
     "spec_property",
+    "classproperty",
     "FrozenInstanceError",
     "Alias",
     "DeprecatedAlias",

spec_classes/types/__init__.py

@@ -3,7 +3,7 @@
 from .attr_proxy import AttrProxy
 from .keyed import KeyedList, KeyedSet
 from .missing import MISSING, EMPTY, SENTINEL
-from .spec_property import spec_property
+from .spec_property import spec_property, classproperty
 from .validated import ValidatedType, bounded, validated
 
 __all__ = (
@@ -19,5 +19,6 @@
     "ValidatedType",
     "bounded",
     "spec_property",
+    "classproperty",
     "validated",
 )