Add to documentation

docs/extension_examples.md

@@ -259,6 +259,138 @@ if __name__ == "__main__":
         print(e)
 ```
 
+#### Frozen Attributes ####
+
+Here's an implementation that allows freezing of individual attributes.
+
+```python
+import ducktools.classbuilder as dtbuild
+
+
+class FreezableField(dtbuild.Field):
+    frozen: bool = False
+
+
+def setattr_generator(cls, funcname="__setattr__"):
+    globs = {}
+
+    flags = dtbuild.get_flags(cls)
+    fields = dtbuild.get_fields(cls)
+
+    frozen_fields = set(
+        name for name, field in fields.items()
+        if getattr(field, "frozen", False)
+    )
+
+    globs["__frozen_fields"] = frozen_fields
+
+    if flags.get("slotted", True):
+        globs["__setattr_func"] = object.__setattr__
+        setattr_method = "__setattr_func(self, name, value)"
+        attrib_check = "hasattr(self, name)"
+    else:
+        setattr_method = "self.__dict__[name] = value"
+        attrib_check = "name in self.__dict__"
+
+    code = (
+        f"def {funcname}(self, name, value):\n"
+        f"    if name in __frozen_fields and {attrib_check}:\n"
+        f"        raise AttributeError(\n"
+        f"            f'Attribute {{name!r}} does not support assignment'\n"
+        f"        )\n"
+        f"    else:\n"
+        f"        {setattr_method}\n"
+    )
+
+    return dtbuild.GeneratedCode(code, globs)
+
+
+def delattr_generator(cls, funcname="__delattr__"):
+    globs = {}
+
+    flags = dtbuild.get_flags(cls)
+    fields = dtbuild.get_fields(cls)
+
+    frozen_fields = set(
+        name for name, field in fields.items()
+        if getattr(field, "frozen", False)
+    )
+
+    globs["__frozen_fields"] = frozen_fields
+
+    if flags.get("slotted", True):
+        globs["__delattr_func"] = object.__delattr__
+        delattr_method = "__delattr_func(self, name)"
+    else:
+        delattr_method = "del self.__dict__[name]"
+
+    code = (
+        f"def {funcname}(self, name):\n"
+        f"    if name in __frozen_fields:"
+        f"        raise AttributeError(\n"
+        f"            f'Attribute {{name!r}} is frozen and can not be deleted'\n"
+        f"        )\n"
+        f"    else:\n"
+        f"        {delattr_method}\n"
+    )
+
+    return dtbuild.GeneratedCode(code, globs)
+
+
+frozen_setattr_field_maker = dtbuild.MethodMaker("__setattr__", setattr_generator)
+frozen_delattr_field_maker = dtbuild.MethodMaker("__delattr__", delattr_generator)
+gatherer = dtbuild.make_unified_gatherer(FreezableField)
+
+
+def freezable(cls=None, /, *, frozen=False):
+    if cls is None:
+        return lambda cls_: freezable(cls_, frozen=frozen)
+
+    # To make a slotted class use a base class with metaclass
+    flags = {"frozen": frozen, "slotted": False}
+
+    cls = dtbuild.builder(
+        cls,
+        gatherer=gatherer,
+        methods=dtbuild.default_methods,
+        flags=flags,
+    )
+
+    # Frozen attributes need to be added afterwards
+    # Due to the need to know if frozen fields exist
+    if frozen:
+        setattr(cls, "__setattr__", dtbuild.frozen_setattr_maker)
+        setattr(cls, "__delattr__", dtbuild.frozen_delattr_maker)
+    else:
+        fields = dtbuild.get_fields(cls)
+        has_frozen_fields = False
+        for f in fields.values():
+            if getattr(f, "frozen", False):
+                has_frozen_fields = True
+                break
+
+        if has_frozen_fields:
+            setattr(cls, "__setattr__", frozen_setattr_field_maker)
+            setattr(cls, "__delattr__", frozen_delattr_field_maker)
+
+    return cls
+
+
+@freezable
+class X:
+    a: int = 2
+    b: int = FreezableField(default=12, frozen=True)
+
+
+x = X()
+x.a = 21
+
+try:
+    x.b = 43
+except AttributeError as e:
+    print(repr(e))
+```
+
 #### Converters ####
 
 Here's an implementation of basic converters that always convert when