Clarify that initialization must be allowed

Author: Enegg

peps/pep-0767.rst

@@ -250,20 +250,24 @@ Initialization
 
 Assignment to a read-only attribute can only occur in the class declaring the attribute.
 There is no restriction to how many times the attribute can be assigned to.
-The assignment can happen only\* in the following contexts:
+The assignment must be allowed in the following contexts:
 
-1. In the body of ``__init__``, on the instance received as the first parameter (likely, ``self``).
-2. In the body of ``__new__``, on instances of the declaring class created via
-   a direct call to a super-class' ``__new__`` method.
-3. In the body of the class.
+1. In ``__init__``, on the instance received as the first parameter (likely, ``self``).
+2. In ``__new__``, on instances of the declaring class created via a call
+   to a super-class' ``__new__`` method.
+3. At declaration in the body of the class.
 
-\*A type checker may choose to allow assignment to read-only attributes on instances
-of the declaring class in ``__new__``, without regard to the origin of the instance.
-(This choice trades soundness, as the instance may already be initialized,
-for the simplicity of implementation.)
+Additionally, a type checker may choose to allow the assignment:
 
-Note that child classes cannot assign to read-only attributes of parent classes
-in any of the aforementioned contexts, unless the attributes are redeclared.
+1. In ``__new__``, on instances of the declaring class, without regard
+   to the origin of the instance.
+   (This choice trades soundness, as the instance may already be initialized,
+   for the simplicity of implementation.)
+2. In ``@classmethod``\ s, on instances of the declaring class created via
+   a call to the class' or super-class' ``__new__`` method.
+
+Note that a child class cannot assign to any read-only attribute of a parent class
+in any of the aforementioned contexts, unless the attribute is redeclared.
 
 .. code-block:: python
 
@@ -296,7 +300,7 @@ in any of the aforementioned contexts, unless the attributes are redeclared.
 
     class SubBand(Band):
         def __init__(self) -> None:
-            self.songs = []  # error: cannot assign to a read-only attribute of base class
+            self.songs = []  # error: cannot assign to a read-only attribute of a base class
 
 When a class-level declaration has an initializing value, it can serve as a `flyweight <https://en.wikipedia.org/wiki/Flyweight_pattern>`_
 default for instances:
@@ -475,10 +479,9 @@ However, caution is advised while using the backported ``typing_extensions.ReadO
 in older versions of Python. Mechanisms inspecting annotations may behave incorrectly
 when encountering ``ReadOnly``; in particular, the ``@dataclass`` decorator
 which `looks for <https://docs.python.org/3/library/dataclasses.html#class-variables>`_
-``ClassVar`` may incorrectly treat ``ReadOnly[ClassVar[...]]`` as an instance attribute.
+``ClassVar`` may mistakenly treat ``ReadOnly[ClassVar[...]]`` as an instance attribute.
 
-In such circumstances, authors should prefer ``ClassVar[ReadOnly[...]]`` over
-``ReadOnly[ClassVar[...]]``.
+To avoid issues with introspection, use ``ClassVar[ReadOnly[...]]`` instead of ``ReadOnly[ClassVar[...]]``.
 
 
 Security Implications