PEP 767: Address feedback & open issues #4559
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Clarify that bare `ReadOnly` is not allowed, and add that to rejected ideas Specify that `type[Protocol]` does not inherit the protocol's attributes Close issue: "Extending Initialization" Bikeshed on some wording Use my real name
| @@ -176,6 +178,7 @@ It can be used at class-level or within ``__init__`` to mark individual attribut | |||
| self.id = id | |||
| self.name: ReadOnly[str] = name | |||
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| Use of bare ``ReadOnly`` (without ``[<type>]``) is not allowed. | |||
| Type checkers should error on any attempt to reassign or ``del``\ ete an attribute | |||
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The meaning of "reassign" here is not clear. (What is an "assignment" and what is a "reassignment"?)
I think it's clarified below, but maybe we should reference that here.
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| Instance Attributes | ||
| ''''''''''''''''''' | ||
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| Assignment to an instance attribute must be allowed in the following contexts: | ||
| Assignment to a read-only instance attribute must be allowed in the following contexts: |
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I think @oscarbenjamin raised an interesting point in the Discourse thread about this. The effects of ReadOnly as described in this PEP (structural typing and variance of generics) only require for soundness that we disallow external mutation of the attribute. Disallowing internal mutation (within methods of the class) is not required for soundness, and is arguably veering too far into Final territory.
It would simplify this section (and type checker implementations) quite a lot if we just specified that ReadOnly describes the external interface of the attribute, and all internal reassignments of a ReadOnly attribute (within methods of the class) are permitted.
ReadOnlyis not allowed, and added that to rejected ideastype[Protocol])📚 Documentation preview 📚: https://pep-previews--4559.org.readthedocs.build/