My proposed text for the PEP is as follows:
"In order for processors of function annotations to work interoperably, they must use a common interpretation of objects used as annotations on a particular function. For example, one might interpret string annotations as docstrings. Another might interpet them as path segments for a web framework. For this reason, function annotation processors SHOULD avoid assigning processor-specific meanings to types defined outside of the processor's framework. For example, a Django processor could process annotations of a type defined in a Zope package, but Zope's creators should be considered the authorities on the type's meaning for the same reasons that they would be considered authorities on the semantics of classes or methods in their packages."
"This implies that the interpretation of built-in types would be controlled by Python's developers and documented in Python's documentation. This is just a best practice. Nothing in the language can or should enforce this practice and there may be a few domains where there is a strong argument for violating it ( e.g. an education environment where saving keystrokes may be more important than easing interopability)."
"In Python 3000, semantics will be attached to the following types: objects of type string (or subtype of string) are to be used for documentation (though they are not necessarily the exclusive source of documentation about the type). Objects of type list (or subtype of list) are to be used for attaching multiple independent annotations."
"Developers who define new metadata frameworks SHOULD choose explicit and unambiguous mechanisms for associating objects with their frameworks. Furthermore, they SHOULD consider that some users may wish to extend their frameworks and should support that. For example, they could use Python 3000 overloaded functions, some form of registry, some kind of interface or some unambiguously recognizable method signature protocol ( e.g. _pytypelib_type_check())."
Paul Prescod
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