[Python-ideas] RFC: PEP: Add dict.__version__

[Andrew Barnert]

On Jan 11, 2016, at 14:56, Terry Reedy <tjreedy at udel.edu> wrote:
> 
> Classes with immutable instances (tuples, numbers, strings, frozen sets, some extension classes, and user classes that take special measures) are exceptions.  So are classes with mutable hashes (lists, sets, dicts, some extension classes, and user classes that override __eq__ and __hash__).

I don't understand your terminology here. What are "classes with mutable hashes"? Your examples of lists, sets, and dicts don't have mutable hashes; they have no hashes. If you write "hash([])", you get a TypeError("unhashable type: 'list'"). And well-behaved extensions classes and user classes that override __eq__ and __hash__ provide immutable hashes and immutable equality to match, or they use __hash__=None if they need mutable equality.

Python can't actually stop you from creating a class with mutable hashes, and even putting instances of such a class in a dict, but that dict won't actually work right. So, there's nothing for a version-guarded dict to worry about there.