The docs of shelve mention that

> Shelf objects support all methods supported by dictionaries. This eases the transition from dictionary based scripts to those requiring persistent storage.

However the `|=` operator is not implemented, preventing a seamless transition from `dict` to `shelve`. So should this be implemented for `Shelf` as well? `|` on the other hand doesn't make much sense.

Otherwise the docs could be updated.

The comment is outdated. Shelf objects also do not support methods copy and fromkey. Creating a new Shelve object without specifying a new underlying database object does not make much sense.

Maybe say that they implement the MutableMapping interface?

>>> sorted(set(dir(dict)) - set(dir(shelve.Shelf)))
['__ior__', '__or__', '__ror__', 'copy', 'fromkeys']
>>> sorted(set(dir(collections.abc.MutableMapping)) - set(dir(shelve.Shelf)))
[]

Right, that seems like a good idea. But `__ior__` could be implemented nevertheless?

> Maybe say that they implement the MutableMapping interface?

Yes, that's a good idea (also add link to the MutableMapping docs).

> Right, that seems like a good idea. But `__ior__` could
> be implemented nevertheless?

Off-hand, I don't see why not.

Adding Brandt to the nosy list to see what he thinks.

One other thought.  While __ior__() could be used as a short-cut for update(), the related __or__() method is more problematic because it would need to create a new underlying DB.  So maybe this is a can worms that we should not open. It would be weird to have __ior__() but not __or__().

+1 for the MutableMapping comment.

We purposely omitted shelve when determining what classes should grow the new operators. Guido's thoughts:

> I definitely think we should leave Shelf alone, it's a toy class from a different era.

(https://bugs.python.org/msg364196)

I personally have no experience with shelve, so I'd rather defer to the judgement of others here.

It's true, having `__ior__` but not `__or__` would probably be weird. In the end it's just "nice to have", but I'm not even sure that this applies. Calling `db.update(...)` is still more explicit than `db |= ...`.  The docs mention that

> This eases the transition from dictionary based scripts to those requiring persistent storage.

For my use cases, however, I always knew right from the beginning that I want object persistence between different runs of a script (e.g. for data analysis, caching the expensive results), so it was always clear that I'm working with a Shelf object and not a dict (i.e. no expectations on the availability of `|=`).

Primarily, this issue was meant to point out the mismatch of docs/implementation and not to get `|=` implemented for `Shelf`. In the end, I think updating the docs is all that is needed.

New changeset c97c2a050cf753003012ae3f08e035326b8f6167 by Serhiy Storchaka in branch 'main':
bpo-43443: Clarify difference between shelve objects and dicts (GH-27004)
https://github.com/python/cpython/commit/c97c2a050cf753003012ae3f08e035326b8f6167

New changeset 563e05743c8f31366db7ef35a00e396c9f442b17 by Miss Islington (bot) in branch '3.10':
bpo-43443: Clarify difference between shelve objects and dicts (GH-27004) (GH-27369)
https://github.com/python/cpython/commit/563e05743c8f31366db7ef35a00e396c9f442b17

New changeset 7482fff297727bb5faf64cc92c73561ccf48feda by Miss Islington (bot) in branch '3.9':
bpo-43443: Clarify difference between shelve objects and dicts (GH-27004) (GH-27370)
https://github.com/python/cpython/commit/7482fff297727bb5faf64cc92c73561ccf48feda