PickleSerializer default protocol=0

[jayvdb]

PickleSerializer default protocol=0 is quite problematic IMO - it is surprising that on modern Python (3), protocol 0 would be the default.

For me this resulted in copyreg._reduce_ex (protocol 0 and 1) raising

a class that defines slots without defining getstate cannot be pickled

It is possible to override this with serializer='pickle:///?protocol=4'

cloudpickle uses default DEFAULT_PROTOCOL = pickle.HIGHEST_PROTOCOL which is dynamically used if the protocol arg at instantiation is None, allowing the caller to modify module attribute DEFAULT_PROTOCOL if they need to. That probably isnt ideal here where the serializer arg can take sub-args.

Perhaps the default should be serializer='pickle:///?protocol=highest', or at least that should be supported to allow easy access to a sane default for most common purposes where cross-python-version pickling isnt needed.

[jayvdb]

I guess protocol=0 may have been selected as it results in text in sqlite instead of blobs. It can be desirable. If so, documentation of the various options would be useful

One way to auto allow that would be to attempt protocol=0, detect failure, and then switch to a higher protocol after failure was detected. Test cases for that will be a little bit tricky, but it would simplify users first experience of the library if they are using slightly complex data values.

[jayvdb]

Also it would be a good idea to detect if ?protocol= is higher than pickle.HIGHEST_PROTOCOL and use a backport if installed. I have tested https://pypi.org/project/pickle4/ and https://pypi.org/project/pickle5/ quite successfully.