Assume that I have a pickle dump - either as a file or just as a string - how can I determine the protocol that was used to create the pickle dump automatically?

And if so, do I need to read the entire dump to figure out the protocol or can this be achieved in O(1)? By O(1) I think about some header information at the beginning of the pickle string or file whose read out does not require processing the whole dump.

Thanks a lot!

EDIT: I have an update on this, apparently the answer given below does not always work under python 3.4. If I simply pickle the value True with protocol 1, sometimes I can only recover protocol 0 :-/

You could roll your own using picketools:

with open('your_pickle_file', 'rb') as fin:op, fst, snd = next(pickletools.genops(fin))proto = op.proto

It appears that a PROTO marker is only written as the first element where the protocol is 2 or greater. Otherwise, the first element is a marker or element that indicates if the protocol is 0 or 1.

Update into kludging even more land:

pops = pickletools.genops(pickle_source)
proto = 2 if next(pops)[0].proto == 2 else int(any(op.proto for op, fst, snd in pops))