Cannot unpickle pickled object [duplicate]

Question

Running Python 3.4.3 on linux.

import pickle
import ssl
context = ssl.SSLContext(ssl.PROTOCOL_TLSv1_2)
pickled = pickle.dumps(context)
unpickled = pickle.loads(pickled)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    unpickled = pickle.loads(pickled)
TypeError: __new__() missing 1 required positional argument: 'protocol'

I don't do much pickling, so forgive me if this is well know.

I had a bit of a look around the only I found this about pickle in the What's new In Python 3.4 which suggests this should work. Other than that just issues pickling between different python versions.

I also tried setting pickle.HIGHEST_PROTOCOL but this didn't help either.

(BTW I've already worked around this issue but I'm curious why it does not work.)

Answer 1

Classes that take extra arguments to __new__ need to provide a __getnewargs__ method to tell pickle what the extra arguments are.

You can monkey-patch SSLProtocol to add a __getnewargs__ method:

ssl.SSLContext.__getnewargs__ = lambda self: (self.protocol,)

The new feature of pickle in Python 3.4 is that it also supports a __getnewargs_ex__ method that can supply keyword arguments for __new__; but classes still need to provide __getnewargs__ and/or __getnewargs_ex__ themselves.