gistfile1.txt

import attr
import pickle


@attr.s(frozen=True)
class A(object):
    x = attr.ib()

@attr.s(slots=True)
class B(object):
    x = attr.ib()

@attr.s(slots=True, frozen=True)
class C(object):
    x = attr.ib()

try:
    pickle.loads(pickle.dumps(A(1)))
except:
    print("A failed.")
else:
    print("A succeeded.")

try:
    pickle.loads(pickle.dumps(B(1)))
except:
    print("B failed.")
else:
    print("B succeeded.")

try:
    pickle.loads(pickle.dumps(C(1)))
except:
    print("C failed.")
else:
    print("C succeeded.")


> python p.py
A succeeded.
B succeeded.
C failed.