Implicit Exception chains are not pickled.

[bernardpazio]

Explicit exceptions those given with the from syntax when raising an exception are pickled, however ones which are called with out this are not.

excepted:

def chain_exception():
    try:
        raise Exception('Foo')
    except Exception as ex:
        try:
            raise Exception('Bar')
        except Exception as e:
            raise Exception('explict') from e

chain_exception()

Exception chain

Traceback (most recent call last):
  File "<ipython-input-14-53e847eef0b4>", line 5, in chain_exception
    raise Exception('Foo')
Exception: Foo

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<ipython-input-14-53e847eef0b4>", line 8, in chain_exception
    raise Exception('Bar')
Exception: Bar

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/bert/.conda/envs/dask-dev/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3343, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-14-53e847eef0b4>", line 12, in <module>
    chain_exception()
  File "<ipython-input-14-53e847eef0b4>", line 10, in chain_exception
    raise Exception('explict') from e
Exception: explict

after pickling:

import pickle
import tblib.pickling_support
tblib.pickling_support.install()

try:
    chain_exception()
except Exception as e:
    pickled_e = pickle.dumps(e, protocol=pickle.HIGHEST_PROTOCOL)

raise pickled_e

Exception chain

Traceback (most recent call last):
  File "<ipython-input-14-53e847eef0b4>", line 8, in chain_exception
    raise Exception('Bar')
Exception: Bar

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/bert/.conda/envs/dask-dev/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3343, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-15-269163ec6143>", line 8, in <module>
    raise pickle.loads(pickled_e)
  File "<ipython-input-15-269163ec6143>", line 2, in <module>
    chain_exception()
  File "<ipython-input-14-53e847eef0b4>", line 10, in chain_exception
    raise Exception('explict') from e
Exception: explict

I believe this is simply due to the fact that __cause__ attribute is pickled but the __context__ one is not. See PEP 3134 for further details on this.