Issue 29466: pickle does not serialize Exception __cause__ field

python3 pickle does not serialize the __cause__ field, as shown by the attached demo program.

True. Attributes __context__, __cause__ and __traceback__ are not pickled. The traceback objects are even not pickleable.

What is worse, some other non-special attributes are lost during pickling. For example name and path attributes of ImportError.

>>> try: import foo
... except Exception as ex: exc = ex
... 
>>> exc.name
'foo'
>>> exc.__reduce__()
(<class 'ModuleNotFoundError'>, ("No module named 'foo'",), {})

Or the value attribute of StopIteration if it was not passed to the constructor.

>>> exc = StopIteration()
>>> exc.value = 42
>>> exc.__reduce__()
(<class 'StopIteration'>, (), {})

Hey, can I work on this?

I get different output for Serhiy's first example now, but the same for the second:

>>> try: import foo
... except Exception as ex: exc = ex
... 
>>> exc.name
'foo'
>>> exc.__reduce__()
(<class 'ModuleNotFoundError'>, ("No module named 'foo'",), {'name': 'foo'})
>>> exc = StopIteration()
>>> exc.value = 42
>>> exc.__reduce__()
(<class 'StopIteration'>, ())
>>>

See also issue43460, issue32696, issue30005.

> Attributes __context__, __cause__ and __traceback__ are not pickled. The traceback objects are even not pickleable.


It's not guaranteed that the __cause__ and __context__ are picklable either.

Should we try to pickle them, or should we document this and suggest traceback.TracebackException for storing or transmitting exception information?

messages: + msg396603
versions: + Python 3.11, - Python 3.6