Hi, when I'm trying to serialize/deserialize python exception object through pickle, I found that deserialize result is not the same as the original object...

My python version is 3.9.1, working os: macOS Big Sur 11.4

Here is minimum reproducing code example:

import pickle

class ExcA(Exception):
    def __init__(self, want):
        msg = "missing "
        msg += want
        super().__init__(msg)

ExcA('bb')   # this will output ExcA("missing bb"), which is good
pickle.loads(pickle.dumps(ExcA('bb')))  # this will output ExcA("missing missing bb"), which is different from `ExcA('bb')`

The default __reduce__ method of Exception returns the arg you pass to the Exception constructor:

>>> a = ExcA("a banana")
>>> a.__reduce__()
(<class '__main__.ExcA'>, ('missing a banana',))
>>> 


This is then pickled, and when unpickled the arg is passed to the ExcA constructor. 

If you want to change this, you can implement __reduce__ on ExcA o return just the part you want it to return.

Pickling customized subclasses can be tricky.  The essential details are here:  https://docs.python.org/3/library/pickle.html#object.__reduce__

Here's some code to get you started.

class ExcA(Exception):
    def __init__(self, want):
        msg = "missing "
        msg += want
        super().__init__(msg)
    def __reduce__(self):
        return (type(self), self.args, self.args)
    def __setstate__(self, state):
        self.args = stat

It would be nice if exception pickling was more friendly to subclasses.  Perhaps, the default should be to store and restore all state including args and a __dict__ if any.