Cannot pickle cyclic ExceptionGroup

[dcolascione]

Bug report

Bug description:

Pickling an exception group with a cycle, like so...

import pickle
inner = Exception("Inner")
outer = ExceptionGroup("outer", [inner])
outer.exceptions[0].foo = outer  
pickle.loads(pickle.dumps(outer))

...produces...

Traceback (most recent call last):
  File "<stdin>", line 5, in <module>
ValueError: second argument (exceptions) must be a non-empty sequence

Maybe BaseExceptionGroup needs a custom reducer that allows for two-phase initialization.

CPython versions tested on:

3.13

Operating systems tested on:

macOS

[serhiy-storchaka]

This is because BaseException.__reduce__() returns BaseException.args which is a tuple of the original arguments. And one of them is a mutable list.

You can have issue with non-recursive ExceptionGroup:

>>> excs = [ValueError()]
>>> eg = ExceptionGroup('group', excs)
>>> eg.args
('group', [ValueError()])
>>> excs.clear()
>>> eg.args
('group', [])
>>> eg
ExceptionGroup('group', [])
>>> eg.exceptions
(ValueError(),)
>>> import pickle
>>> pickle.loads(pickle.dumps(eg))
ValueError: second argument (exceptions) must be a non-empty sequence

I think there may be similar issues with other exceptions too.

The BaseException exception constructor is split on two parts, __new__() and __init__(), and args is set in both of them.

[serhiy-storchaka]

We can change the args attribute so the second item is a tuple, but this will not work in general case, for ExceptionGroup subclasses. For example:

class MyExceptionGroup(ExceptionGroup):
    def __new__(cls, code, msg, exceptions, **kwargs):
        self = ExceptionGroup.__new__(cls, msg, exceptions, **kwargs)
        self.code = code
        return self

The args attribute of the MyExceptionGroup instance will have three items -- code, msg, and exceptions. Now exceptions is the third item, not the second item, but ExceptionGroup.__init__() cannot know this.