Description

dcolascione

opened on Aug 17, 2025

Issue body actions

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

Activity

1. dcolascione

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Aug 17, 2025
2. StanFromIreland

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Aug 18, 2025
3. StanFromIreland

   added this to Pickle and copy issues 🥒on Aug 18, 2025
4. serhiy-storchaka

   self-assigned this

   on Aug 18, 2025
5. serhiy-storchaka commented on Aug 18, 2025

   

   serhiy-storchaka

   on Aug 18, 2025

   Member

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   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.

   Reacted by Daniel Colascione
6. serhiy-storchaka commented on Aug 18, 2025

   

   serhiy-storchaka

   on Aug 18, 2025

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   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.

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