[New-bugs-announce] [issue46431] Trouble subclassing ExceptionGroup

[Petr Viktorin]

New submission from Petr Viktorin <encukou at gmail.com>:

I want to test a web application by crawling every reachable page. If an error occurs, I need to keep track of the page the error occured at (and additional info like what links were followed to get to the page, so a `__note__` string isn't enough). This info is in an object I'll call Task.
To use the improvements from PEP 654 but also keep extra info, I tried to make a subclass of ExceptionGroup:

```python
class MultiError(ExceptionGroup):
    def __init__(self, failed_tasks):
        super.__init__(
            f"{len(tasks)} tasks failed",
            [t.exception for t in failed_tasks],
        )
        self.tasks = tasks
        # ... and set __note__ on each exception for nice tracebacks
```

but this fails with a rather opaque message:

```python
>>> class Task: exception = AssertionError() # enough of a Task for this example 
... 
>>> MultiError([Task(), Task()])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: function takes exactly 2 arguments (1 given)
```

Knowing about `__new__` and following a note in the docs, I'm able to fix this, but It's not obvious.
Before suggesting stuff, I'll ask: Am I doing something weird, or should this be made easier/more obvious?


Another issue I ran into: the list of exceptions is stored in the publicly-named but undocumented attribute `exceptions`. Am I allowed to use it?

----------
messages: 410942
nosy: iritkatriel, petr.viktorin
priority: normal
severity: normal
status: open
title: Trouble subclassing ExceptionGroup
versions: Python 3.11

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<https://bugs.python.org/issue46431>
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