[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:


New changeset 072f4a473e861c6c987650f08990c0ed1f76715f by Irit Katriel in 
branch 'main':
bpo-46431: use raw string for regex in test (GH-30901)
https://github.com/python/cpython/commit/072f4a473e861c6c987650f08990c0ed1f76715f


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Change by Irit Katriel :


--
pull_requests: +29080
pull_request: https://github.com/python/cpython/pull/30901

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:

Thank you Petr.

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:


New changeset b18fd54f8c27e4b2aac222e75ac58aa85e5a7988 by Irit Katriel in 
branch 'main':
bpo-46431: Add example of subclassing ExceptionGroup. Document the message and 
exceptions attributes (GH-30852)
https://github.com/python/cpython/commit/b18fd54f8c27e4b2aac222e75ac58aa85e5a7988


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:


New changeset 573b54515740ce51dcf2402038a9d953aa6c317f by Irit Katriel in 
branch 'main':
bpo-46431: improve error message on invalid calls to BaseExceptionGroup.__new__ 
(GH-30854)
https://github.com/python/cpython/commit/573b54515740ce51dcf2402038a9d953aa6c317f


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Change by Irit Katriel :


--
pull_requests: +29035
pull_request: https://github.com/python/cpython/pull/30854

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Guido van Rossum]

Guido van Rossum  added the comment:

So, a PR with that fix would be nice?

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:

This is the fix:

-if (!PyArg_ParseTuple(args, "UO", , )) {
+if (!PyArg_ParseTuple(args,
+  "UO:BaseExceptionGroup.__new__",
+  ,
+  )) {


Then we get

TypeError: BaseExceptionGroup.__new__() takes exactly 2 arguments (1 given)

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:

The error message isn't always this bad:

>>> class Base:
... def __new__(cls, a, b, c):
...cls.newargs = (a,b,c)
... 
... 
>>> class Derived(Base):
... def __init__(self, x):
... super().__init__(x)
... self.initargs = (x,)
... 
>>> Derived(1)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: Base.__new__() missing 2 required positional arguments: 'b' and 'c'

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Change by Irit Katriel :


--
keywords: +patch
pull_requests: +29033
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/30852

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Guido van Rossum]

Guido van Rossum  added the comment:

Is this a matter of adding some documentation showing how it should be done? 
Even if we don't want to encourage such subclassing, we do have APIs to support 
it, and I think we ought to show how to do it, given that you have to 
coordinate a bunch of things.

How feasible is it to improve the error? (If that error is what you always get 
when __init__ and __new__ don't agree, maybe that could be a separate, more 
general issue?)

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Change by Irit Katriel :


--
nosy: +gvanrossum, yselivanov

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:

> Is there any other danger in not overriding it?

No issue as long as you don't use split()/subgroup() or except*.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Petr Viktorin]

Petr Viktorin  added the comment:

> can you just assign the task to a field on the exception other than __note__?

That might work, but I'm afraid of touching namespaces I don't own. If the 
subclass is feasible, I'd rather go with that.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Petr Viktorin]

Petr Viktorin  added the comment:

Thanks for looking into it!

> If you don't define derive the superclass constructor is used, which means 
> you get something of type ExceptionGroup, not your subclass.

That might be fine in my case (for a MVP at least). Is there any other danger 
in not overriding it?
I see the docs say "A subclass needs to override it", but it might be better to 
enumerate the perils, or if such a class is unusable, not allow creating it.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:

For your use case - can you just assign the task to a field on the exception 
other than __note__?  The only reason we needed __note__ as an official feature 
is because we want the interpreter's traceback code to use it. But I think you 
can assign any field to an exception and then use it in your app:

e.webapp_task = task

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:

Re the exceptions attribute - I don't think there's a reason not to document 
it, I can add that (it is mentioned in the PEP).

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Irit Katriel]

Irit Katriel  added the comment:

We probably do need better documentation for subclassing ExceptionGroup.

When you subclass an ExceptionGroup you want to make sure that split() and 
subgroup() (which are used by except*) will continue working, usually by 
defining a derive() method:

https://docs.python.org/3.11/library/exceptions.html#BaseExceptionGroup.derive

If you don't define derive the superclass constructor is used, which means you 
get something of type ExceptionGroup, not your subclass.


I don't know whether it's a good idea to make it easier to define a subclass 
that doesn't support split()/except* because ti changes the constructor 
signature without providing derive().

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue46431] Trouble subclassing ExceptionGroup

[Petr Viktorin]

New submission from Petr Viktorin :

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 "", line 1, in 
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

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com