transaction.on_commit() proposal

[Adam Johnson]

Hi

I just wrote a blog post on a late-binding bug that can occur with using transaction.on_commit() with a function or lambda within a loop. THe post shows how using functools.partial() solves this. See the post here: https://adamj.eu/tech/2022/08/22/use-partial-with-djangos-transaction-on-commit/

I propose that transaction.on_commit() be modified to take args and kwargs to pass to the callback function. This would mean it effectively constructs a partial() "under the hood":

transaction.on_commit(send_mail, subject=f"...", recipient_list=[user.email], ...)

I think this would be simpler than asking developers to beware of the late-binding problem and use partial(), always or when appropriate.

This change would make transaction.on_commit() similar to Python's atexit.register(), which also takes a callback to run at a later point, and optional args/kwargs to pass it: https://docs.python.org/3.10/library/atexit.html#atexit.register

The only backwards-compatibility concern I can see is the `using` argument, which would not be passed to the underlying function. I don't see this as a huge concern, since it would be the only argument that isn't passed through, and there doesn't seem to be any reason to add further arguments. Users who need to pass a kwarg called 'using' could always construct a partial() themselves.

Thoughts?

Adam

[Carlton Gibson]

Related ticket from this weekend, suggesting a docs change: 

https://code.djangoproject.com/ticket/33939

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[Adam Johnson]

I'd also be okay with a docs change, although this wouldn't help avoid late-binding bugs quite so much.

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