Add attributes to tomllib.TOMLDecodeError

[hukkin]

Feature or enhancement

Proposal:

Add the same attributes to tomllib.TOMLDecodeError that json.JSONDecodeError also has, namely: msg, doc, pos, lineno, colno. This will help user facing apps show customized errors to users.

This draft PR in Tomli shows the proposed changes.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/69546

Linked PRs

• gh-126175: Add attributes to TOMLDecodeError. Deprecate free-form __init__ args #126428

[hukkin]

Pinging codeowners @hauntsaninja and @encukou

Thoughts on my concern in Discourse about the change breaking TOMLDecodeError init with no arguments would be much appreciated:

Unfortunately, this changes signature of TOMLDecodeError so breaks any users instantiating the error. Not sure why anyone would do that though? Some less breaking options are using keyword arguments, or dynamically injecting attributes after instantiation.

[nineteendo]

I would use keywords arguments defaulting to None. Also see PEP-387.

[picnixz]

Unfortunately, this changes signature of TOMLDecodeError so breaks any users instantiating the error. Not sure why anyone would do that though? Some less breaking options are using keyword arguments, or dynamically injecting attributes after instantiation.

You can instantiate those errors if you create a custom tomllib extension (e.g., to handle extended syntax not supported by TOML but you still want to use the same exceptions for that).

I would use keywords arguments defaulting to None

Agreed.

[hukkin]

Thanks all for the feedback! I've updated my PoC draft PR in Tomli to be fully backwards compatible. It also adds a deprecation warning for use of positional arguments, which is simple to remove if folks think it's unnecessary. With it I think we could eventually move to the same exact signature that json.JSONDecodeError has.