add exception hierarchy pytests

[davidhewitt]

WIP solution for #3452

The goal here is to get examples and documentation working for how to create a Python exception hierarchy and interact with it from Rust. This will probably go through a few rounds of rebases as features / bugfixes inspired by it will be opened as separate PRs.

[davidhewitt]

I wrote a bug here which confused me for a bit; #[new] ignores signature.

Either this should be a hard error or it should be equivalent to #[new] #[pyo3(signature = (*_args, **_kwargs))].

[davidhewitt]

@stinodego did you have any luck with an exception hierarchy on the polars side? I think this is a baseline which you might be able to start from, I think the most practical way forward is that if you try implementing what you want starting from this, please report back on your experience and we can fix any issues or papercuts.

[stinodego]

@stinodego did you have any luck with an exception hierarchy on the polars side? I think this is a baseline which you might be able to start from, I think the most practical way forward is that if you try implementing what you want starting from this, please report back on your experience and we can fix any issues or papercuts.

I haven't worked on this since I opened the issue - I'll take this as a template and try to set it up for Polars! Sometime next week probably. Will let you know my findings.

[stinodego]

Apologies for getting back to this so late. When I try to adapt this for Polars, I get an error that PyClass isn't implemented for PyException. Any idea how to solve this? I must be doing something wrong.

[davidhewitt]

Is polars using the abi3 feature by any chance? That currently doesn't support subclassing native types (although I intend to fix that in the mid term).

[stinodego]

Ah that's right - we are using the abi3-py38 feature.

I have been able to get a basic version of a hierarchy working through the create_exception macros - I didn't realize it would allow me to do basically what I had intended:

create_exception!(polars.exceptions, PolarsBaseError, PyException);
create_exception!(polars.exceptions, ComputeError, PolarsBaseError);
create_exception!(polars.exceptions, ColumnNotFoundError, PolarsBaseError);

Though the setup in this PR definitely looks nicer and more powerful.

[davidhewitt]

In the long run I would like to deprecate the create_exception macro and only support the #[pyclass] route as proposed here, but you highlight a good point here that I probably need to solve #1344 first.