Allow creating an Exception hierarchy

[stinodego]

I'd like to be able to create an exception hierarchy in PyO3. In Python I would write something like:

class PolarsError(Exception):
    ...

class OutOfBoundsError(PolarsError):
    ...

Then users could catch the specific error thrown by the code (OutOfBoundsError), or use catch more general exception type (PolarsError).

Right now, I'm using the create_exception! macro to define the exceptions, as mentioned in the user guide. However, this does not allow for specifying a hierarchy.

I've found some discussion on subclassing exceptions:

• pyclass: support extending Exception types #1591
• Add a proc macro for exceptions #295 (comment)
• Add exceptions to modules #732

...from this, it seems like my wish is already possible. However, I cannot find any code examples of how this is supposed to work. If it is already possible, could someone point me in the right direction? If not, consider this a feature request!

[davidhewitt]

Hello, sorry for the slow reply.

This should definitely be possible and really this just needs proper documenting and testing, after which point I'd like to deprecate the create_exception! macro so we just have one blessed way of doing things.

I've opened #3475 which begins building such a test for an exception hierarchy, let's collaborate there to make sure that all the functionality you want is supported.