pretty python exception traceback in rust

[jmrgibson]

The print method on PyErr (https://pyo3.rs/master/doc/pyo3/struct.PyErr.html#method.print) is super handy but relies on stderr being available on the python side. When embedding rust + python into another application this is not always available and so it fails silently. It would be better if we had a way of getting the string of the error or the traceback as a string in rust, so that we can use it with whatever logging/error infrastructure is already set up on the rust side (eg log!, the failure crate, etc)

At the moment I've been doing some variation of py.eval("str(err)") or py.eval(import traceback; # etc) which works, but is cumbersome.

[kngwyu]

When embedding rust + python into another application this is not always available and so it fails silently.

Hmm, I haven't thought about such a situation 🤔
Thank you for your feedback!

[flaper87]

At the moment I've been doing some variation of py.eval("str(err)") or py.eval(import traceback; # etc) which works, but is cumbersome.

Do you have a full example of how you're extracting the traceback string? I'd like to do the same.

It'd be great to have a way to extract the traceback str representation built-in into PyErr

[hfingler]

Same here, need to extract the error since stderr is not available :(

[davidhewitt]

I've done a lot of reworking of errors for 0.12, so it would be nice to have an API for this included in this release if possible.

[sebpuetz]

Is this issue blocked by other work on PyErr or can this be done independently of your planned changes?

[davidhewitt]

I think it's easiest to do it once #1115 merges, if implementing display isn't enough. I was wondering about adding a format_traceback() function. But not worth doing right now as basically the whole PyErr implementation is mid-rewrite.

[davidhewitt]

For those who discover this thread: the new reworked PyErr now implements Display, so it's easy enough to get the error type and value.

That Display doesn't currently show the traceback, so as discussed above I'm considering adding a format_traceback() function. But I think this function will pretty much call import traceback; traceback.format_exception(), so I'm wondering whether we really want this API in the pyo3 core (it's easy for users to re-implement).

So I'd like a little bit more time to think about exactly what that API should be, if we do include it.

[davidhewitt]

Alternatively we can use PyTraceback_Print here, but that needs a file object as the second argument. (We'd potentially want to implement a custom file object for performance.)