return valid Python Exceptions when js2python is applied to javascript errors

[hoodmane]

This is intended to resolve #888, necessary for PR #880 to work correctly.
Work in progress.

Currently there is a serious issue: if the Javascript Error was created without new, this will crash the Pyodide runtime. I have absolutely no idea why.

[hoodmane]

I think this is ready now @oeway.

[oeway]

Looks good!

I have two questions related to this: 1) have you figured why it's crashing when using Error without new? 2) if we do a round trip from js to python then back to js, will we get the same error object?

[hoodmane]

1. have you figured why it's crashing when using Error without new?

Yes I was incorrectly using a C Python API causing undefined behavior. This particular symptom was bad luck. It is fixed.

2. if we do a round trip from js to python then back to js, will we get the same error object?

Yes. See test here:
https://github.com/iodide-project/pyodide/blob/ad90a07b5ea6003e2947f0486641292cb53ac42a/src/tests/test_pyodide.py#L41

[oeway]

Great, thank you!

[hoodmane]

@rth I think this is ready to be merged unless you have further concerns?

[rth]

Thanks for working on this @hoodmane and thank you for the review @oeway !

Two comments below, otherwise LGTM.

Please also add an entry to docs/changelog.md.

[hoodmane]

I also added an entry for JsException in the api docs. To be clear, I'm envisioning code like:

from js import js_async_func
from pyodide import JsException
async def wrapper(arg):
   try:
      await js_async_func(arg)
   except JsException:
      raise ValueError(f"Unknown thunk {arg}")

[rth]

Could you please resolve conflicts following #909 ? Otherwise should be good to merge assuming CI passes.

I have a doubt, would this address #636 or not?

[hoodmane]

This does not fix #636. To do that we should:

• Add hiwire_seterr(id) which will do PyErr_SetObject(JsProxy_new_error(id)).
• Adjust hiwire_call and hiwire_call_member to use try + catch and call hiwire_setexc(hiwire_new(err)) for any caught error.

I can implement that as a separte PR after this is merged, or I could add it to this PR, whichever you prefer.

Error conversion in the other direction is also not very good: currently if python code throws an error, it prints stuff to stdout directly, but it should instead be possible to catch the error in javascript. To get the errors printed to stdout, we should explicitly catch them and call some separate print function.

[hoodmane]

I think the title "Raise Exceptions when js2python is applied to javascript errors" is misleading: js2python doesn't raise an error at all, it returns an exception that can be raised. These are very different things:

f raises an error:

def f():
  raise Exception("hi")

f returns an error:

def f():
  return Exception("hi")

[rth]

I had to shorten it as it was too long for a commit message. Didn't mean to rename the issue only the commit. I agree that's not ideal.

[hoodmane]

Thanks!