VoilaExecutePreprocessor.allow_errors always True

[jeffyjefflabs]

If a cell throws an exception, the cells after it are still being executed. Setting allow_errors explicitly to False (which is the default anyway) doesn't affect it.

[jtpio]

Good catch, thanks @jeffyjefflabs.

Voila handles CellExecutionError here:

voila/voila/execute.py

Line 148 in 919d57d

except CellExecutionError as e:

Which seems to be shadowing the allow_errors flag from nbconvert.

[martinRenou]

I am working on this today

[martinRenou]

It comes from #186, I feel like this PR should not have been merged as it shadows the CellExecutionError generated by nbconvert when allow_errors is set to False (see https://github.com/jupyter/nbconvert/blob/master/nbconvert/preprocessors/execute.py#L477)

[martinRenou]

Although I agree with the initial intention of this PR. Maybe we should set allow_errors to True by default in Voila?

[maartenbreddels]

I agree, since this looks to be controllable from nbconvert, voila should not add similar behavior. Maybe allow_error should be set to True on debug?

[martinRenou]

What I have in mind is to overwrite the allow_errors default value to True in Voila, so that Voila does not show a 500 error by default if the execution failed.

And in the case of having allow_errors to True (default), errors are only shown in debug mode.

[maartenbreddels]

Sound good I think, maybe we can use this pattern:

voila/voila/exporter.py

Line 62 in 919d57d

def default_config(self):

[martinRenou]

Completely overwriting the trait would allow writing the right documentation in voila --help-all:

allow_errors = Bool(
    True,
    help=dedent(
        """
        If `True` (default), execution errors are ignored and the execution
        is continued until the end of the notebook. Output from
        exceptions is included in the cell output only if Voila is run with --debug.
        If `False`, when a cell raises an error the
        execution is stopped and a `CellExecutionError`
        is raised.
        """
    )
).tag(config=True)

[maartenbreddels]

I remember this being an issue actually, not 100% sure it is related: jupyter/nbconvert#1056 (comment)

[martinRenou]

I am not sure I understand the comment on your link, what was the issue?

[maartenbreddels]

If you add --allow-errors to aliases (like in jupyter/nbconvert@e78f02e) traitlets gets confused which class to alias this to. But as long as we don't make it an alias we should be fine.

[jeffyjefflabs]

Thanks for looking into this! I tried your PR #530 and it works for me. I like keeping allow_errors=False by default; for most of our use cases we definitely don't want to try to keep going after an error.