validate_config_values attempts to append to tuple

[luketudge]

Describe the problem

The config value nb_custom_formats can be a string, as documented here. If it is, then validate_config_values() converts it to a tuple here, presumably in order to achieve a consistent format in which the caller is bundled with a dictionary of arguments. However, a few lines later nb_custom_formats() then attempts to .append() to this tuple here, resulting in an exception.

Steps to reproduce.

Add something like this to conf.py:

nb_custom_formats = {
  ".mysuffix": "mylibrary.converter_function"
}

Get an error a bit like this at validation:

Handler <function validate_config_values at [...]> for event 'config-inited' threw an exception (exception: 'tuple' object has no attribute 'append')

Link to your repository or website

No response

Provide steps to reproduce

.

Describe your environment.

No response

The version of Python you're using

3.8.10

Your operating system

Ubuntu 20.04.2

Versions of your packages

Jupyter Book      : 0.11.2
External ToC      : 0.2.2
MyST-Parser       : 0.13.7
MyST-NB           : 0.12.3
Sphinx Book Theme : 0.1.1
Jupyter-Cache     : 0.4.2
NbClient          : 0.5.3

Additional context

No response

[welcome[bot]]

Thanks for opening your first issue here! Engagement like this is essential for open source projects! :hugs:
If you haven't done so already, check out EBP's Code of Conduct. Also, please try to follow the issue template as it helps other community members to contribute more effectively.
If your issue is a feature request, others may react to it, to raise its prominence (see Feature Voting).
Welcome to the EBP community! :tada: