allow trailing : in directives for YAML compliance

[hamelsmu]

This is to allow people to work with Quarto a bit easier. People can add a colon after their nbdev directives so it won't break Quarto. This is the most minimal solution I can think of to unblock folks. I tested this as follows:

• I added a unit test to make sure directives are still extracted correctly, and did some tests on nbdev
• I tested this on a stand alone quarto project and confirmed that adding a trailing: alleviated Quarto's rendering issue.

This issue keeps cropping on the Quarto side.

This is a workaround for this issue: quarto-dev/quarto-cli#3152

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[hamelsmu]

@cscheid just to confirm, if nbdev directives allowed a trailing : would that allow Quarto to do what it needs to do?

IIUC

foo: is equivalent to foo: null and foo: is valid YAML.

cc: @jjallaire

[cscheid]

(With the caveat that I don't have nbdev experience!)

Yes, I believe that would solve the problem on quarto's side. The particular problem isn't when "export" exists by itself on a cell. The problem happens like this:

```{python}
#| export
#| echo: false

print("code")
```

That causes the entire "cell front matter" to become invalid yaml, because export is not a valid key specifier (or whatever the YAML terminology is in this case). (#| export by itself, funnily enough, does work, because a singleton string is a valid YAML object.)

[jjallaire]

Note that while #| export by itself is valid YAML (the parser doesn't choke) it is interpreted as a string not an object so could cause other problems downstream.

#| export: by itself is valid but the value evaluates to null (which is a bit different than the intended usage which is #| export: true. I guess though if this directive is only used by nbdev then it's entirely fine for it to be interpreted on the quarto side as null.

[hamelsmu]

This is a good point

which is a bit different than the intended usage which is #| export: true

I suppose we might want to allow for that as well in a follow on PR, if folks want to abide by the semantic meaning of YAML.

Jeremy, WDYT?

[hamelsmu]

@jph00 mind taking a look? Thank you

[jph00]

Thanks @hamelsmu , I think this is a reasonable workaround for now. Maybe in the future we should consider stuff like #|export foo to be the same as #|export: foo?

[cscheid]

Fantastic. We'll make sure export: works on our side with the CLI and tooling. Thank you!!