jupytext.write doesn't error if `fmt` is unknown, but the output file has a known extension

Just playing around with MyST -> ipynb -> MyST right now. The ipynb syncing works beautifully! I love it

I am having a hard time getting MyST code cells to render as ipynb code cells.

E.g., this markdown:

---
jupytext:
  formats: ipynb,md:myst
  text_representation:
    extension: .md
    format_name: myst
    format_version: '0.6'
    jupytext_version: 1.4.1
kernelspec:
  display_name: Python 3
  language: python
  name: python3
---

```{code-cell} python3
print('hi')
```


```{code-cell} python3
print('hi')
```

has its ```{code-cell} ``` read in as markdown instead of being converted to Python code cells.

I tried adding double-spaces between them but that didn’t do the trick. Anything I am missing?

(I also noticed a weird thing where the YAML header of the MyST markdown file would be read into the notebook as a raw cell at the top, rather than in the notebook metadata…is that expected behavior?)

[mwouts]

Thanks @choldgraf , that is a good point… I agree that jupytext.write should raise an error in that case. I’ll see how to add a test and fix that.

[choldgraf]

ahhh yes, I believe that myst-parser was a key issue here. I’ve upgraded it and it now works much better! Amazing 😃

One thing that I think exacerbated my problem, if I write the notebook to markdown and give a fmt= value that jupytext doesn’t have, then jupytext still writes the format just fine. E.g., this works:

I think what it is doing is inferring the output type from the .md extension of teh output, because if I remove the .md then it throws an error. However, I think jupytext should throw a warning if it doesn’t understand the fmt= value, and instead is going to infer from the file extension. This was confusing me, because I was writing jupytext.write(ntbk, "out.md", fmt="myst"), when I should have been writing jupytext.write(ntbk, "out.md", fmt="md:myst"), but because no error or warning was thrown, it was just defaulting to “regular” markdown.