Incorrect .ipynb > .py > .ipynb roundtrip in simple and common cases

[lebigot]

If we have an .ipynb notebook that contains a first Markdown cell:

First Markdown text.

Then a second Markdown cell with two paragraphs:

Second Markdown text…

… with a second paragraph.

Then converting to .py and then back to .ipynb fails: the second cell is considered (commented out) code. This is a problem, because this situation is very likely to happen.

Also, if the first Markdown cell is a header (# This is a header), then the second Markdown cell is considered code through the roundtrip, which is a problem. Having a header cell and then some Markdown cell is another, common situation, for notebook.

PS: I tested this with the grst/ipymd version of ipymd, which apparently didn't touch this part of the code. The pip version (of rossant/ipymd) fails with:

  File "/…/anaconda/envs/py2/lib/python2.7/site-packages/ipymd/formats/notebook.py", line 80, in read
    assert nb['nbformat'] >= 4
TypeError: string indices must be integers, not str