Testing differences across two pre-run notebooks / paths

[psychemedia]

One of the ways we use nbval is to test legacy notebooks in new environments to see if there are any breaking changed to notebooks arising from environment updates.

It would be really handy to be able to generate nbval reports for pre-run versions of the same notebooks already executed in different environments.

For example:

• compare two notebooks: py.test --nbval-diffnb my_notebook_in env1.ipynb my_notebook_in env2.ipynb — use env1 notebook as reference and text env2 notebook outpurs against it;
• compare notebooks with common names in two directories: py.test --nbval-diffpath env1_nb_path/ env2_nb_path/ — use notebooks in env1 path as reference values, and text similarly notebbooks from env2 path.