ENH: Nested .npz files

[rherault-pro]

In Short

This is a demo code for having nested NPZ files.

What ?

When a dict variable with only str keys is given to np.savez, it is not converted to array of dtype=object nor pickled but rather used to create a child npz file that will be embedded in the parent npz file.
Of course that can be recursive.

Why ?

• Having structured data without the use of dtype object
• If saving a tree structure made out of valid dicts with standard arrays as leaves, no need to use pickle (and so avoid python2/python3 unpickle problems)

Comments

The writing part (modification of function _savez + new function _is_valid_dict ) is quite clean.

The reading part (modification of class NpzFile) can gain in quality, but that would mean deeper changes int the way getitem handles field types and in the way the class handles its parent zip file: currently we can't pass a byte stream to the NpzFile constructor but only a real file, that induces the use of temp files when having nested npz files.

[rherault-pro]

This pull request is not compatible with pull request #6540.
Nevertheless this proposal will be easier/cleaner to implement after pull request #6540.

[homu]

☔ The latest upstream changes (presumably #7133) made this pull request unmergeable. Please resolve the merge conflicts.

[mattip]

This has not been updated nor generated comments in quite a while. I will close it, but if there is interest in pursuing this please reopen or open a new PR.

[mattip]

@rkern just making sure you saw that we closed this.