Apple's freebie tools -- Pages, Numbers, Keynote, Garage Band, etc. -- are wonderful things. I really like Numbers. I'm tolerant of Pages. I've used Pages to write books and publish them to the Apple Bookstore. (Shameless Plug: Pivot to Python.)
These tools have a significant problem. Protobuf.
Once upon a time, Numbers used an XML-based format. This was back in '09, I think. At some point, version 10 of Numbers (2013?) switched to Protobuf.
I had already unwound XLSX and ODS files, which are XML. I had also unwound Numbers '09 in XML. I had a sense of what a spreadsheet needed to look like.
The switch to Protobuf also meant using Snappy compression. Back in 2014? I worked out my own version of the Snappy decompression algorithm in pure Python. I think I knew about python-snappy but didn't want the complex binary dependency. I wrote my own instead.
I found the iWorkFileFormat project. From this, and a lot of prior knowledge about the XML formats, I worked out a way to unpack the protobuf bytes into Python objects. I didn't leverage the formal protobuf definitions; instead I lazily mapped the objects to a dictionary of keys and bytes. If a field had a complex internal structure, I parsed the subset of bytes.
(I vaguely recall the Protobuf definitions are in XCode somewhere. But. I didn't want to write a protobuf compiler to make a pure-Python implementation. See the protobuf project for what I was looking for, but didn't have at the time.)
Which brings us to today's discovery.
State of the Art
Someone has taken the steps necessary to properly unpack Numbers files. See numbers-parser. This has first-class snappy and protobuf processing. It installs cleanly. It has an issue, and I may try to work on it.
I'm rewriting my own Stingray Reader with intent to dispose of my own XLSX, ODS, and Numbers processing. These can (and should) be imported separately. It's a huge simplification to stand on the shoulders of giants and write a dumb Facade over their work.
Ideally, all the various spreadsheet parsing folks would adopt some kind of standard API. This could be analogous to the database API used by SQL processing in Python. The folks with https://www.excelpython.org or http://www.python-excel.org might be a place to start, since they list a number of packages.
The bonus part? Seeing my name in the Credits for numbers-parser. That was delightful.
At some point, I need to make a coherent pitch for a common API with permits external JSON Schema as part of extracting data from spreadsheets.
First. I need to get Stingray Reader into a more final form.