Add cattrs to unstructure/structure and allow (de)serialization with json or similar

[anthrotype]

this allows to do things like

import orjson
from ufoLib2.converters import default_converter as c
from ufoLib2.objects import Font

font = Font.open("NotoSans-Regular.ufo")

# dumping a UFO to json
data = orjson.dumps(c.unstructure(font))
with open("notosans-regular.json", "wb") as f:
    f.write(data)

# or loading a UFO from json
with open("notosans-regular.json", "rb") as f:
    data = f.read()
font2 = c.structure(orjson.loads(data), Font)

assert font == font2

Still WIP, as I need to add tests. Added tests

[anthrotype]

hm gotta figure out py37 compat (next week)

[anthrotype]

@madig I think this is ready for another review pass, at your leisure. Thanks

[anthrotype]

By the way, all this is generic and not tied to json or any specific serialization format. Here's how it'd work with msgpack for example. The only difference below is, instead of using ufoLib2's default_converter, we create a new GenConverter, register our custom structure/unstructure hooks, and allow_bytes=True (for msgpack can encode those natively, unlike the our default_converter which is geared toward json and we need to encode bytes as base64).

import io
import cattr
import ufoLib2
import ufoLib2.converters
import msgpack

conv = cattr.GenConverter(omit_if_default=True)
ufoLib2.converters.register_hooks(conv, allow_bytes=True)

font = ufoLib2.Font.open("...ufo")

buf = io.BytesIO()

msgpack.pack(conv.unstructure(font), buf)

buf.seek(0)

font2 = conv.structure(msgpack.unpack(buf), Font)

assert font == font2

[madig]

The LayerSet does not seem to get unstructured?

import json

import ufoLib2
import ufoLib2.converters

u = ufoLib2.Font.open("tests/data/MutatorSansBoldCondensed.ufo/")
d = ufoLib2.converters.default_converter.unstructure(u)

j = json.dumps(d)

TypeError: Object of type LayerSet is not JSON serializable

[anthrotype]

The LayerSet does not seem to get unstructured?

that is very weird, the tests pass, and I added more that explicitly test json (de)serialization. It must be a local setup issue of yours.

[madig]

It was a fluke, sorry for the noise.

Something different: How should other libraries handle serialized dumps? Consider e.g. intermediate build steps in a pipeline, where currently a Designspace might reference normal UFOs but should reference JSON dumps instead. Should e.g. designspaceLib be made aware of the new format or should this rather be done elsewhere?

[anthrotype]

we are not designing any "new format" here. This is not tied to json particularly either. Clients of ufoLib2 can do whatever they like with this, this is just enabling them to more easily (de)construct ufoLib2 classes to something that can be (de)serialized. We can figure those thing out elsewhere

[madig]

Right, so we use it externally to the established libraries, where tooling has to deliberately make use of it.