refactor: add @final to contents, types, and forms

[agoose77]

This type hint makes it clear (and, in future, enforces) that contents, types, and forms are all final non-inheritable types.

[codecov]

Codecov Report

Merging #2033 (46ea4ec) into main (08a099a) will increase coverage by 0.08%.
The diff coverage is 98.48%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/typing.py	88.88% <75.00%> (-2.78%)	⬇️
src/awkward/contents/bitmaskedarray.py	68.02% <100.00%> (+0.10%)	⬆️
src/awkward/contents/bytemaskedarray.py	88.54% <100.00%> (+0.02%)	⬆️
src/awkward/contents/emptyarray.py	72.22% <100.00%> (+0.15%)	⬆️
src/awkward/contents/indexedarray.py	77.66% <100.00%> (+0.05%)	⬆️
src/awkward/contents/indexedoptionarray.py	88.67% <100.00%> (+0.01%)	⬆️
src/awkward/contents/listarray.py	90.45% <100.00%> (+0.01%)	⬆️
src/awkward/contents/listoffsetarray.py	80.20% <100.00%> (+0.02%)	⬆️
src/awkward/contents/numpyarray.py	91.05% <100.00%> (+0.01%)	⬆️
src/awkward/contents/recordarray.py	84.05% <100.00%> (+0.03%)	⬆️
... and 24 more

[jpivarski]

A warning from PyLint (they easily get buried):

typing.final is not supported by all versions included in the py-version setting

Is there a way to import it as backports for Python versions as early as 3.7?

[jpivarski]

That's right: the Content, Form, and Type subclasses should not be subclassed; they're final.

So are ak.types.ArrayType, ak.record.Record, and the ak.index.Index subclasses (e.g. ak.index.Index64). These are not subclasses of the above, but are closely related.

I suppose you can't annotate Content, Form, and Type (the superclasses) to say that they can't have subclasses beyond those that are defined in our codebase. (I think Scala has a way to say that: they have a lot of module-level/namespace-level type qualifiers.)

Also, this would only prevent people from defining subclasses and then type-checking them, right? We're not preventing people from hacking things as a test—just formally declaring something as "okay for production," right?

[agoose77]

This is done, it just needs a fix for mypy on older Python versions. I think the issue is how we import typing_extensions and typing in awkward.typing. It seems like there's no easy way to say "import everything from typing and typing_extensions (if appropriate), where typing_extensions takes priority". I think I might have to explicitly import the appropriate types from typing_extensions, but I'll properly dig into this next week.

I suppose you can't annotate Content, Form, and Type (the superclasses) to say that they can't have subclasses beyond those that are defined in our codebase. (I think Scala has a way to say that: they have a lot of module-level/namespace-level type qualifiers.)

Unfortunately not, but I think we'll be OK at least with constraining the immediate subclasses.

Also, this would only prevent people from defining subclasses and then type-checking them, right? We're not preventing people from hacking things as a test—just formally declaring something as "okay for production," right?

Right, this is not runtime-enforced at all.