Add __slots__ to all types and nodes

[sobolevn]

Quick review of changes:

1. I had to move all class-level values into self. definitions, because class attributes and incompatible with __slots__. This seems like a good idea for me, because there was no reason for them to be class-level. I can even imagine several bugs based on over-use of class-level attributes mutation. It might take more memory in some cases though.
2. I've added several TODO items to op: str, it seems like Enum would be a better fit there

Closes #11195

[97littleleaf11]

Personally speaking I like this modification!

[emmatyping]

Would you be able to provide some benchmarks? You might try running the daemon and use the --perf-stats-file flag and have mypy check itself (or any other larger code base you have around)

[JukkaL]

It might take more memory in some cases though.

__slots__ are ignored in release builds (which are compiled using mypyc), so they shouldn't have any (measurable) performance or memory use impact. I think that their only benefits in most use cases are these:

1. catch invalid attribute assignments when doing local dev/test without having to run mypy (these would be caught in CI at the latest, though)
2. makes it impossible to have a mutable initializer.

I think that the main drawbacks are these:

1. some extra verbosity
2. there's an extra thing that needs to be updated when adding/deleting an attribute
3. class-level defaults are not supported
4. __slots__ is a somewhat obscure feature and some contributors might not know about it

I don't have a strong opinion, but catching invalid attribute assignments is somewhat nice during development and can save some time when debugging (but pretty rarely). What do others think?

[sobolevn]

Since now mypy supports __slots__ check after #10801, I think that this might be very useful during development.

I think that the main drawbacks are these:
some extra verbosity
there's an extra thing that needs to be updated when adding/deleting an attribute

Totally.

class-level defaults are not supported

I consider this as a benefit, actually 🙂
Class level defaults can be tricky: user might accidentally change the default of instances by changing it for a type. This problem is quite common. I am not sure what mypyc does in this case, but it can surely happen during development.

slots is a somewhat obscure feature and some contributors might not know about it

I hope more people will learn about this feature! Moreover, #10801 is now a thing. I personally love it! And @attr.s(slots=True) is my goto-method to create new classes 🙂

I personally think it was more confusing for some types to have __slots__ and for some types to not have them.

[sobolevn]

Would you be able to provide some benchmarks?

@ethanhs do you mean https://github.com/python/mypy/blob/3bdef9fe6d401f3d2e0cacf4964bd315550c3394/mypyc/test-data/run-bench.test or do you have any other specific benchmark in mind?

[97littleleaf11]

It's easier to understand code structure with __slots__ for new contributors like me.

[emmatyping]

do you mean https://github.com/python/mypy/blob/3bdef9fe6d401f3d2e0cacf4964bd315550c3394/mypyc/test-data/run-bench.test or do you have any other specific benchmark in mind?

I was thinking you could just time running mypy on itself, but I didn't realize mypyc doesn't use slots, so it probably doesn't matter.

[JukkaL]

Looks like nobody is against this, so lets do it!

Using slots elsewhere may not be worth it, though, since mypyc basically gives all compiled classes an implicit __slots__.