Support for Final-as-default, like Rust

[willhansen]

Feature

If I want to follow "best practices", I've got to put a Final[ ] wrapper around most type hints I use. Could there be a mypy flag to assume every type hint is final unless it's got a Mut[ ] wrapper?

Would probably require a search-replace script to convert a project between the modes, but that doesn't sound too bad.

Pitch

Variables being constant when you won't need to change them is generally regarded as a good idea, right? This would make it easier, like Rust does.

I don't know about any of the mypy internals, but this doesn't strike me as too big a change?

Interface change would be a new --final-as-default flag.

[sobolevn]

I love this idea :)

I think that it needs to be clarified, that the flag should be something like --implicit-final-vars, because final might also mean classes, methods, etc.

[takeda]

Yes! I'm making myself to use Final in my code frequently and most of the time the variables aren't really mutated, so having a mode where Final would be implicit would be really helpful.