[MrJohz]

As an alternative perspective, I've found mypy to be pretty poor in comparison to other rich type schemes. Particularly in comparison to the Javascript/Typescript ecosystem, it really feels like taking a huge step backwards.

Partly that's because the ecosystem support isn't there - lots of libraries don't have types, or have types that behave oddly, or even require their own plugin for mypy. I suspect that's going to slowly change over time, but I feel like the infrastructure doesn't feel as strong as it did in the early days of Typescript, and the gradual change feels even more gradual.

Partly it's just that the syntax is painful as soon as you want to do anything remotely complex. For example, generics are defined in different ways depending on what you're making generic, and the TypeVar system is usable, but ugly and very unintuitive. There's also missing syntax for things like inline type assertions, which aren't great, but are often useful for mixing static and dynamic code together.

And I think partly it's also that Python has a much higher level of dynamism in the type system - lots of "clever" things are possible in Python that just wouldn't be possible in the same way in Javascript - which means that mypy has a much more difficult time in trying to describe the idiomatic Python patterns that people are actually using. In Typescript, figuring out the correct types feels like an extension of writing natural Javascript code and understanding the flow of data through the program. With mypy, it feels like I'm restricted to writing the subset of Java-like code that conforms to the type checker.

It's a disappointing feeling, because I like static types, and I had hoped it would help solve the problem of large codebases in Python. But in my last project it became so painful to use, and felt like it was adding so little (and preventing so few actual issues), that I kind of regretted pushing for it.

I hope a lot of these problems are just teething issues, and that over time it'll get better. But right now I would be very cautious about using it in production.

[dragonwriter]

> - lots of "clever" things are possible in Python that just wouldn't be possible in the same way in Javascript

JS has Object.setPrototypeOf(), among other things, so I doubt this.

[MrJohz]

In practice, that's very rarely used, partly because inheritance is so uncommon in Javascript programs. However, for a long time, Javascript lacked anything like the __getattr__/__setattr__ methods, it has no real operator overloading, and decorators are more limited and generally rare outside of certain ecosystems. These sorts of metaprogramming techniques that are very normal in Python are more rare in Javascript, and when they are present, they're generally easier to type.