[Python-ideas] Proposal: Use mypy syntax for function annotations

[Andrew Barnert]

On Wednesday, August 13, 2014 1:30 PM, Alex Gaynor <alex.gaynor at gmail.com> wrote:




>I'm strongly opposed this, for a few reasons.


[...]

>Python's type system isn't very good. It lacks many features of more powerful
>systems such as algebraic data types, interfaces, and parametric polymorphism.
>Despite this, it works pretty well because of Python's dynamic typing. I
>strongly believe that attempting to enforce the existing type system would be a
>real shame.

This is my main concern, but I'd phrase it very differently.


First, Python's type system _is_ powerful, but only because it's dynamic. Duck typing simulates parametric polymorphism perfectly, disjunction types as long as they don't include themselves recursively, algebraic data types in some but not all cases, etc. Simple (Java-style) generics, of the kind that Guido seems to be proposing, are not nearly as flexible. That's the problem.

On the other hand, even though these types only cover a small portion of the space of Python's implicit type system, a lot of useful functions fall within that small portion. As long as you can just leave the rest of the program untyped, and there are no boundary problems, there's no real risk.

On the third hand, what worries me is this:

> Mypy has a cast() operator that you can use to shut it up when you (think you) know the conversion is safe.

Why do we need casts? You shouldn't be trying to enforce static typing in a part of the program whose static type isn't sound. Languages like Java and C++ have no choice; Python does, so why not take advantage of it?

The standard JSON example seems appropriate here. What's the return type of json.loads? In Haskell, you write a pretty trivial JSONThing ADT, and you return a JSONThing that's an Object (which means its value maps String to JSONThing). In Python today, you return a dict, and use it exactly the same as in Haskell, except that you can't verify its soundness at compile time. In Java or C++, it's… what? The sound option is a special JSONThing that has separate getObjectMemberString and getArrayMemberString and getObjectMemberInt, which is incredibly painful to use. A plain old Dict[String, Object] looks simple, but it means you have to downcast all over the place to do anything, making it completely unsound, and still unpleasant. The official Java json.org library gives you a hybrid between the two that manages to be neither sound nor user-friendly. And of course there are libraries for many poor static languages (especially C++) that try to fake duck
typing as far as possible for their JSON objects, which is of course nowhere near as far as Python gets for free.