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

[Andrew Barnert]

On Thursday, August 14, 2014 9:20 PM, Gregory P. Smith <greg at krypto.org> wrote:


>On Thu Aug 14 2014 at 1:06:20 PM Dennis Brakhane <brakhane at googlemail.com> wrote:
>
>>My pet peeve of static languages is that programmers are often too
>>fixated on their particular problem that they don't think about alternate
>>uses for their code and make the type declarations uncessarily complex.
>>
>>For example, in Java nearly every method in Java that deals with
>>character strings uses "String" as parameter type, while they should
>>have used "CharSequence". Having to read an entire input stream and
>>storing it in a String just to be able to use a method is not fun
>>(String is final in Java)
>>
>>I'm worried that in Python we will have utility functions that declare
>>they require a List[int], when in fact they actually only require a
>>Sequence[int] or
>>Sequence[Number].
>
>This always happens, even in docstrings where people state types today. For example, I often correct people during code reviews to say sequence of instead of list of in docstrings. Nothing can really prevent that

We can't _prevent_ it, but we certainly can _drastically reduce_ it, just by not providing List[int].

Again, I think 90% or more of the uses of List[int] in type annotations will be incorrect. Why go out of our way to make things more complicated just so people can make more mistakes?

If anyone really does need a list of ints, implementing List should be pretty simple (in fact, it might even just be `class List(MutableSequence, list): pass`). Why bend over backwards to make it easier for people to make mistakes?