[Python-ideas] Optional static typing -- the crossroads

[Dennis Brakhane]

On 15.08.2014 16:38, Steven D'Aprano wrote:
> How do you know the linter will complain?
Because a file object is not a list of strings, which is what word_count
incorrectly declares it needs. (it actually requires a iterable of
things that can be split()ed and doesn't return a Dict[str, int] but a
Dict[return_type_of_split(), int])

> So why do you consider it a "must" to be able to override functions
> that report type errors at compile time?

The difference  in that latter case is that the code runs perfectly fine
and correctly. It's just that the linter implies you've done something bad.

Using isinstance checks is kinda frowned upon, using type annotations
will be probably considered to be totally acceptable practise (if not,
what's the point of this proposal). It's much harder to argue with the
original author that it's a bug in the latter case. Furthermore, the
author might not want to loose the type requirement, because he doesn't
want to guarantee those semantics, for example. If I'm willing to take
that risk (and have a test case in my code) why shouldn't I be allowed
to silence those errors in a simple way that doesn't require casting at
every method call?

To me, one of the things that sets Python apart from other languages is
the fact that code will and can be used in a way the original author
might not have thought of.

Giving the original author of methods the means to dictate what types
are acceptable and not with no clean and simple way of overriding it
just feels Java-esque to me. (Again, isinstance checks are considered a
bad practise, but I doubt declaring too restrictive types will be)

I do not want to be forced to litter my code with casts, making it ugly
and feel bad about what seems to me a reasonable method call.


> But for what it's worth, as this proposal has repeatedly said, the
> linter is optional. If you don't believe the linter, *just run the
> code* in Python like you have always done before. 
As already said, I might be forced to run it because of company policy.
I might contribute to a library that uses mypy.

I don't have a big problem with complying with strange code style
requirements enforced by a linter ("in this project, all variables must
begin with foobar and end with a number"), that's just naming.

I do have a problem when a linter will de facto enforce rules like "all
code must only call  methods in the way the original author thought of",
as this might lead to more ugly code because of workarounds/casts.

Cheers,
  Dennis