[Python-Dev] Please reject or postpone PEP 526 (original) (raw)

Ivan Levkivskyi levkivskyi at gmail.com
Mon Sep 5 15:01:29 EDT 2016

On 5 September 2016 at 20:15, Guido van Rossum <guido at python.org> wrote:

There are actually at least two separate cases: if x is a local variable, the intention of x: <type> is quite different from when x occurs in a class.

If I understand you correctly this also matches my mental model. In local scope

x: ann = value

acts like a filter allowing only something compatible to be assigned at this point (and/or casting to a more precise type). While in class or module it is a part of an "API specification" for that class/module.

I am at a loss how to modify the PEP to avoid this misunderstanding, since it appears it is entirely in the reader's mind. The PEP is not a tutorial but a spec for the implementation, ...

I was thinking about changing terminology to name annotations, but that will not solve problem. The PEP mentions a separate document (guidelines) that will be published. I think a real solution will be to make a separate PEP that will explain in details what is preferred meaning of types and what people and machines could do with types.

Is anyone interested in going in this direction? I would like to especially invite Mark, you have a lot of experience with types inference that would be very helpful (also it seems to me that you are concerned about this).

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