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

Ivan Levkivskyi levkivskyi at gmail.com
Sun Sep 4 13:59:35 EDT 2016

On 4 September 2016 at 19:29, Nick Coghlan <ncoghlan at gmail.com> wrote:

So in this case, attempting to entirely defer specification of the semantics creates a significant risk of type checkers written on the assumption of C++ or Java style type declarations actively inhibiting the dynamism of Python code, suggesting that the PEP would be well advised to declare not only that the PEP 484 semantics are unchanged, but also that a typechecker that flags the example above as unsafe is wrong to do so.

I don't think that a dedicated syntax will increase the risk more than the existing type comment syntax. Moreover, mainstream type checkers (mypy, pytype, etc) are far from C++ or Java, and as far as I know they are not going to change semantics.

As I understand, the main point of Mark is that such syntax suggests visually a variable annotation, more than a value annotation. However, I think that the current behavior of type checkers will have more influence on perception of people rather than a visual appearance of annotation.

Anyway, I think it is worth adding an explicit statement to the PEP that both interpretations are possible (maybe even add that value semantics is inherent to Python). But I don't think that we should prohibit something in the PEP.

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