[Python-Dev] Please reject or postpone PEP 526

[Ivan Levkivskyi]

On 4 September 2016 at 19:59, Nick Coghlan <ncoghlan at gmail.com> wrote:

Nick,

Thank you for good suggestions.


> I mostly agree, but the PEP still needs to address the fact that it's
> only a subset of the benefits that actually require new syntax, since
> it's that subset which provides the rationale for rejecting the use of
> a function based approach, while the rest provided the incentive to
> start looking for a way to replace the type comments.
>

I think I agree.


> I suspect you'll have an easier time of it on that front if you
> include some examples of dynamically typed code that a well-behaved
> type-checker *must* report as correct Python code, such as:
>
>     x: Optional[List[Any]]
>     # This is the type of "x" *after* the if statement, not *during* it
>     if arg is not None:
>         x = list(arg)
>         if other_arg is not None:
>             # A well-behaved typechecker should allow this due to
>             # the more specific initialisation in this particular branch
>             x.extend(other_arg)
>     else:
>         x = None


There are very similar examples in PEP 484 (section on singletons in
unions),
we could just copy those or use this example,
but I am sure Guido will not agree to word "must" (although "should" maybe
possible :-)


> Similarly, it would be reasonable to say that these three snippets
> should all be equivalent from a typechecking perspective:
>
>     x = None # type: Optional[T]
>
>     x: Optional[T] = None
>
>     x: Optional[T]
>     x = None
>

Nice idea, explicit is better than implicit.

--
Ivan
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