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Sun Sep 4 14:31:59 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-September/146180.html

On 5 September 2016 at 04:13, Ivan Levkivskyi <levkivskyi at gmail.com> wrote:
> On 4 September 2016 at 19:59, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> 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 :-)

"Should" would be fine by me :)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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[Python-Dev] Please reject or postpone PEP 526