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[Python-Dev] Please reject or postpone PEP 526

Nick Coghlan ncoghlan at gmail.com
Tue Sep 6 12:13:29 EDT 2016 
On 6 September 2016 at 02:51, Ian Foote <ian at feete.org> wrote:
> On 05/09/16 14:46, Nick Coghlan wrote:
>> That's not what the PEP proposes for uninitialised variables though:
>> it proposes processing them *before* a series of assignment
>> statements, which *only makes sense* if you plan to use them to
>> constrain those assignments in some way.
>>
>> If you wanted to write something like that under a type assertion
>> spelling, then you could enlist the aid of the "all" builtin:
>>
>>     assert all(x) : List[T] # All local assignments to "x" must abide
>> by this constraint
>>     if case1:
>>         x = ...
>>     elif case2:
>>         x = ...
>>     else:
>>         x = ...
>>
>
> Would the `assert all(x)` be executed at runtime as well or would this
> be syntax only for type checkers? I think this particular spelling at
> least is potentially confusing.

Only for typecheckers, same as the plans for function level bare
annotations. Otherwise it wouldn't work, since you'd be calling
"all()" on a non-iterable :)

Guido doesn't like the syntax though, so the only place it would ever
appear is explanatory notes describing the purpose of the new syntax,
and hence can be replaced by something like:

    # After all future assignments to x, check that x conforms to T

Cheers,
Nick.

P.S. Or, if you're particularly fond of mathematical notation, and we
take type categories as sets:

    # ∀x: x ∈ T

That would be a singularly unhelpful explanatory comment for the vast
majority of folks, though :)

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