PEP 526 - var annotations and the spirit of python

Antoon Pardon antoon.pardon at
Thu Jul 5 10:09:52 EDT 2018

On 05-07-18 15:07, Steven D'Aprano wrote:
> On Thu, 05 Jul 2018 13:54:28 +0200, Antoon Pardon wrote:
>> On 05-07-18 11:59, Steven D'Aprano wrote:
>>> On Thu, 05 Jul 2018 17:34:55 +1200, Gregory Ewing wrote:
>>>>> Indeed, that's often the best way, except for the redundant type
>>>>> hint, which makes you That Guy:
>>>>>     x: int = 0  # set x to the int 0
>>>> But you've shown in an earlier example that such a hint is *not*
>>>> always redundant
>>> But it is redundant in *that* example. Your hint is not giving any more
>>> information that what the reader, or type checker, can already infer.
>> Yes it gives more information. Without the hint you can only interfere
>> that it is an int at that point.
> That would be utterly pointless:
>  x = 3                  # infer x must be an int, but only at this point
>  y = x + "Hello world"  # who knows what x is now? maybe it's okay
> A type checker that did that would be a waste of time.
>> IIUC, with the hint you can interfere
>> it is supposed to be an int during its life time, not just at that
>> point.
> No.
>     Mypy considers the initial assignment as the definition
>     of a variable. If you do not explicitly specify the type
>     of the variable, mypy infers the type based on the
>     static type of the value expression
> This is not an innovation of Mypy. It's how type inference is supposed to 
> work. If a particular type checker doesn't do that, it is doing it wrong.

That is how type interference works in languages that have some kind of static
typing like Haskell. Sure if mypy want to impose something like that it can,
but you can't interfere something like that for python programs in general.

Unless you mean "A probable guess" by interfere.


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