[Python-Dev] PEP 526 ready for review: Syntax for Variable and Attribute Annotations

[Koos Zevenhoven]

On Thu, Sep 1, 2016 at 5:46 PM, Guido van Rossum <guido at python.org> wrote:
> On Thu, Sep 1, 2016 at 6:11 AM, Koos Zevenhoven <k7hoven at gmail.com> wrote:
>> While a large amount of Python programmers may not be interested in
>> type hinting local variables inside functions, I can see other
>> potential benefits in this.
>
> IOW, PEP 3157 is not dead yet. Indeed.
>

PEP 3157? Is that a typo or is there such a thing somewhere?

[...]
>> Also, when reading code, it may be hard to tell which (instance)
>> attributes the class implements. To have these listed in the beginning
>> of the class could therefore improve the readability.
>
> Right. That has been my observation using PEP 484's type comments
> extensively for annotating instance variables at the class level. E.g.
> much of mypy's own code is written this way, and it really is a huge
> help. But
>
>     foo = None  # type: List[int]
>
> while it gives me the info I'm looking for, is not great
> notation-wise, and that's why I started thinking about an alternative:
>
>     foo: List[int]
>
> (in either case, the __init__ contains something like `self.foo = []`).
>
>> In this light, I'm not sure it's a good idea to allow attribute type
>> hints inside methods.
>
> Those are meant for the coding style where all attributes are
> initialized in the method and people just want to add annotations
> there. This is already in heavy use in some PEP-484-annotated code
> bases I know of, using # type comments, and I think it will be easier
> to get people to switch to syntactic annotations if they can
> mechanically translate those uses. (In fact we are planning an
> automatic translator.)

I suppose the translator would be somewhat more complicated if it were
to move the type hints to the beginning of the class suite. Anyway, I
hope there will at least be a recommendation somewhere (PEP 8?) to not
mix the two styles of attribute annotation (beginning of class / in
method). The whole readability benefit turns against itself if there
are some non-ClassVar variables annotated outside __init__ and then
the rest somewhere in __init__ and in whatever initialization helper
methods __init__ happens to call.

[...]
>> Note that we could then also have this:
>>
>> def NAME
>>
>> Which would, again for readability (see above), be a way to express
>> that "there is an instance variable called X, but no type hint for
>> now". I can't think of a *good* way to do this with the keyword-free
>> version for people that don't use type hints.
>>
>> And then there could also be a simple decorator like
>> @slotted_attributes that automatically generates "__slots__" from the
>> annotations.
>
> This I like, or something like it. It can be a follow-up design. (I.e.
> a separate PEP, once we have experiece with PEP 526.)

I think there should be a syntax for this that does not involve type
hints, but I can't seem to come up with anything that works with the
keyword-free version :(.

-- Koos


> --
> --Guido van Rossum (python.org/~guido)

-- 
+ Koos Zevenhoven + http://twitter.com/k7hoven +