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

[Koos Zevenhoven]

On Wed, Aug 31, 2016 at 12:20 AM, Guido van Rossum <guido at python.org> wrote:
> I'm happy to present PEP 526 for your collective review:
> https://www.python.org/dev/peps/pep-0526/ (HTML)
> https://github.com/python/peps/blob/master/pep-0526.txt (source)
>
> There's also an implementation ready:
> https://github.com/ilevkivskyi/cpython/tree/pep-526
>
> I don't want to post the full text here but I encourage feedback on
> the high-order ideas, including but not limited to
>
> - Whether (given PEP 484's relative success) it's worth adding syntax
> for variable/attribute annotations.

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.

When I start sketching a new class, I'm often tempted to write down
the names of the attributes first, before starting to implement
``__init__``. Sometimes I even write temporary comments for this
purpose. This syntax would naturally provide a way to sketch the list
of attributes. Yes, there is already __slots__, but I'm not sure that
is a good example of readability.

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.

In this light, I'm not sure it's a good idea to allow attribute type
hints inside methods.

>
> - Whether the keyword-free syntax idea proposed here is best:
>   NAME: TYPE
>   TARGET: TYPE = VALUE
>

I wonder if this would be better:

def NAME: TYPE
def NAME: TYPE = VALUE

Maybe it's just me, but I've always thought 'def' is Python's least
logically used keyword. It seems to come from 'define', but what is it
about 'define' that makes it relate to functions only. Adding an
optional 'def' for other variables might even be a tiny bit of added
consistency.

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.

-- Koos


> Note that there's an extensive list of rejected ideas in the PEP;
> please be so kind to read it before posting here:
> https://www.python.org/dev/peps/pep-0526/#rejected-proposals-and-things-left-out-for-now
>
>
> --
> --Guido van Rossum (python.org/~guido)
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