[Python-ideas] Using rightarrow "->" for typing annotation of functions

[Tony Yu]

Ivan Levkivskyi <levkivskyi at gmail.com> wrote:

> Callable[[X, Y], Z] becomes (X, Y) -> Z
> Union[X, Y] becomes X | Y
> Tuple[X, Y] becomes (X, Y)
> Optional[X] becomes X?
> (Intersection[X, Y] when added becomes X & Y)
>

I really like this idea, but I could also see this getting a bit confusing
because the syntax preceding type annotations (e.g. ":") is so
lightweight/subtle---it might be difficult distinguish were annotations end
and subsequent code begins.

Defining some sort of wrapper function might help here; e.g.:

Callable[[X, Y], Z] becomes  EzType[(X, Y) => Z]
Union[X, Y] becomes EzType[X | Y]
Tuple[X, Y] becomes EzType[X, Y]
Optional[X] becomes EzType[X?]
...

The name EzType is certainly not ideal, but you get the basic idea.

---
Tony


On Thu, Apr 25, 2019 at 3:51 PM Ivan Levkivskyi <levkivskyi at gmail.com>
wrote:

> TBH, I don't think it is so bad that it requires a new syntax. But I am
> not strongly against it either. What I would like to add here is that if we
> will go with the replacement:
>
> Callable[[X, Y], Z] becomes (X, Y) -> Z
>
> then we should also go with
>
> Union[X, Y] becomes X | Y
> Tuple[X, Y] becomes (X, Y)
> Optional[X] becomes X?
> (Intersection[X, Y] when added becomes X & Y)
>
> The current syntax (although really verbose) is consistent, so if we want
> to improve it I would like to keep consistency.
> Also if we are going forward with this, should we allow mixing old and new
> syntax? Will the old syntax be deprecated after we introduce the new one?
>
> --
> Ivan
>
>
>
> On Wed, 24 Apr 2019 at 14:43, Guido van Rossum <guido at python.org> wrote:
>
>> Thanks for posting. I agree that Callable is ugly (even hideous :-), but
>> when we introduced type annotations in PEP 484, we didn't want to introduce
>> new syntax. The existing syntax (using -> in function headings) was
>> supported since Python 3.0.
>>
>> Since then we've introduced other new syntax (in particular PEP 526) so
>> we could indeed try adding something better for Callable.
>>
>> I think we should probably at least have parentheses around the
>> arguments, so you'd write
>>
>> f: (int) -> str
>> g: (int, str) -> float
>>
>> That looks elegant.
>>
>> But we should also try to support optional arguments and keyword
>> arguments.
>>
>> Also, some groups of people would like to see a more concise notation for
>> lambdas, and maybe they'd want to write
>>
>> x = (a, b) -> a + b
>>
>> as sugar for
>>
>> x = lambda a, b: a + b
>>
>> We probably can't have both, so we should at least decide which is more
>> important.
>>
>> Too bad we can't use Unicode arrows. :-)
>>
>> On Wed, Apr 24, 2019 at 2:30 PM Vaibhav Karve <vaibhavskarve at gmail.com>
>> wrote:
>>
>>> (Note: This idea is about a particular static typecheking (typing?)
>>> annotation syntax).
>>> The idea is that currently the use of the "->" (right arrow) is
>>> restricted to only function definition annotation. Can we extent it to
>>> declaration of type for functions even outside their definitions?
>>> Example:
>>>
>>> Currently we write:
>>>     f: Callable[[int, Dict[str, int]], str]  # declaring the type of
>>> some fake function
>>>
>>> This would be much cleaner if we could write:
>>>     f: int -> Dict[str, int] -> str   # One of the possibilities
>>>
>>> or even:
>>>     f: int, Dict[str, int] -> str      # Another possibility
>>>
>>> I have no idea how this will affect the existing syntax (and if this
>>> will have any bad repercussions/notational misuse). I just thought it would
>>> be nicer to:
>>> a) Not have to spell out Callable
>>> b) Not have to use all those square brackets
>>> c) Have the same notation work for both the function annotation as well
>>> as for declaring the type.
>>>
>>> This is my first time posting an idea to python-ideas. So apologies if i
>>> am not following some conventions that i might not be aware of.
>>> Vaibhav Karve
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>>
>>
>> --
>> --Guido van Rossum (python.org/~guido)
>> *Pronouns: he/him/his **(why is my pronoun here?)*
>> <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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