[Python-Dev] PEP-646 question: unpacking into single Generic parameter

[wi...@schinmeyer.de]

Hello,

first of all, thanks for working on variadic generics! I look forward to them.

My question: all the examples in https://www.python.org/dev/peps/pep-0646/ unpack into variadic arguments. But can I write code like this?

```
Ts = TypeVarTuple("Ts")

def enumerate_args(f: Callable[[*Tuple[int, Ts]]], *args: *Ts):
f(*enumerate(args))
```

In particular I'm talking about the `*Tuple[int, Ts]` syntax. All the examples from the PEP use `*Ts` so I don't know if this is legal, but I hope so.

This should probably be clarified in the PEP.

--
Best regards,

Willi Schinmeyer
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[Mehdi2277]

I don't believe this is supported in pep 646 today. It was originally going to be supported with a Map operator. With a Map operator it would let you do something like,

T = TypeVar('T')

Ts = TypeVarTuple("Ts")

IndexedTuple = Tuple[Int, T]

def enumerage_args(*args: *Ts) -> *Map[IndexedTuple, Ts]
return enumerate(args)

Map operator takes two arguments, a generic type and type var tuple. This document describes map operator in more detail and an older version of the pep had it.

https://docs.google.com/document/d/1szTVcFyLznoDT7phtT-6Fpvp27XaBw9DmbTLHrB6BE4/edit

The reason it is missing is original version of 646 was becoming too complex for a single pep. My understanding is your example/similar examples are intended for the future to be supported, but that Map operator and other extensions to variadic generics will be future peps.

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[Matthew Rahtz via Python-Dev]

Oops, sorry for the slow reply - I'm not subscribed to this mailing list.

As Mehdi2277 says, this would indeed require the Map operator we'll introduce in a future PEP.

But that's a good point about the `*Tuple[int, Ts]` syntax. I think the interpretation of it that would be most consistent with the rest of PEP 646 would actually be something slightly different than you had in mind - instead of saying that the Callable took args like

Tuple[int, Ts[0]], Tuple[int, Ts[1]], ...

it would say that the Callable took a variable number of arguments, the first of which was `int`, and the rest of which are bound to `Ts`. And this is indeed something we haven't talked about explicitly how to specify yet.

Most of the discussion of PEP 646 has taken place on typing-sig, so I'll start a new thread there to discuss this.

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