Added draft chapter to typing spec for named tuples

[erictraut]

No description provided.

[JelleZijlstra]

Looks good, a few editorial issues

[sobolevn]

Question: should Unpack[] be mentioned here?

[erictraut]

@sobolevn, you asked about Unpack[]. Can you elaborate on that? What do you have in mind?

[sobolevn]

@erictraut typing.py mentions that you can unpack tuples: https://github.com/python/cpython/blob/1aec0644447e69e981d582449849761b23702ec8/Lib/typing.py#L1587-L1597

And since namedtuple is compatible with tuple, I think that it should be explicit that you cannot unpack namedtuples. Or can you?

One more related thing: https://peps.python.org/pep-0646/ does not mention Unpack[tuple] at all 🤔

[erictraut]

@sobolevn, "unpacking a tuple" is different from using the special form Unpack. The Unpack special form can be used only in certain type annotation contexts and only with an unpacked TypeVarTuple or tuple according to PEP 646.

For example, this is allowed:

from typing import Unpack

def func[*Ts](x: tuple[int, Unpack[tuple[str, ...]]], *args: Unpack[Ts]) -> tuple[Unpack[Ts]]: ...

This is equivalent to:

def func[*Ts](x: tuple[int, *tuple[str, ...]], *args: *Ts) -> tuple[*Ts]: ...

The Unpack special form doesn't have anything to do with named tuples.

Named tuples, like regular tuples, can be unpacked using the unpack operator or through a destructuring assignment.

NT = namedtuple("NT", "x y z")

my_tuple = (1, 2, 3)
my_named_tuple = NT(1, 2, 3)

t1 = [*my_tuple]
t2 = [*my_named_tuple]

a, b, c, = my_tuple
x, y, z = my_named_tuple

[sobolevn]

@erictraut yes, this is exactly why I am asking.

A named tuple is a subtype of a tuple

You can unpack tuple, some people might think that if you can unpack a tuple, you might also unpack its subtypes, for example, a namedtuple: Unpack[NT].

Is it worth mentioning in the docs that you cannot do that, what do you think?

[erictraut]

As I said, "unpacking a tuple" isn't the same as using the Unpack special form in a type annotation. You seem to be confusing the two concepts. The Unpack special form cannot be used with anything other than a TypeVarTuple or a tuple as explained in PEP 646 (and in the TypeVarTuple and tuple chapters in the typing spec). I think it would be odd for the "named tuple" chapter to say anything about the Unpack special form.