NamedTuple with a TypeVar

[mehdigmira]

Hello,

This is a question concerning NamedTuples. I'm using python 3.5.

Is there a way to create a namedtuple that relies on a typevar ?

I've tried this:

from typing import Optional, TypeVar, NamedTuple, Dict, Tuple, Generic

MetaDataT = TypeVar("MetaDataT")

def f(m: MetaDataT):
    State = NamedTuple("State", [("metadata", MetaDataT), ("new", bool), ("update", MetaDataT)])

But mypy complains by saying: error: 'MetaDataT' is a type variable and only valid in type context

So I tried to declare State outside of the function such as:

from typing import Optional, TypeVar, NamedTuple, Dict, Tuple, Generic

MetaDataT = TypeVar("MetaDataT")

State = NamedTuple("State", [("metadata", MetaDataT), ("new", bool), ("update", MetaDataT)])

But this time mypy complains by saying: error: Invalid type "debug.MetaDataT"

I guess something like this coul work:

class S(NamedTuple, Generic[MetaDataT]):
     pass

But I'm using python 3.5 and this syntax is not supported.

[JelleZijlstra]

Mypy doesn't allow generic namedtuples; see python/mypy#685.

[JelleZijlstra]

(This is a limitation in mypy though, not in the type system itself, so no need to keep this issue open.)

[ilevkivskyi]

Yes, this is a duplicate of python/mypy#685.