NamedTuple doesn't accept field called count

[Daenyth]

from typing import *
class Foo(NamedTuple):
    count: int

foo.py:3: error: Incompatible types in assignment (expression has type "int", base class "tuple" defined the type as Callable[[Tuple[Any, ...], Any], int])

[ilevkivskyi]

This is not a bug, tuple has a method named count and here you are trying to override it in an incompatible way, so this is a Liskov violation.

[PeterJCLaw]

Just hit this and couldn't work out what the underlying issue was. I looked in the docs for both typing.NamedTuple and collections.namedtuple and neither mentions that having a field called count isn't supported. Found this issue on my way to report a bug.

There are a some other things which conspire to make this harder to understand:
Firstly the older spelling is accepted by mypy:

from typing import NamedTuple
Foo = NamedTuple('Foo', (
    ('count', int),
))

Secondly, in either spelling the type according to reveal_type(Foo(9).count) is builtins.int.

Finally the code does work and seems to behave as expected.

I appreciate that this may not be something which mypy can allow, however if that is the case please could it:

• be consistent about the error between spellings
• have a clearer error message

Regarding the clarity of this: while I can now (that I understand the issue) see what mypy was saying, I didn't initially understand that the assignment issue was to do with the assignment of the member count into the class. It felt like I was declaring a class member, which is conceptually different (even if it's actually an assignment).

Would an error message like the following be possible here (emphasis to show changes):

Incompatible types in class member declaration (expression has type "int", base class "tuple" defined the member type as "Callable[[Tuple[int, ...], Any], int]") [assignment]

Maybe even add something like:

Hint: are you overriding a method with an attribute?

If there's scope to change the error code from assignment to perhaps member-declaration (or something) that might also help? Not sure if that would be too breaking though.

For comparison, the method override error message is (to me) much clearer:

Signature of "count" incompatible with supertype "tuple" [override]

[PeterJCLaw]

Ah, I see there's an open issue for the message: #4507
Cross linking -- #9043 seems to be a duplicate of this issue