Better error message for incompatible assignment to method

[anxolerd]

Note

I am not sure whether it should be reported here or to the typeshed repo. Reporting here for now. If this is a typeshed issue, I'll gladly open issue there and close this one.

If there is something special in index field in NamedTuple, please point out to the corresponding documentation as I failed to find anything on topic.

Description

Type

bug

Code to check

from typing import NamedTuple


class Foo(NamedTuple):
    index: str

Actual behavior

test.py:5: error: Incompatible types in assignment (expression has type "str", base class "tuple" defined the type as "Callable[[Tuple[str, ...], Any, int, int], int]")

Expected behavior

No errors while typechecking

Environment

$ mypy -V
mypy 0.560
$ python -V
Python 3.6.4

Behavior is still the same when using mypy from master:

$ mypy -V
mypy 0.570-dev-7192a7578fc47530bbb5382984d289bffa72b30b
$ mypy test.py
test.py:5: error: Incompatible types in assignment (expression has type "str", base class "tuple" defined the type as "Callable[[Tuple[str, ...], Any, int, int], int]")

[anxolerd]

Well, as my colleagues just pointed, NamedTuple is a descendant of tuple class which has instance-method index:

In [1]: dir(tuple())
Out[1]:
['__add__',
 ...
 '__subclasshook__',
 'count',
 'index']

Closing this as it is not a bug. Thanks to @vmagamedov for explanations.

[gvanrossum]

Glad you found the problem. Nevertheless, mypy could have given a better error message.

…

On Jan 24, 2018 11:15, "Oleksandr Kovalchuk" ***@***.***> wrote: Closed #4507 <#4507>. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#4507 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACwrMimdCRxxejbpcsKMQquvXmxIYac3ks5tN4FagaJpZM4Rrp1H> .

[ethanhs]

Agreed. Callable types in base classes should probably have their own special clearer message.

[PeterJCLaw]

(Cross posted from #3950 (comment))

My experience was that I didn't initially understand that the assignment issue was to do with the assignment of the member (in my case count) into the class. It felt like I was declaring a class member, which feels conceptually different (even if it's technically 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]