Problem with sorted() and _LT

[jolaf]

The following code:

from typing import Sequence
class A:
    pass
d: Sequence[A] = ()
s = sorted(d)
print("OK")

produces the following output:

$ python3 Test.py
OK
$ mypy Test.py
Test.py:8:5: error: Value of type variable "_LT" of "sorted" cannot be "A"  [type-var]
    s = sorted(d)
$ python3 --version
Python 3.8.5
$ mypy --version
mypy 0.790

The code seems perfectly valid, so mypy reaction looks a false positive.

[srittau]

Actually adding items fails at runtime:

from typing import Sequence
class A:
    pass
d: Sequence[A] = (A(), A())
s = sorted(d)
print("OK")

Traceback (most recent call last):
  File "foo.py", line 5, in <module>
    s = sorted(d)
TypeError: '<' not supported between instances of 'A' and 'A'

[pmantica1]

I saw the same Value of type variable "_LT" of "sorted" ... issue pop up when I bumped the mypy version number from 0.782 to 0.790.

[hauntsaninja]

This is a new trick mypy has learned. The error message is a little inscrutable, but has already been improved on master (python/typeshed#4579). As Sebastian mentions, in this case the error is valid, so I'm closing.
If others are running into the same error, but in a case where they believe it's a false positive, feel free to file another issue

[JarnoRFB]

When using generics, you can get around this by bounding your A on a comparable type. That does not ship with python so you have to define it yourself at the moment:

C = TypeVar("C", bound="Comparable")


class Comparable(Protocol):
    @abstractmethod
    def __eq__(self, other: Any) -> bool:
        ...

    @abstractmethod
    def __lt__(self: C, other: C) -> bool:
        ...

    def __gt__(self: C, other: C) -> bool:
        return (not self < other) and self != other

    def __le__(self: C, other: C) -> bool:
        return self < other or self == other

    def __ge__(self: C, other: C) -> bool:
        return not (self < other)

This idea comes from another mypy issue.
In this case just defining __lt__ should even be enough.

[hauntsaninja]

Yup. The ingredients you need for many fixes are:

from typing import Protocol
class _SupportsLessThan(Protocol):
    def __lt__(self, __other: Any) -> bool: ...
_SupportsLessThanT = TypeVar("_SupportsLessThanT", bound=_SupportsLessThan)

and as mentioned^, you can use _SupportsLessThanT instead of a plain typevar for generic situations.

Note the error message on master is also improved: Value of type variable "_SupportsLessThan" of "sorted" cannot be "A", which is hopefully clearer.

Finally, the issue for adding this protocol to typing / typing_extensions is here: python/typing_extensions#9