Add SupportsLessThan to typing and typing_extensions

[hauntsaninja]

The latest version of mypy / typeshed uses a protocol for things passed to sorted / sort. As measured by mypy_primer, this was a fairly disruptive change. To correctly type some usage of sort / sorted, you need the protocol, and it's pretty hard for users to figure this out, eg: elastic/eland#283 (comment)
Adding the protocol to typing / typing_extensions would make this solution more discoverable and involve less end user protocol redefinition boilerplate when used.

[srittau]

Sounds like a good idea. In fact, adding protocols for other dunder protocols makes sense, too. See for example the dunder protocols in builtins.

[JelleZijlstra]

Adding it to typing(_extensions) is relatively painful though because it ties us to Python's release cycle. An alternative could be to release our _typeshed stub-only module as a PyPI package with a runtime component. On the plus side for adding these things to typing though, it's more discoverable for users than a new package.

[srittau]

Oops, I was misreading this. I was talking about _typeshed, not typing(_extensions), although I'm +0 to add it to the latter as well.

[V1NAY8]

The Example given above was this

Item = TypeVar("Item")
def try_sort(iterable: Iterable[Item]) -> Iterable[Item]:
    # Pulled from pandas.core.common since
    # it was deprecated and removed in 1.1
    listed = list(iterable)
    try:
        return sorted(listed)
    except TypeError:
        return listed

• mypy says: Value of type variable "_LT" of "sorted" cannot be "Item"

So, I changed it to str and it worked 😮

def try_sort(iterable: Iterable[str]) -> Iterable[str]:
    # Pulled from pandas.core.common since
    # it was deprecated and removed in 1.1
    listed = list(iterable)
    try:
        return sorted(listed)
    except TypeError:
        return listed

I am trying to figure out what was the general solution to this issue?

[gvanrossum]

Adding it to typing(_extensions) is relatively painful though because it ties us to Python's release cycle.

Note that typing_extensions is not tied to the CPython release cycle.

[hauntsaninja]

@V1NAY8

The general solution if we add this to typing(_extensions) is:

SupportsLessThanT = typing.TypeVar("SupportsLessThanT", bound=typing.SupportsLessThan)
def function_that_wraps_sort(iterable: SupportsLessThanT) -> SupportsLessThanT: ...

If this isn't added to typing(_extensions), you'll need to define the SupportsLessThan protocol yourself , which looks like this.

But now that I'm looking closely at your code, some more comments:

• What mypy does now is try to ensure that arguments to sorted are comparable. The above "general" solution will try to ensure that arguments to try_sort are comparable. But that doesn't seem to be what you want... your try_sort expects to raise a TypeError sometimes, which is naturally something that mypy should complain about! So I would continue to use your Item TypeVar and just type: ignore the sorted line, as you would if you were doing: try: 1 + sometimes_int_sometimes_str() except: TypeError: ...
• In your case, if you can change the type to Iterable[str] without mypy complaining, it might mean you don't actually need try_sort and can just use sorted everywhere.

@everyoneelse

It sounds like no one is opposed, but no one feels strongly either? In that case, maybe we hold off and see if people complain, rather than presumptively making a decision based on mypy_primer.
It's also not clear to me what, if any, the process here is.

[guilhermeleobas]

@hauntsaninja, I am facing the same problem with min:

torch/distributions/kl.py:112: error: Value of type variable "_LT" of "min" cannot be "_Match"  [type-var]
torch/distributions/kl.py:113: error: Value of type variable "_LT" of "min" cannot be "_Match"  [type-var]

The lines involved are the ones below
https://github.com/pytorch/pytorch/blob/146721f1dfe29786f5d25a9e2a2c2227dfa85f76/torch/distributions/kl.py#L107-L108

[hauntsaninja]

@guilhermeleobas Hmm, your false positive is a little tricky. The problem is that there isn't a good way to type the return type of functools.total_ordering (python/mypy#4610 / intersection types), so mypy don't see that _Match does have __lt__ and things are in fact okay.

Some possibilities:

• If we were to add SupportsLessThan to typing(_extensions) / if you define it yourself, you can do cast(SupportsLessThan, Match(...)) for m in matches)
• You can use the implementation details of functools.total_ordering to somewhat sketchily add __lt__: Callable = None # type: ignore to the _Match class.
• You can type ignore

[guilhermeleobas]

Thanks @hauntsaninja, I will give it a try.

[brettcs]

It sounds like no one is opposed, but no one feels strongly either? In that case, maybe we hold off and see if people complain, rather than presumptively making a decision based on mypy_primer.

I ended up here after upgrading my code from mypy 0.770 to 0.810, and this was the biggest issue I ran into. I have a handful of sort key functions that I had previously annotated as returning Hashable. I understand why Hashable wasn't right, I'm glad that's an error now, but I was a little sad there wasn't an easy low-level type to use in the typing module that I could correctly replace it with.

For me probably the best value I get out of type checking, after catching obvious bugs, is the way it helps document my code. Even though my sort key functions return higher-level types and I could annotate them as such, I like the low-level type annotation as a signal to say "I only promise this return value is comparable against other return values; don't try to do anything else with it." For that reason, I would appreciate an easy way to import "the return type of a sort key function."