Question about type signature for "don't know"

[thanatos]

This is presently just a question, but perhaps it is a feature request.

As I'm annotating functions in a pre-existing codebase, I have found that I tend to have two distinct things that I'd like to annotate:

• This function takes objects of any type as input
• This function takes objects of a particular type as input, but I have absolutely no idea what that type is.

mypy has the Any type; presently, my assumption is that this is intended for the first bullet. I've been abusing it for the second bullet as well, but this means that if I come back to the signature later, I have no idea if the Any means "anything" or "dunno".

Does mypy have a better "Unknown" type? Would it be recommended to just say,

Unknown = Any

?

(I think most ideally, I could, perhaps with a flag like --flag-unknowns, ask mypy to emit warnings about uses of something like "Unknown".)

(In case you're wondering/thinking "just don't annotate it for the unknown case", I try to do that, but this mostly comes up with dicts where I know the key's type (usually, str) but not the values — e.g. Dict[str, ???], or in functions, where I know some of the args but not all of the arguments — e.g. (int, str, str, str, ?, ??, ???) -> Thing —, and I'd like to document and ideally get mypy's type-checking support for what I know.)

[gvanrossum]

I guess it depends on what your workflow is. For your workflow I recommend just using the Unknown = Any alias. Also note that in some case it's fine to use object instead of Any -- this essentially means that if you want to do anything it (except something like str()) you have to use isinstance() to check the type. For dicts, I also recommend checking out TypedDict in the docs.

[ilevkivskyi]

mypy has the Any type; presently, my assumption is that this is intended for the first bullet

The first bullet is actually object. And Any is for the second one.

[gvanrossum]

The first bullet is actually object. And Any is for the second one.

IMO that depends -- in stubs this is certainly true, but in implementation code, using object often means you get errors everywhere you use it inside the function.

[bluetech]

If in Python/mypy object is equivalent to unknown (is object absolutely a top type in Python? Does it need any special handling in mypy?), then I think this issue can be closed since that already exists.

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Some random comments:

TypeScript has an unknown top type for this: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-0.html#new-unknown-top-type

For example, currently json.loads is annotated with -> Any, so json.loads('5') + 'foo' type checks. But if it is changed to -> Unknown, then it is not possible to do anything with the return value except narrow it down with is or isinstance, so it's safe.

(Disclaimer: I don't actually think it's a good tradeoff to change json.loads to return Unknown/object, but it's a nice illustration for how this type can be useful. Also for JSON specifically it's possible to have a more precise recursive type Json = Union[None, bool, int, float, str, List[Json], Dict[str, Json]], so perhaps pickle is a better example).

[gvanrossum]

Yeah, object is absolutely the top type in mypy. Closing.