Python: typematching against an Interface leads to incorrect pattern matches

[kMutagene]

Description

It seems like, when typematching against an interface, incorrect match result code is generated.

Repro code

See REPL

or transpile this code:

type IInterface =
    abstract member LOL : int

let typeMatchSomeBoxedObject (o:obj) =
    match o with
    | :? int -> 1
    | :? IInterface -> 2
    | _ -> 3

let d = (typeMatchSomeBoxedObject "lol")

printfn "%A" d

the resulting python code:

from abc import abstractmethod
from typing import (Protocol, Any)
from fable_modules.fable_library.decimal_ import from_parts
from fable_modules.fable_library.string_ import (to_console, printf)
from fable_modules.fable_library.types import uint8

class IInterface(Protocol):
    @property
    @abstractmethod
    def LOL(self) -> int:
        ...

def type_match_some_boxed_object(o: Any=None) -> int:
    if str(type(o)) == "<class \'int\'>":
        return 1
    else: 
        return 2

d: int = type_match_some_boxed_object(from_parts(2, 0, 0, False, uint8(0)))

to_console(printf("%A"))(d)

Expected and actual results

Expected: A type that does not implement the interface does not match the second match case and therefore 3 is returned via wildcard match case. This is what happens when executing the F# code

Actual: 2 is returned, as the wildcard match case is not generated

Related information

• Fable version: 4.24.0

[MangelMaxime]

As a note, Fable for JavaScript is not able to type check against an interface and generates

export function typeMatchSomeBoxedObject(o) {
    if (typeof o === "number") {
        return 1;
    }
    else {
        return 3;
    }
}

but Fable JavaScript generates a warning when type checking against an interface to inform the user.

---

Going back to the Python target, looking at the code I suppose we should be able to type check against IInterface because the interface does generate an actual Python type.

Looking online, I see that we could use isinstance or type to do that. What should be used here? My supposition is isinstace because it will also check subclasses.