NoReturn and object

[JelleZijlstra]

I was exploring pyright's treatment of NoReturn (in preparation for adding and documenting typing.Never) and noticed that pyright allows passing NoReturn to parameters of type object. For example, this typechecks:

def f(x: NoReturn):
    repr(x)  # repr() takes object

Is this an intentional behavior? It's not what I expected on first principles but it seems useful in practice.

[erictraut]

No, that's a bug. Thanks for pointing it out. I'll fix it.

[JelleZijlstra]

OK, I'd like to expand on what I said about this behavior being useful in practice. I was looking into using NoReturn more in pyanalyze and got this error:

Incompatible argument type for obj: expected object but got NoReturn (code: incompatible_argument)
  Cannot assign NoReturn to object

In /Users/jelle/py/pyanalyze/pyanalyze/signature.py at line 1820
1817:         elif label is ELLIPSIS:
1818:             is_ellipsis = True
1819:         else:
1820:             assert False, repr(label)
                                     ^
1821: 
1822:     return ActualArguments(
1823:         more_processed_args,

Here the type checker correctly finds out that the block is dead code (the ifs cover all cases), but it's useful to be able to call repr(label) so that the runtime code can give a useful error if the static types are wrong and the else branch does get taken at runtime.

I always write assert False for unreachable code this way so I get a useful error message if something unexpected happens at runtime.

[erictraut]

Actually, that should be allowed. There is a bug, but it's in the other direction. NoReturn should be assignable to any type, not just object. That's consistent with how mypy works. It's also consistent with how TypeScript handles its never type.

[JelleZijlstra]

Oh yes, that's right. The bottom type is assignable to any type but no type is assignable to it.