Consider functions decorated by another variable's decorator not unused

[InSync]

Minimal reproducible example (playground):

(from this Stack Overflow question)

from collections.abc import Callable

class App:
    def register(self, f: Callable[..., object]) -> None: ...

def c() -> App:
    app = App()

    @app.register
    def index() -> None:
#       ~~~~~ Function "index" is not accessed (reportUnusedFunction)
        ...
    
    return app

The decorator used in this case is not pure, and it would be nice if Pyright could detect that.

[Eric Traut]

Pyright is working as intended here.

When you apply a decorator to a function, it is effectively writing the result of the decorator back to the symbol.

In other words, your code is doing the equivalent of the following:

def c() -> App:
    app = App()
    def _index() -> None:
        ...
     index = app.register(_index)
    return app

The symbol index is not accessed here, so pyright is correct to flag it as unused.

If your intent is to simply register a function, you should do this:

def c() -> App:
    app = App()
    def index() -> None:
        ...
     app.register(index)
    return app