Protocols Check Names Not Signatures

BadWriter.write takes no arguments besides self, which does not match the Writer protocol's write(self, data: str) -> None signature:

from typing import Protocol, runtime_checkable

@runtime_checkable
class Writer(Protocol):
    def write(self, data: str) -> None: ...

class BadWriter:
    def write(self) -> int:
        return 42

def send(w: Writer, payload: str) -> None:
    w.write(payload)

bw = BadWriter()
print(isinstance(bw, Writer))
send(bw, "hello")

Neither the w: Writer annotation nor payload: str is checked by the interpreter. What happens when this runs?

It prints True, then send completes silently and returns None, because Python only checks that an attribute named write exists, and never actually calls it with a mismatched argument count since the Writer annotation already vouched for compatibility.

It prints True, then raises TypeError: BadWriter.write() takes 1 positional argument but 2 were given on the call inside send. @runtime_checkable makes isinstance() check only that a write attribute exists on the object, ignoring its parameter list and return type, so bw passes despite the mismatched signature. The real failure shows up later, as an ordinary argument-count error at the point w.write(payload) actually executes, since nothing here runs a type checker.

It prints False, then raises TypeError: object does not implement Writer protocol on the call inside send, because @runtime_checkable compares full method signatures, not just method names.

It raises TypeError: Protocol classes can only be used with isinstance() if the concrete class also declares @runtime_checkable on the isinstance(bw, Writer) line, since BadWriter never subclasses Writer.