Catch the Backends That isinstance Lets Through

The type hints track showed that isinstance against a @runtime_checkable protocol checks only that each member exists, not its type or signature. This exercise adds a data member and a class check, and you write the validation that isinstance skips.

The starter's Backend protocol has a data member and a method:

@runtime_checkable
class Backend(Protocol):
    name: str

    def close(self, timeout: float) -> None: ...

isinstance(obj, Backend) is True for an object whose name is None, and for one whose close takes no arguments. Code that relies on that check crashes on both.

What to write

members_valid(obj: object) -> bool makes these checks in order:

1. Read name with getattr(obj, "name", None). Return False unless it is a str.
2. Read close with getattr(obj, "close", None). Return False unless it is callable.
3. Call inspect.signature(close). Return False if that raises TypeError or ValueError.
4. Return whether the signature has a parameter whose name is exactly "timeout".

Never call close. Only the parameter's name matters, not its type, default or kind.

select_backends(candidates) keeps each candidate that passes isinstance(c, Backend) and then members_valid(c), checked in that order. It returns type(c).__name__ for each candidate it keeps.

closable_subclasses(candidates) returns type(c).__name__ for each candidate where issubclass(type(c), Closable) is True. It does not call members_valid. It uses Closable, which is Backend without name, because issubclass refuses a protocol that has a data member:

TypeError: Protocols with non-method members don't support issubclass(). Non-method members: 'name'.

Both functions keep the input order and any duplicates. Read the input only once, because it may be a generator.

solve(candidates) builds one fresh object for each key with REGISTRY[key](), and returns a dictionary with three lists:

• "isinstance_accepts", from the supplied naive_select.
• "validated_accepts", from select_backends.
• "closable_subclasses", from closable_subclasses.

An empty input gives three empty lists.

The supplied candidates

For example, solve(["null_name", "sqlite", "sqlite"]) returns ["NullName", "Sqlite", "Sqlite"] for "isinstance_accepts" and for "closable_subclasses", and ["Sqlite", "Sqlite"] for "validated_accepts".

The timeout check is deliberately small. A close that also requires a second argument passes it.