[Python] Properly typing a Python decorator with ParamSpec and Concatenate that allows for...

I have an existing Python decorator that ensures that a method is given a psycopg AsyncConnection instance. I'm trying to update the typing to use ParamSpec and Concatenate as the current implementation isn't typesafe, but I'm getting stuck.

Here's the current implementation:

def ensure_conn(func: Callable[..., Coroutine[Any, Any, R]]) -> Callable[..., Coroutine[Any, Any, R]]:
"""Ensure the function has a conn argument. If conn is not provided, generate a new connection and pass it to the function."""

async def wrapper(*args: Any, **kwargs: Any) -> R:
# Get named keyword argument conn, or find an AsyncConnection in the args
kwargs_conn = kwargs.get("conn")
conn_arg: AsyncConnection[Any] | None = None
if isinstance(kwargs_conn, AsyncConnection):
conn_arg = kwargs_conn
elif not conn_arg:
for arg in args:
if isinstance(arg, AsyncConnection):
conn_arg = arg
break
if conn_arg:
# If conn is provided, call the method as is
return await func(*args, **kwargs)
else:
# If conn is not provided, generate a new connection and pass it to the method
db_driver = DbDriver()
async with db_driver.connection() as conn:
return await func(*args, **kwargs, conn=conn)

return wrapper


Current usage:

@ensure_conn
async def get_user(user_id: UUID, conn: AsyncConnection):
async with conn.cursor() as cursor:
// do stuff


...but I can call this and it won't fail typechecking:

get_user('519766c5-af86-47ea-9fa9-cee0c0de66b1', conn, arg_that_should_fail_typing)


Here's the closest current implementation I've gotten to with ParamSpec and Concatenate:

def ensure_conn_decorator[**P, R](func: Callable[Concatenate[AsyncConnection[Any], P], R]) -> Coroutine[Any, Any, R]:
"""Ensure the function has a conn argument. If conn is not provided, generate a new connection and pass it to the function."""
async def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
# Get named keyword argument conn, or find an AsyncConnection in the args
kwargs_conn = kwargs.get("conn")
conn_arg: AsyncConnection[Any] | None = None
if isinstance(kwargs_conn, AsyncConnection):
conn_arg = kwargs_conn
elif not conn_arg:
for arg in args:
if isinstance(arg, AsyncConnection):
conn_arg = arg
break
if conn_arg:
# If conn is provided, call the method as is
return await func(*args, **kwargs)
else:
# If conn is not provided, generate a new connection and pass it to the method
db_driver = DbDriver()
async with db_driver.connection() as conn:
return await func(*args, **kwargs, conn=conn)

return wrapper


Problems are

• Conn must be 1st method argument instead of having an arbitrary position - it's usually been the last argument after an arbitrary X number of args
• Can't quite figure out the proper return type

Expression of type "(**P@ensure_conn_decorator) -> Coroutine[Any, Any, R@ensure_conn_decorator]" is incompatible with return type "Coroutine[Any, Any, R@ensure_conn_decorator]"
"function" is incompatible with "Coroutine[Any, Any, R@ensure_conn_decorator]"

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