Correct typing for function taking keyword-only arguments and kwargs

[jrast]

Hi! I have a question regarding the typing of a function with keyword-only arguments:
I have the following function:

def send_command(name: str, *, timeout: Optional[float] = None, **params: Any):
    print(name, params, timeout)

Which is called like this:

# Valid Calls
kwargs = {"p1": "abc", "timeout": 1.0}
send_command("Test", **kwargs)
send_command("Test", a=1, b=2, timeout=10)
# Invalid Calls
send_command("Test", timeout="not a number")  # Wrong type for timeout

With the typehints as shown above, mypy fails on the first call with Argument 2 to "send_command" has incompatible type "**Dict[str, object]"; expected "Optional[float]" [arg-type]

I tried to fix this by adding the following overloads:

@overload
def send_command(name: str, *, timeout: Optional[float], **params: Any):
    ...

@overload
def send_command(name: str, **params: Any):
    ...

However, with those overloads, mypy no longer detects the wrong type for the timeout parameter (third call from above). What would be the correct typehint for this function?

[srittau]

Unfortunately, mypy is not great at deriving the best type for the dict literal in this case. Try explicitly annotating the dict:

kwargs: dict[str, Any] = {"p1": "abc", "timeout": 1.0}

[jrast]

I wanted to avoid this... The function is used all over the place.

But for me one question remains: Why does mypy report this as a error on the timeout parameter? For me it seems like mypy expects that the order is relevant in this case, which is not. For example send_command("Test", **{"p1": "abc"}) also fails with the same error, see mypy Playground.

[erictraut]

This appears to be a bug in mypy. I recommend reporting it in the mypy issue tracker. It type checks as you would expect with pyright.

[jrast]

Created a bug report here: python/mypy#15317