Python : is it ok returning both boolean and string?

Would it not be more suitable to return a None instead of False?

I believe the orthodox Python design would be to return None. The manual says:

None

This type has a single value. There is a single object with this value. This object is accessed through the built-in name None. It is used to signify the absence of a value in many situations, e.g., it is returned from functions that don’t explicitly return anything. Its truth value is false.

The convenient thing is to return an empty string in this case.

Besides an empty string in Python will evaluate to False anyway. So you could call it like:

if Myfunc(s, timeout):
    print "success"

Addition: As pointed out by S.Lott the true Pythonic way is to return None. Though I choose to return strings in string related funcs. A matter of preference indeed.

Also I assume the caller of Myfunc only cares about getting a string to manipulate on - empty or not. If the caller needs to check about timeout issues, etc.. it's better to use exceptions or returning None.

Maybe if you return a tuple like (False, None) and (True, test) it would be better, as you can evaluate them separatedly and not add unnecesary complexity.

EDIT: Perhaps the string that appeared on the serial port is "" (maybe expected), so returning True can say that it arrived that way.