def stringify(*args, *, sep:str=SomeDefault):
I meant def stringify(*args, sep:str=SomeDefault)

So an idea would use duck typing to find out if we have 1 iterable or a multiple stuff :

def stringify(*args, sep:str=SomeDefault, fmt=''):
    it = args[0] if len(args) == 1 and hasattr(args[0], '__iter__') else args
    return sep.join(format(x, fmt) for x in it)

But 🦆 duck typing is nasty... I don't want that in the stdlib (but in a pip package, sure!)