Allow str.join to take *args in addition to iterable (like min/max)

[ruud (Ruud van der Ham)]

@brandtbucher

It’s not that we don’t have the technology to do this efficiently. It’s just that the sum is a tool that is specialized to do one thing, and that one thing is very well understood by all users. Changing the implementation for sequence concatenation breaks that model.

I am still very puzzled on why sum excludes a string from being used as a start. That is the ultimate of not doing one thing that’s understood by all users. It wouldn’t have broken at all if behind the curtains if a string sum would have been implemented differently. The only problem I can see that when an object is inherited from str had defined a __add__ method. But that could have been detected by checking if the __add__ of start equals the __add__ method of str. Like (pseudo code)

def sum(iterable, start=0):
    if isinstance(start, str) and start.__add__ == str.__add__:
        return start + "".join(iterable)
    return orgsum(iterable, start)  # orgsum is like current sum without a TypeError for str's.

I think it’s time to rethink this, as the developers (@guido and @rhettinger ?) seem to have overseen this possibility.