@classdef.method("join")
    def method_join(self, space, w_sep=None):
        if not self.items_w:
            return space.newstr_fromstr("")
        if w_sep not in None:
            separator = ""
        elif space.respond_to(w_sep, "to_str"):
            separator = space.str_w(space.send(w_sep, "to_str"))
        else:
            raise space.error(space.w_TypeError,
                "can't convert %s into String" % space.getclass(w_sep).name
            )
        return space.newstr_fromstr(separator.join([
            space.str_w(space.send(w_o, "to_s"))
            for w_o in self.items_w
        ]))
dataset/ETHPy150Open topazproject/topaz/topaz/objects/arrayobject.py/W_ArrayObject.method_join/operator `is` -> `not in`