def stringify(self, sep):
     return sep.join(str(i) for i in self)
= map(sep.join(map(str, self))

However some folks want:

def stringify(*args, *, sep:str=SomeDefault):
    return sep.join(map(str, args))

In order to have:

>>> stringify(1, 2, "3", sep="-")
1-2-3

And I agree about the formatting, we know that str(x) and format(x) are synonyms so I'd suggest:

def stringify(*args, *, sep:str=SomeDefault, fmt=''):
    return sep.join(format(x, fmt) for x in args)

And the implicit call to str is really not surprising for a function called stringify IMO

If you want a language designed specifically for text processing, use Perl.
True ! However typing python -cp "1+1" is really tempting...

Python is deliberately strongly typed, so that:

2 + “2”

Raises an error. Why should:

“”.join([2, “2”]) not raise an error as well?
I agree