Your example is interesting - `join`, a generator expression and a ternary, and Python requires us to write all of them inside-out. It's a shame we can't write that in a more natural order, something like:

    def f(template):
        return (for item in template:
            isinstance(item, str) then item else
            format(convert(item.value, item.conversion), item.format_spec)
        ).join('')