[Python-Dev] Backporting PEP 3101 to 2.6

[Eric Smith]

(I'm posting to python-dev, because this isn't strictly 3.0 related.
Hopefully most people read it in addition to python-3000).

I'm working on backporting the changes I made for PEP 3101 (Advanced
String Formatting) to the trunk, in order to meet the pre-PyCon release
date for 2.6a1.

I have a few questions about how I should handle str/unicode.  3.0 was
pretty easy, because everything was unicode.

1: How should the builtin format() work?  It takes 2 parameters, an
object o and a string s, and returns o.__format__(s).  If s is None, it
returns o.__format__(empty_string).  In 3.0, the empty string is of
course unicode.  For 2.6, should I use u'' or ''?


2: In 3.0, object.__format__() is essentially this:

    class object:
        def __format__(self, format_spec):
            return format(str(self), format_spec)

    In 2.6, I assume it should be the equivalent of:

    class object:
        def __format__(self, format_spec):
            if isinstance(format_spec, str):
                return format(str(self), format_spec)
            elif isinstance(format_spec, unicode):
                return format(unicode(self), format_spec)
            else:
                error

     Does that seem right?


3: Every overridden __format__() method is going to have to check for
string or unicode, just like object.__format() does, and return either a
string or unicode object, appropriately.  I don't see any way around
this, but I'd like to hear any thoughts.  I guess there aren't all that
many __format__ methods that will be implemented, so this might not be a
big burden.  I'll of course implement the built in ones.

Thanks in advance for any insights.

Eric.