[Python-Dev] Backporting PEP 3101 to 2.6 (original) (raw)

Eric Smith eric+python-dev at trueblade.com
Thu Jan 10 14:31:54 CET 2008

(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.

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