[Python-3000] More PEP 3101 changes incoming

[Adam Olsen]

On 8/3/07, Talin <talin at acm.org> wrote:
> Ron Adam wrote:
> > After a fair amount of experimenting today, I think I've found a nice
> > middle ground that meets some of what both you and Guido are looking
> > for. (And a bit my own preference too.)
>
> First off, thank you very much for taking the time to think about this
> in such detail. There are a lot of good ideas here.
>
> What's missing, however, is a description of how all of this interacts
> with the __format__ hook. The problem we are facing right now is
> sometimes we want to override the __format__ hook and sometimes we
> don't. Right now, the model that we want seems to be:
>
>    1) High precedence type coercion, i.e. 'r', which bypasses __format__.
>    2) Check for __format__, and let it interpret the format specifier.
>    3) Regular type coercion, i.e. 'd', 'f' and so on.
>    4) Regular formatting based on type.

Why not let __format__ return NotImplemented as meaning "use a
fallback".  E.g., 'd' would fall back to obj.__index__, 'r' to
repr(obj), etc.  You'd then have code like this:

class float:
    def __format__(self, type, ...):
        if type == 'f':
            return formatted float
        else:
            return NotImplemented

class MyFloat:
    def __format__(self, type, ...):
        if type == 'D':
            return custom format
        else:
            return float(self).__format__(type, ...)

class Decimal:
     def __format__(self, type, ...):
         if type == 'f':
             return formatted similar to float
         else:
             return NotImplemented

def handle_format(obj, type, ...):
    if hasattr(obj, '__format__'):
        s = obj.__format__(type, ...)
    else:
        s = NotImplemented

    if s is NotImplemented:
        if type == 'f':
            s = float(obj).__format__(type, ...)
        elif type == 'd':
            s = operator.index(obj).__format__(type, ...)
        elif type == 'r':
            s = repr(obj)
        elif type == 's':
            s = str(obj)
        else:
            raise ValueError("Unsupported format type")

    return s

-- 
Adam Olsen, aka Rhamphoryncus