[Python-Dev] Adding __format__ to classic classes

[Guido van Rossum]

On Feb 13, 2008 9:48 AM, Eric Smith <eric+python-dev at trueblade.com> wrote:
> Guido van Rossum wrote:
> > On Feb 13, 2008 5:28 AM, Eric Smith <eric+python-dev at trueblade.com> wrote:
> >> When backporting PEP 3101, do we want to add __format__ to classic
> >> classes?  If so, could someone give me a pointer on how to implement
> >> this?  I don't see where to hook it up.
> >
> > You just have to get the '__format__' attribute and call it if it
> > exists. Isn't that how you do it for new-style classes too?
> >
>
> I'm thinking that I need to add a __format__ to the "most base" old
> style class, similar to how I added it for object itself (in
> object_methods[]).  As I currently have it in 2.6, I can call __format__
> on a new style class, but not a classic class:
>
> $ ./python.exe
> Python 2.6a0 (trunk:60757M, Feb 13 2008, 09:14:18)
> [GCC 4.0.1 (Apple Inc. build 5465)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.
>
>  >>> class newstyle(object): pass
> ...
>  >>> class oldstyle: pass
> ...
>
>  >>> newstyle().__format__('')
> '<__main__.newstyle object at 0x3d4d90>'
>
>  >>> oldstyle().__format__('')
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> AttributeError: oldstyle instance has no attribute '__format__'
>
>  >>>
>
> So my question is, to what do I need to add __format__ so that classic
> classes will have a default implementation?
>
> My knowledge of how classic classes are implemented is weak, so I don't
> know where to add this.

Ah, I see.

There is no root class of the classic class hierarchy (that's why
we're nixing it in 3.0 :-).

The customary pattern, used everywhere from len() to setattr(), is to
first check for an (instance) attribute with a special name
(__format__), and if that isn't found, to use a hard-coded fallback
implementation. For len() the fallback raises an exception; for
setattr() it puts the value in the instance __dict__.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)