method to intercept string formatting % operations

[Jean-Michel Pichavant]

bradallen wrote:
> Hello,
>
> For container class derived from namedtuple, but which also behaves
> like a dictionary by implementing __getitem__ for non-integer index
> values, is there a special reserved method which allows intercepting %
> string formatting operations? I would like for my container type to
> behave appropriately depending on whether the string formatting
> operation has a string like this :
>
> "whatever %s yadayada" % mycontainer  # needs to act like a tuple
>
> "whatever %(key)s yadayada" % mycontainer  # needs to act like a
> dictionary
>
> I looked through the Python data model docs at <http://docs.python.org/
> reference/datamodel.html#emulating-container-types> but only found
> this:
>
> "If the left operand of a % operator is a string or Unicode object, no
> coercion takes place and the string formatting operation is invoked
> instead."
>
> Thanks!
>
>   
class toto:
    def __getitem__(self, key):
        return 'paf'
    def __str__(self):
        return 'pif'
    def toTuple(self):
        return (1,2,3)


1/ print '%s %s %s' % toto().toTuple()
'1 2 3'
2/ print '%(key)s ' % toto()
'paf'
3/ print '%s' % toto()
'pif'


1/ I don't know how to spare the explicit toTuple conversion, supporting 
tuple() would be tedious
2/ thanks to __getitem__ (duck typing)
3/ thanks to __str__

Anyway why would you want to use the tuple form ? it's beaten in every 
aspect by the dictionary form.


JM

"If you need something call me, I'll tell you how to live without it" 
(Coluche, about politicians)