Making string-formatting smarter by handling generators?

Gabriel Genellina gagsl-py2 at
Thu Feb 28 03:27:35 CET 2008

En Wed, 27 Feb 2008 20:40:04 -0200, Tim Chase  
<python.list at> escribió:

>>> Note that your problem has nothing to do with map itself.
>>> String interpolation using % requires either many individual
>>> arguments, or a single *tuple* argument. A list is printed
>>> as itself.
> Just as an exercise to understand this better, I've been trying
> to figure out what allows for this interpolation.  It doesn't
> seem to be via duck-typing:
>    >>> # what does a tuple have that a list doesn't
>    >>> tuple_set = set(dir(tuple()))
>    >>> list_set = set(dir(list()))
>    >>> tuple_set - list_set
>    set(['__getnewargs__'])
>    class LikeATuple(list):
>      def __getnewargs__(self, *args, **kwargs):
>        pass # now have everything dir(tuple()) has
>    f = LikeATuple((1,2,3))
>    print "%s, %s, %s" % f
> However, if I create a class that derives from a tuple(), it
> seems to work fine:
>    class MyTuple(tuple):
>      pass
>    f = MyTuple((1,2,3))
>    print "%i, %i, %i" % f
> it works fine.  Is there some secret attribute that I'm missing?

Yes: the class itself :)
The code checks whether the right argument is a tuple (or subclass of), or  
a generic mapping. Checking *only* for tuples allows one to usually write  
"%s" % thing, and only bother to write "%s" % (thing,) when such `thing`  
could actually be a tuple.
Mappings don't have that problem, because the left operand must be written  
in a different form so Python knows when to treat the right operand as a  
See my previous response to Steven D'Aprano.

Gabriel Genellina

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