string interpolation mystery in Python 2.6

[Alan G Isaac]

On 9/11/2009 9:42 PM, Steven D'Aprano wrote:
> However, I must admit I'm perplexed why the original example is calling
> __unicode__() in the first place! Given the line:
>
> raise self.severe('Problems with "%s" directive path:\n%s: %s.'
>      % (self.name, error.__class__.__name__, error))
>
> it looks to me like it should be calling error.__str__() not
> error.__unicode(). Making the suggested edit:
>
> raise self.severe('Problems with "%s" directive path:\n%s: %s.'
>      % (self.name, error.__class__.__name__, str(error)))
>
> should have no effect. But it (apparently) does. This brings us back to
> Alan's original question:
>
> "MYSTERY: how can "%s"%error be different from "%s"%str(error) in Python
> 2.6?"


George Brandl explained it to me this way:

         It's probably best explained with a bit of code:

         >>> >>> class C(object):
         ...  def __str__(self): return '[str]'
         ...  def __unicode__(self): return '[unicode]'
         ...
         >>> "%s %s" % ('foo', C())
         'foo [str]'
         >>> "%s %s" % (u'foo', C())
         u'foo [unicode]'
         
         I.e., as soon as a Unicode element is interpolated into a string, further
         interpolations automatically request Unicode via __unicode__, if it exists.

Pretty subtle ...

Cheers,
Alan Isaac