String interpolation % operates on unicode strings directly without calling the __str__ method. In Python 2.5 and 2.6:

>>> class K(unicode):
...     def __str__(self): return "Surprise!"
...
>>> u"%s" % K("some text")
u'some text'

but subclasses of str do call __str__:

>>> class K(str):
...     def __str__(self): return "Surprise!"
...
>>> "%s" % K("some text")
'Surprise!'

In Python 3.1, the above example for subclassing str operates like the unicode example, i.e. it fails to call __str__.

The documentation for string interpolation states that str() is called for all Python objects.

http://docs.python.org/library/stdtypes.html#string-formatting-operations

If the behaviour for unicode (2.5/2.6, str in 3.1) is considered correct, then the documentation should be updated to say that unicode is an exception to the rule. Otherwise the behaviour is incorrect, and it should call __str__ the same as everything else.

I've assumed that the documentation is correct, and that "%s"%obj should call __str__ for unicode objects as well as everything else.

Attached in a test file.

In Python 2.x, __unicode__ is called instead of __str__.  Whether that's "correct" behavior, I'm not sure, but at least it is consistent with "{}".format(K()).

In Python 3.x, __unicode__ doesn't exist and __str__ isn't called; but for "{}".format(K()), it is.  Therefore I think here a fix should be made in any case.

Benjamin, what do you think?

I agree with Georg. I think 2.x is arguably correct, and 3.1 is broken.

It looks like this has already been fixed in 3.2. It's not immediately obvious why that is, I'll have to look at the code more than the quick glance I just gave it.

Python 3.1.1+ (release31-maint:77299, Jan  4 2010, 08:27:32) 
[GCC 4.1.2 20070626 (Red Hat 4.1.2-13)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class MyStr(str):
...     def __str__(self): return "Surprise!"
... 
>>> '%s' % MyStr('foo')
'foo'
>>> '{}'.format(MyStr('foo'))
'Surprise!'

Python 3.2a0 (py3k:80525M, Apr 27 2010, 05:19:53) 
[GCC 4.1.2 20070626 (Red Hat 4.1.2-13)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class MyStr(str):
...     def __str__(self): return "Surprise!"
... 
>>> '%s' % MyStr('foo')
'Surprise!'
>>> '{}'.format(MyStr('foo'))
'Surprise!'

It's probably a duplicate of issue #1583863.

Yes, that's the cause, thanks for finding that issue. It's actually fixed in 3.1.2, I just hadn't updated my local copy.

Closing, since there's nothing to fix here. The 2.6 behavior is correct, and the 3.x behavior that was broken has been fixed.