You must be running an older version of Python. This does work in Python 3.1.1+:
$ python3
Python 3.1.1+ (r311:74480, Nov 2 2009, 14:49:22)
[GCC 4.4.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> '{0} is not'.format('That that is not')
'That that is not is not'
You will, however, get this error in Python 2.5.4:
$ python2.5
Python 2.5.4 (r254:67916, Jan 20 2010, 21:44:03)
[GCC 4.4.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> '{0} is not'.format('That that is not')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'format'
This feature seems to have been backported to Python 2.6, so you won't get this error there. You must be running Python < 2.6.