Found on 2.6.2 and 2.6.4:

I expect that printing an uninitialized variable from a defaultdict should work.  In fact it does with old-style string formatting.  However, when you try to do it with new-style string formatting, it raises a KeyError.

>>> import collections
>>> d = collections.defaultdict(int)
>>> "{foo}".format(d)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'foo'
>>> "%(foo)d" % d
'0'

str.format() does not take a mapping object. You want:

>>> "{0[foo]}".format(d)
'0'

Oops, thanks.  I should have known that.  However, should this work?  This is what initially led me to file this ticket.  My initial example was a bad one.

>>> from collections import defaultdict
>>> d = defaultdict(int)
>>> d['bar'] += 1
>>> "{bar}".format(**d)
'1'
>>> "{foo}".format(**d)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'foo'

No, because that's not the behavior of keyword arguments.

See also issue 6081.