I just added a new "unicode_writer" API for the issue #14687 (Optimize str%tuple for the PEP 393) which helps to make "str%tuple" between 25% and 30% faster.

Attached patch replaces PyAccu API with the unicode_writer API for str.format().

Python 3.2:

1000000 loops, best of 3: 0.198 usec per loop
100000 loops, best of 3: 11.3 usec per loop
10000000 loops, best of 3: 0.167 usec per loop
1000000 loops, best of 3: 0.494 usec per loop

Python 3.3:

1000000 loops, best of 3: 0.293 usec per loop
10000 loops, best of 3: 20.2 usec per loop
1000000 loops, best of 3: 0.219 usec per loop
1000000 loops, best of 3: 0.909 usec per loop

Python 3.3 + patch (speed up of the patch):

1000000 loops, best of 3: 0.226 usec per loop (-22%)
100000 loops, best of 3: 14.8 usec per loop (-26%)
1000000 loops, best of 3: 0.219 usec per loop (0%)
1000000 loops, best of 3: 0.658 usec per loop (-27%)

Oh, I forgot the benchmark script:
------------
$ cat ~/bench_format.sh 
./python -m timeit \
    -s 'fmt="{}:"; arg="abc"' \
    'fmt.format(arg)'
./python -m timeit \
    -s 'N=200; L=3; fmt="{}"*N; args=("a"*L,)*N' \
    'fmt.format(*args)'
./python -m timeit \
    -s 's="x=%s, y=%u, z=%x"; args=(123, 456, 789)' \
    's.format(*args)'
./python -m timeit \
    -s 's="The {k1} is {k2} the {k3}."; args={"k1": "x", "k2": "y", "k3": "z"}' \
    's.format(**args)'
------------
(based on the one used for #14687, see msg159822)

New changeset 7be716a47e9d by Victor Stinner in branch 'default':
Close #14716: str.format() now uses the new "unicode writer" API instead of the
http://hg.python.org/cpython/rev/7be716a47e9d

New changeset ab500b297900 by Victor Stinner in branch 'default':
Issue #14716: Change integer overflow check in unicode_writer_prepare()
http://hg.python.org/cpython/rev/ab500b297900

New changeset 01581e8b50f2 by Victor Stinner in branch 'default':
Backout ab500b297900: the check for integer overflow is wrong
http://hg.python.org/cpython/rev/01581e8b50f2