[issue14744] Use _PyUnicodeWriter API in str.format() internals

[Serhiy Storchaka]

Serhiy Storchaka <storchaka at gmail.com> added the comment:

Not quite honest contrexample:

./python -m timeit -s "f='[{}]'.format;s='A'*100" "f(s)"

Python 3.3: 1000000 loops, best of 3: 1.67 usec per loop
Python 3.3 + dont_overallocate.patch: 100000 loops, best of 3: 2.01 usec per loop

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<http://bugs.python.org/issue14744>
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