bench_bytes_format.py: micro-benchmark testing a few formats. Some tests are focused on the implementation of _PyBytesWriter to ensure that the optimization is efficient.

Except of a single test (which is not really revelant, it takes less than 500 nanoseconds), all tests are faster.

The b"xxxxxx %s" % b"y" test confirms that the optimization disabling overallocation for the last write is effective.

Results:

Common platform:
Timer info: namespace(adjustable=False, implementation='clock_gettime(CLOCK_MONOTONIC)', monotonic=True, resolution=1e-09)
Python unicode implementation: PEP 393
CPU model: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Platform: Linux-4.1.6-200.fc22.x86_64-x86_64-with-fedora-22-Twenty_Two
CFLAGS: -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes
Timer: time.perf_counter
Bits: int=32, long=64, long long=64, size_t=64, void*=64

Platform of campaign orig:
SCM: hg revision=1aae9b6a6929 tag=tip branch=default date="2015-10-09 01:34 -0400"
Timer precision: 64 ns
Python version: 3.6.0a0 (default:1aae9b6a6929, Oct 9 2015, 11:33:56) [GCC 5.1.1 20150618 (Red Hat 5.1.1-4)]
Date: 2015-10-09 11:34:11

Platform of campaign writer:
SCM: hg revision=fc2c11a19ae1+ tag=tip branch=default date="2015-10-09 11:48 +0200"
Timer precision: 61 ns
Python version: 3.6.0a0 (default:fc2c11a19ae1+, Oct 9 2015, 12:16:16) [GCC 5.1.1 20150618 (Red Hat 5.1.1-4)]
Date: 2015-10-09 12:16:31

---------------------------+------------+--------------
use smaller buffer         |       orig |        writer
---------------------------+------------+--------------
b"hello %s" % b"world"     |  13 ns (*) |   12 ns (-5%)
b"hello %-100s" % b"world" | 158 ns (*) |  98 ns (-38%)
b"x=%d" % 123              |  13 ns (*) |         12 ns
b"x=%f" % 1.2              |  13 ns (*) |         13 ns
b"x=%100d" % 123           | 156 ns (*) |  166 ns (+7%)
---------------------------+------------+--------------
Total                      | 353 ns (*) | 301 ns (-15%)
---------------------------+------------+--------------

-------------------------------------------------+-------------+---------------
"hello %s" % long_string                         |        orig |         writer
-------------------------------------------------+-------------+---------------
fmt = b"hello %s"; arg = b"x" * 10; fmt % arg    |   98 ns (*) |   86 ns (-12%)
fmt = b"hello %s"; arg = b"x" * 100; fmt % arg   |   85 ns (*) |          87 ns
fmt = b"hello %s"; arg = b"x" * 10**3; fmt % arg |  298 ns (*) |  208 ns (-30%)
fmt = b"hello %s"; arg = b"x" * 10**5; fmt % arg |  4.8 us (*) |  4.39 us (-9%)
-------------------------------------------------+-------------+---------------
Total                                            | 5.28 us (*) | 4.77 us (-10%)
-------------------------------------------------+-------------+---------------

---------------------------------------+-------------+---------------
b"xxxxxx %s" % b"y"                    |        orig |         writer
---------------------------------------+-------------+---------------
fmt = b"x" * 10 + b"%s"; fmt % b"y"    |   99 ns (*) |   81 ns (-18%)
fmt = b"x" * 100 + b"%s"; fmt % b"y"   |  189 ns (*) |   87 ns (-54%)
fmt = b"x" * 10**3 + b"%s"; fmt % b"y" | 1.12 us (*) |  209 ns (-81%)
fmt = b"x" * 10**5 + b"%s"; fmt % b"y" | 88.4 us (*) | 8.49 us (-90%)
---------------------------------------+-------------+---------------
Total                                  | 89.8 us (*) | 8.87 us (-90%)
---------------------------------------+-------------+---------------

----------------------------------------------------------+-------------+---------------
%f                                                        |        orig |         writer
----------------------------------------------------------+-------------+---------------
n = 200; fmt = b"%f" * n; arg = tuple([1.2]*n); fmt % arg | 37.2 us (*) | 29.6 us (-21%)
----------------------------------------------------------+-------------+---------------

------------------------------------------------------------+-------------+---------------
%i                                                          |        orig |         writer
------------------------------------------------------------+-------------+---------------
n = 200; fmt = b"%f" * n; arg = tuple([12345]*n); fmt % arg | 49.4 us (*) | 42.8 us (-13%)
------------------------------------------------------------+-------------+---------------

-------------------------+-------------+---------------
Summary                  |        orig |         writer
-------------------------+-------------+---------------
use smaller buffer       |  353 ns (*) |  301 ns (-15%)
"hello %s" % long_string | 5.28 us (*) | 4.77 us (-10%)
b"xxxxxx %s" % b"y"      | 89.8 us (*) | 8.87 us (-90%)
%f                       | 37.2 us (*) | 29.6 us (-21%)
%i                       | 49.4 us (*) | 42.8 us (-13%)
-------------------------+-------------+---------------
Total                    |  182 us (*) | 86.3 us (-53%)
-------------------------+-------------+---------------