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In Python 3.3, I added _PyUnicodeWriter API to factorize code handling a Unicode "buffer", just the code to allocate memory and resize the buffer if needed.
I propose to do the same with a new _PyBytesWriter API. The API is very similar to _PyUnicodeWriter:
 * _PyBytesWriter_Init(writer)
 * _PyBytesWriter_Prepare(writer, count)
 * _PyBytesWriter_WriteStr(writer, bytes_obj)
 * _PyBytesWriter_WriteChar(writer, ch)
 * _PyBytesWriter_Finish(writer)
 * _PyBytesWriter_Dealloc(writer)
The patch changes ASCII, Latin1, UTF-8 and charmap encoders to use _PyBytesWriter API. A second patch changes CJK encoders.
I did not run a benchmark yet. I wrote a patch to factorize the code, not the make the code faster.
Notes on performances:
 * I peek the "small buffer allocated on the stack" idea from UTF-8 encoder, but the smaller buffer is always 500 bytes (instead of a size depending on the Unicode maximum character of the input Unicode string)
 * _PyBytesWriter overallocates by 25% (when overallocation is enabled), whereas charmap encoders doubles the buffer:
 /* exponentially overallocate to minimize reallocations */
 if (requiredsize < 2*outsize)
 requiredsize = 2*outsize;
 * I didn't check if the allocation size is the same with the patch. min_size and overallocate attributes should be set correctly to not make the code slower.
 * The code writing a single into a _PyUnicodeWriter buffer is inlined in unicodeobject.c. _PyBytesWriter API does not provide inlined function for the same purpose.