[issue17742] Add _PyBytesWriter API

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Could you please provide the benchmarks results? I am afraid that it may hit a performance. Results on Windows are especially interesting.

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components: +Interpreter Core
stage:  -> patch review
type:  -> enhancement

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17742>
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