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Update FileIO comments, documentation to match implementation#129011
@cmaloney

Description

Documentation

Discuss thread: https://discuss.python.org/t/change-open-write-to-guarantee-all-bytes-will-be-written-or-an-exception-will-be-thrown/71082

  1. FileIO behaves differently than its code comments around read and readall. Update the code comments to match current behavior
    • PEP 475, reads are retried in some cases
    • .readall() makes multiple system calls by design
  2. Add documentation to open() builtin that buffering=0, which currently just says "disables buffering") changes how .write() behaves and may result in data loss as a result of a partial write (ex. Corrupt .pyc files stay on disk after failed writes #126606).
    • TextIO and BufferedIO (which are gotten via commonly used open('README.rst'), open('README.rst', 'rb')) retry partial writes providing a user a guarantee either all bytes will be written or an exception will be thrown.
    • FileIO is "Raw I/O" and by PEP-3116 design, as documented, and currently implemented does not retry partial writes.
    • Most the time, buffering=0 currently speeds up writing a file, but it can also result in corrupted files, ex. Corrupt .pyc files stay on disk after failed writes #126606 from using FileIO directly
    • I would like to try and change the behavior of buffering=0 to use BufferedIO but with a 0 sized buffer, and that is on my roadmap but will be a while (people use the flag for a reason / get benefit!). This will mean open() always returns an object which implements "Write all or throw exception" behavior.
    • Add warning / document existing behavior on open() in the meantime.

Linked PRs