[Python-ideas] Hooks into the IO system to intercept raw file reads/writes (original) (raw)

Paul Moore p.f.moore at gmail.com
Mon Feb 2 15:53:36 CET 2015

There's a lot of flexibility in the new layered IO system, but one thing it doesn't allow is any means of adding "hooks" to the data flow, or manipulation of an already-created io object.

For example, when a subprocess.Popen object uses a pipe for the child's stdout, the data is captured instead of writing it to the console. Sometimes it would be nice to capture it, but still write to the console. That would be easy to do if we could wrap the underlying RawIOBase object and intercept read() calls[1]. A subclass of RawIOBase can do this trivially, but there's no way of replacing the class on an existing stream.

The obvious approach would be to reassign the "raw" attribute of the BufferedIOBase object, but that's readonly. Would it be possible to make it read/write? Or provide another way of replacing the raw IO object underlying an io object?

I'm sure there are buffer integrity issues to work out, but are there any more fundamental problems with this approach?

Paul

[1] Actually, it's not that easy, because subprocess.Popen objects are insanely hard to subclass - there are no hooks into the pipe creation process, and no way to intercept the object before the subprocess gets run (that happens in the init method). But that's a separate issue, and also the subject of a different thread here.

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