I'm having some trouble with flake8 when it's invoked from another process, such as in vim. Sometimes its output goes missing - I just get back an empty string. The problem vanishes if I PYTHONUNBUFFERED=1, so I suspect it is related to how flake8 creates multiple processes. I looked through the code and could see that it creates multiprocessing sub-processes with daemon=True. Since these sub-processes perform output on stdout, I believe they need to flush it before telling the main thread that they are done, otherwise there is a risk their output will be lost entirely if they are terminated when the main process exits. I find my problem goes away if I add "sys.stdout.flush()" to the finally block in reporter.py's process_main(). I think this is a good thing to do for the reason I described above. However, I'm not *entirely* sure I understand what's going on. Every single time I run flake8 from the command-line, all its output appears as normal. So while there's a theoretical possibility that output buffers might not be flushed, that doesn't actually seem to be what's happening here. My suspicion was that the parent process was exiting first, then the calling process (vim using the system() function) saw it had exited and stopped reading its output, then the child process flushed its output, but it was too late. That would explain why output appears on the command-line, but is sometimes missed by vim. But I can't actually figure out a way for this to happen. The parent process should always terminate its daemon children as part of its exit handler. So as far as I can tell they should all be dead by the time it exits, and either they have flushed or their output is lost. I posted about my problems on the Syntastic google group. You can find a bit more detail about my original problem and what I tried there: https://groups.google.com/forum/#!topic/vim-syntastic/Pdnp43Ok-3Y Does anyone better understand what might be going on here? Regards, Weeble.