So, the patch failed with both my original file and the pastebin one - perhaps due to indentation of Hacker News, so I manually applied since it did seem pretty straightforward - honestly given how short the file was, it would have taken up the same amount of space here as the diff I think, but hopefully I applied it correctly. Manual copy/paste in python always worries me w/ the significant white-space thing (one of our friends accidentally DoS'd our server with his first python script due to that, but that was back when mixing tabs and spaces didn't throw an error by default), but I probably did it right.

And with that out of the way. This one seems to mostly work!

So python3 did not significantly change handling this sort of byte stream and while Mercurial folks might well have had their own woes, I have no idea what the issues were in all those prior attempts with this file.

... that said, it does do one odd thing (following is output on launching):

    /usr/lib/python3.12/subprocess.py:1016: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
        self.stdout = io.open(c2pread, 'rb', bufsize)

And yet, I can't spot any issues in gameplay, yet, caused by this, so I'm inclined to let it pass? But, it does make me wonder if later on, I might hit issues...

At least for now, I'm going to tentatively say it seems fine. Hm. You know what. Let me try with some more obvious things that might fail if the buffer size is wrong.

So. Now I'm wondering why, given how relatively minor this change is (aside from the odd error message, and the typical python3 changes just one slightly modified line and one inserted line), why did so many pythonistas have so much difficulty over the many years I asked about this? I mean, I only formed my opinion that maybe there was a problem with python3 byte/string handling to just how many attempts there were... Were they trying to do things in a more idiomatic python3 fashion? Did the python3 APIs change? Does the error hint at something more concerning? Well, whatever. Clearly it's (mostly) fine now. And my carefully tweaked nethack profile is safe if python2 is removed without needing to make my own stream filter. Yay! Thanks!

... further updates.. ok there are a few issues.

1) the warning

2) there's an odd ghost flicker that jumps around the nethack level as if a cursor is appearing - does not happen in the python2 one.

3) on quitting it no longer it no longer exits gracefully and I have to ctrl-c the script.

4) It is much slower to render. the python2 one draws a screen almost instantly for most uses (although still a bit slower than not filtered, at least on this computer, for things that change a lot, like video). This one ripples down - that might explain the ghost flickering in ② and might be related to the buffer warning. This becomes much more noticeable with BBSes although it is usually fine in nethack. You can see the difference on a simpler testcase without setting up a BBS account by streaming a bit more data at once say by running: ibmfilter curl ascii.live/nyan

So, clearly not perfect but.. eh. functional? Still far better than prior attempts, and at least it mostly works with nethack.