How do I get a valid subtitle from a container?

[Timtam]

I need to either entirely demux a subtitle stream from a video container so that I can put it into e.g. an .ass file and it will be valid ass, or transcode a subtitle into a fixed target format like .srt and output that into a valid file.
I thus tried opening a video file and decoding its subtitle stream (ass) and encoding it into a new file with a .srt stream:

import av

cont = av.open("test.mp4")
cont2 = av.open("test.srt", "w")
srtstream = cont2.add_stream("srt")

for f in cont.decode(subtitles=0):
    srtpackets = srtstream.encode(f)

    cont2.mux(srtpackets)
cont2.close()

Expected behavior

I expected calling container.decode() returning frames.

Actual behavior

When decoding a subtitle stream though, its instead returning SubtitleSet objects, which the stream encoder doesn't know to convert them to actual frames.

Traceback:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\scripts\loudspeaker\test.py", line 10, in <module>
    srtpackets = srtstream.encode(f)
  File "av\stream.pyx", line 152, in av.stream.Stream.encode
TypeError: Cannot convert av.subtitles.subtitle.SubtitleSet to av.frame.Frame

Investigation

I tried decoding the subtitle stream itself, like this:

substream = cont.streams.subtitles[1]

frames = substream.decode()

That however crashes the interpreter silently.

I thought about taking the, I suppose, frames, which can be found in SubtitleSet.rects, and simply writing them to a file one by one, but I don't know if that will actually give a correct .ass file in the end.

So is there a way to get either a valid .ass file (demux the ass stream from my container) or, even better, transcode the ass subtitle into a srt one?

[WyattBlue]

Right now, Stream.encode() only works with Frame and not SubtitleSet, even for SubtitleStream specifically. I would like to add this feature in a future version of PyAV.

In the meantime, I don't think it's possible to encode subtitles how you're doing that. One hack would be to manually write an "srt" file, although I don't think PyAV could mux a subtitle either.