After some searching, the following command works to split my input video into shots, and save the first frame from each shot.

ffmpeg -i input_vid.mp4 -filter:v "select='gt(scene,0.1)',showinfo" -vsync 0 frames/%05d.jpg

When run from the terminal, it prints a line to what I think is stderr for each of the extracted frames, like so

Parsed_showinfo_1 @ 0x7f0198003540] n: 884 pts: 132552 pts_time:2209.2  duration: 2 duration_time:0.0333333 fmt:yuv420p cl:left sar:0/1 s:480x360 i:P iskey:0 type:P checksum:F2D8FCAA plane_checksum:[2D2F32C1 6E0164EC 832064FD] mean:[16 128 128] stdev:[0.0 0.0 0.0]

I want to run this from Python, and also capture the output so I can get the timestamps of the extracted frames (e.g. pts_time:2209.2 in the example above). But when try it in a subprocess.run, I get the following error, and no files are written to frames/.

>> FFMPEG_PATH = imageio_ffmpeg.get_ffmpeg_exe()
>> x = subprocess.run([FFMEPG_PATH, "-i", "input_vid.mp4", '-filter:v ' "select=" "'gt(scene,0.1)'" ",showinfo", "-vsync", "0", r"frames/%05d.jpg"], capture_output=True)
>> print(x.stderr.decode())
  ffmpeg version 4.2.2-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 8 (Debian 8.3.0-6)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libdav1d --enable-libxvid --enable-libzvbi --enable-libzimg
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input_vid.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2013-06-21T09:09:58.000000Z
  Duration: 00:37:10.88, start: 0.000000, bitrate: 394 kb/s
    Stream #0:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 480x360, 296 kb/s, 30 fps, 30 tbr, 60 tbn, 60 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 96 kb/s (default)
    Metadata:
      creation_time   : 2013-06-21T09:10:07.000000Z
      handler_name    : IsoMedia File Produced by Google, 5-11-2011
[NULL @ 0x58e6e80] Unable to find a suitable output format for '0'
0: Invalid argument

If I remove the filter argument, it does extract frames, but there are too many, it’s important to be able to specify a threshold gt(scene,0.1). It also doesn’t capture any output that tells me the timestamps:

>> x = subprocess.run([FFMEPG_PATH, "-i", "input_vid.mp4", "-vsync", "0", "frames2/%05d.jpg"], capture_output=True)
>> print(x.stderr.decode())
ffmpeg version 4.2.2-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 8 (Debian 8.3.0-6)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libdav1d --enable-libxvid --enable-libzvbi --enable-libzimg
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input_vid.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2013-06-21T09:09:58.000000Z
  Duration: 00:37:10.88, start: 0.000000, bitrate: 394 kb/s
    Stream #0:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 480x360, 296 kb/s, 30 fps, 30 tbr, 60 tbn, 60 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 96 kb/s (default)
    Metadata:
      creation_time   : 2013-06-21T09:10:07.000000Z
      handler_name    : IsoMedia File Produced by Google, 5-11-2011
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> mjpeg (native))
Press [q] to stop, [?] for help
[swscaler @ 0x6af2200] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to 'frames2/%05d.jpg':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    encoder         : Lavf58.29.100
    Stream #0:0(und): Video: mjpeg, yuvj420p(pc), 480x360, q=2-31, 200 kb/s, 30 fps, 30 tbn, 30 tbc (default)
    Metadata:
      handler_name    : VideoHandler
      encoder         : Lavc58.54.100 mjpeg
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=66922 fps=1571 q=24.8 Lsize=N/A time=00:37:10.73 bitrate=N/A speed=52.4x    
video:310586kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

>> print(x.stdout.decode())

It seems like there are two problems: something to do with the filter argument, and capturing the output that contains the timestamps. I’ve tried various ways of writing the nested string quotes in the filter argument: triple quotes, escaping, changing the order of double and single quotes, and concatenating separate strings like shown above.