outputformat string is not respected properly when the final file required merging

[atticus-sullivan]

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2020.09.20
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

$ youtube-dl --version
2020.09.20
$ youtube-dl -v -i -f bestvideo+bestaudio --merge-output-format mkv --restrict-filenames -o '%(uploader)s-_-%(title)s.tmp' "https://www.youtube.com/watch?v=Zn4oSL0_8ho"
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', '-i', '-f', 'bestvideo+bestaudio', '--merge-output-format', 'mkv', '--restrict-filenames', '-o', '%(uploader)s-_-%(title)s.tmp', 'https://www.youtube.com/watch?v=Zn4oSL0_8ho']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, pref UTF-8
[debug] youtube-dl version 2020.09.20
[debug] Python version 3.8.5 (CPython) - Linux-5.4.66-1-lts-x86_64-with-glibc2.2.5
[debug] exe versions: ffmpeg 4.3.1, ffprobe 4.3.1, rtmpdump 2.4
[debug] Proxy map: {}
[youtube] Zn4oSL0_8ho: Downloading webpage
[youtube] Zn4oSL0_8ho: Downloading MPD manifest
[debug] Invoking downloader on 'https://manifest.googlevideo.com/api/manifest/dash/expire/1600712423/ei/h5poX9P0KI-x1gK75Lv4CA/ip/2003%3Acb%3Af735%3Afc00%3A7c9a%3Ac747%3A5c83%3A5d31/id/667e2848bd3ff21a/source/youtube/requiressl/yes/playback_host/r5---sn-h0jeln7e.googlevideo.com/mh/os/mm/31%2C26/mn/sn-h0jeln7e%2Csn-4g5ednsk/ms/au%2Conr/mv/m/mvi/5/pl/36/hfr/all/as/fmp4_audio_clear%2Cwebm_audio_clear%2Cwebm2_audio_clear%2Cfmp4_sd_hd_clear%2Cwebm2_sd_hd_clear/initcwndbps/1490000/vprv/1/mt/1600690731/fvip/5/keepalive/yes/itag/0/sparams/expire%2Cei%2Cip%2Cid%2Csource%2Crequiressl%2Chfr%2Cas%2Cvprv%2Citag/sig/AOq0QJ8wRQIhALcCzyoRgp0Du0tEDOFuI7ozDums3TEYjg2XLzCoYFNyAiB-Ail0BuGniZW3lLZWgax4ZDLWVaY07B6JU88tZRkZLg%3D%3D/lsparams/playback_host%2Cmh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps/lsig/AG3C_xAwRQIgMdN5mtSMgSSXeZyaKYc-a79c2ByVvHA_xIP9168uYb4CIQDnGUsvcUznVGFPBPARW70Pr_2zEOw1uqnVO5atl5TLQw%3D%3D'
[dashsegments] Total fragments: 40
[download] Destination: M4-_-M4_-_Not_Good_Enough_Official_Video.tmp.f137
[download] 100% of 90.06MiB in 00:37
[debug] Invoking downloader on 'https://manifest.googlevideo.com/api/manifest/dash/expire/1600712423/ei/h5poX9P0KI-x1gK75Lv4CA/ip/2003%3Acb%3Af735%3Afc00%3A7c9a%3Ac747%3A5c83%3A5d31/id/667e2848bd3ff21a/source/youtube/requiressl/yes/playback_host/r5---sn-h0jeln7e.googlevideo.com/mh/os/mm/31%2C26/mn/sn-h0jeln7e%2Csn-4g5ednsk/ms/au%2Conr/mv/m/mvi/5/pl/36/hfr/all/as/fmp4_audio_clear%2Cwebm_audio_clear%2Cwebm2_audio_clear%2Cfmp4_sd_hd_clear%2Cwebm2_sd_hd_clear/initcwndbps/1490000/vprv/1/mt/1600690731/fvip/5/keepalive/yes/itag/0/sparams/expire%2Cei%2Cip%2Cid%2Csource%2Crequiressl%2Chfr%2Cas%2Cvprv%2Citag/sig/AOq0QJ8wRQIhALcCzyoRgp0Du0tEDOFuI7ozDums3TEYjg2XLzCoYFNyAiB-Ail0BuGniZW3lLZWgax4ZDLWVaY07B6JU88tZRkZLg%3D%3D/lsparams/playback_host%2Cmh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps/lsig/AG3C_xAwRQIgMdN5mtSMgSSXeZyaKYc-a79c2ByVvHA_xIP9168uYb4CIQDnGUsvcUznVGFPBPARW70Pr_2zEOw1uqnVO5atl5TLQw%3D%3D'
[dashsegments] Total fragments: 21
[download] Destination: M4-_-M4_-_Not_Good_Enough_Official_Video.tmp.f251
[download] 100% of 2.96MiB in 00:04
[ffmpeg] Merging formats into "M4-_-M4_-_Not_Good_Enough_Official_Video.tmp.mkv"
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i file:M4-_-M4_-_Not_Good_Enough_Official_Video.tmp.f137 -i file:M4-_-M4_-_Not_Good_Enough_Official_Video.tmp.f251 -c copy -map 0:v:0 -map 1:a:0 file:M4-_-M4_-_Not_Good_Enough_Official_Video.tmp.temp.mkv
Deleting original file M4-_-M4_-_Not_Good_Enough_Official_Video.tmp.f137 (pass -k to keep)
Deleting original file M4-_-M4_-_Not_Good_Enough_Official_Video.tmp.f251 (pass -k to keep)

Description

When I download above Video, after merging I get a file named ....tmp.mkv despite the fact that I've used the output-formatstring '%(uploader)s-_-%(title)s.tmp'.
I'd expect that the final outputfile would be named in that specific case "M4-_-M4_-_Not_Good_Enough_Official_Video.tmp".

I consider this a bug, but maybe this behaviour is known and even wanted. In that case I want to mention that this makes scripting around with youtube-dl really hard, since one cannot get the final filenames of the files produced by youtube-dl

In my scripting case it would be enough to have the precise final filename to work with the output file, but when using --get-filename the filename that is outputted, is not the right one under every circumstances. When using something like -f bestvideo+bestaudio, the extension might change due to merging.
In my specific case I'd get
M4-_-M4_-_Not_Good_Enough_Official_Video.mp4 as output of
youtube-dl -i -f bestvideo+bestaudio --restrict-filenames -o '%(uploader)s-_-%(title)s.%(ext)s' --get-filename "https://www.youtube.com/watch?v=Zn4oSL0_8ho"
which is not right, since when downloading the resulting file (with
youtube-dl -i -f bestvideo+bestaudio --restrict-filenames -o '%(uploader)s-_-%(title)s.%(ext)s' "https://www.youtube.com/watch?v=Zn4oSL0_8ho") will be named
"M4-_-M4_-_Not_Good_Enough_Official_Video.mkv")

[dstftw]

This is expected behavior. Extension should not be hardcoded in output template.

[atticus-sullivan]

@dstftw is it then expected behaviour too that the filename outputted with the flag --get-filename isn't the right one? (or should I open a new issue for this?)