More advanced output formatting (fallbacks, etc)

[Mattwmaster58]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2019.11.22
• I've searched the bugtracker for similar feature requests including closed ones

Description

I think that it would be helpful to allow more flexible output file formatting. Currently, the only way to format the output is with Python %-string interpolation. I think if we implemented formatting with Python f-strings it would allow for more advanced output formatting operations. For example:

Playlist 1:

• video 1: {'title': 'video1', 'track': 'NA'}
• video 2: {'title': 'video2', 'track': 'Video #2'}

Suppose you wanted to download the videos in the playlist in the format %(track)s.%(ext)s, and fallback to %(title)s.%(ext)s if track is NA. With the current system, it's not possible to do this. With f-strings you do something like this:

{t[track] if t[track] != 'NA' else t[title]}.{t[ext]}

Admittedly, it's quite a bit more verbose, but allows for more flexible output formatting. I think such a big change to youtube-dl isn't ideal, especially since in most cases it will only make output filename formatting will become more closely, so I think it would be best for this feature to exist in parallel with the current implementation.

I realize that f-strings are only supported in Python 3.6+, so I'm not sure how/if we would be able to implement this, or if there's a better method of doing this altogether.

[Mattwmaster58]

@dstftw Would you mind pointing me to the relevant issue? I search a few different variations of the title but couldn't find anything related.