Why is "-f mp4" needed on some systems but not others?

[joeschmoe40]

Observe:

(On "system A", running Linux)
$ youtube-dl.py --version
2015.04.28
$ youtube-dl.py -j 'https://www.youtube.com/watch?v=K5QMjtgWdG0' | jq .url
null
$

Now, on "system B", another Unix-y system, running the same version of youtube-dl, the above command returns a valid/useful url. As far as I can tell, the only material difference between the two systems is that system A is running Python 2.7.3 while system B is running 2.7.2. Yet, it is system A that fails.

Now, I've tracked this down, and figured out that it has to do with video formats and that in the failure mode, the problem is that the "url" field is wrapped inside an array (I think that's the correct JSON term for what I think of as a "structure") and thus the very simple "jq" program doesn't find it. Unfortunately, I don't know enough "jq" to know how to parse into the structures and extract the "url" filed(s) therefrom.

But, I have figured out that if I add the option "-f mp4" to the command line (just before the "-j"), then the problem goes away (i.e. I get a valid URL from jq).

So, my questions are:

1. What could account for the different behavior between the two systems?
   and
2. Is there anything wrong with fixing it by adding in "-f mp4"?

[jaimeMF]

youtube-dl now default to download and merge DASH video and audio if ffmpeg/avconv is installed, so if one of your systems has it it will try to download them. You can extract them from the requested_formats field (jq .requested_formats) or use -f best (see the last paragraph at the formats documentation).

[joeschmoe40]

youtube-dl now default to download and merge DASH video and audio if ffmpeg/avconv is installed, so if one of your systems has it it will try to download them.

Yep. That was it. The system in question has ffmpeg/avconv. Pretty weird behavior, but I got it now.

You can extract them from the requested_formats field (jq .requested_formats) or use -f best (see the last paragraph at the formats documentation).

"-f best" works (and looks better than -f mp4).
But "jq .requested_formats" still returns null for me. Hmmm...

[jaimeMF]

But "jq .requested_formats" still returns null for me. Hmmm...

It only works for systems with ffmpeg (or if you use -f bestvideo+bestaudio). If you are using -f best, you need to run jq .url.

[joeschmoe40]

It only works for systems with ffmpeg (or if you use -f bestvideo+bestaudio). If you are using -f best, you need to run jq .url.

I got that, but I just typo'd. So, no worries.
BTW, the magic bullet is:

$ youtube-dl.py -j 'https://www.youtube.com/watch?v=K5QMjtgWdG0' | jq '.requested_formats[]|.url'
$