Extract video and audio using the python package

[revolter]

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Description of your issue, suggested solution and other information

How can I get an URL to the best video and audio format using the python package? I am currently using:

import youtube_dl

input_link = 'https://www.youtube.com/watch?v=rvWf62D1jw0'

with youtube_dl.YoutubeDL() as yt_dl:
    video_info = yt_dl.extract_info(input_link, download=False)

1. I can't find documentation for the returned dictionary, so, for some URLs, there is a top level url key returned, for some, there isn't.
2. What is the difference between formats and requested_formats?
3. How could I know if the URL from a format in one of the mentioned lists contains video and audio?
4. As, for the example link, some only contain video while some only contain audio. If I request them to get merged, how will the file be returned? I would like to avoid writing it to disk at all, even in a temporary file.

[dstftw]

1. https://github.com/rg3/youtube-dl#embedding-youtube-dl
2. formats contains all formats, requested_formats - formats selected with format selector.
3. By not having acodec or vcodec set to 'none'.
4. File will not be returned.