How does youtube-dl generate direct link?

Question

I'm wondering how youtube-dl generate direct link to video. I know that with youtube-dl --get-url link I can get this, but I want to know how this process goes. (From downloading html page to getting link). Is there a way to check this out?
Youtube-dl is open-source so I guess it is, but I just don't know where specifically I should look.
Thanks in advance

Answer 1

youtube-dl uses classes called InfoExtractor to make it possible to download videos from different sites. The info extractor for youtube videos is in /youtube_dl/extractor/youtube.py.

This class is quite complex, since it deals with logging in users and different kinds of videos and channels etc. I think that the relevant part is:

url = proto + '://www.youtube.com/watch?v=%s&gl=US&hl=en&has_verified=1&bpctr=9999999999' % video_id

Where the video_id is extracted by a big regex:

_VALID_URL = r"""(?x)^
                 (
                     (?:https?://|//)                                    # http(s):// or protocol-independent URL
                     (?:(?:(?:(?:\w+\.)?[yY][oO][uU][tT][uU][bB][eE](?:-nocookie)?\.com/|
                        (?:www\.)?deturl\.com/www\.youtube\.com/|
                        (?:www\.)?pwnyoutube\.com/|
                        (?:www\.)?yourepeat\.com/|
                        tube\.majestyc\.net/|
                        youtube\.googleapis\.com/)                        # the various hostnames, with wildcard subdomains
                     (?:.*?\#/)?                                          # handle anchor (#/) redirect urls
                     (?:                                                  # the various things that can precede the ID:
                         (?:(?:v|embed|e)/(?!videoseries))                # v/ or embed/ or e/
                         |(?:                                             # or the v= param in all its forms
                             (?:(?:watch|movie)(?:_popup)?(?:\.php)?/?)?  # preceding watch(_popup|.php) or nothing (like /?v=xxxx)
                             (?:\?|\#!?)                                  # the params delimiter ? or # or #!
                             (?:.*?&)?                                    # any other preceding param (like /?s=tuff&v=xxxx)
                             v=
                         )
                     ))
                     |youtu\.be/                                          # just youtu.be/xxxx
                     |(?:www\.)?cleanvideosearch\.com/media/action/yt/watch\?videoId=
                     )
                 )?                                                       # all until now is optional -> you can pass the naked ID
                 ([0-9A-Za-z_-]{11})                                      # here is it! the YouTube video ID
                 (?!.*?&list=)                                            # combined list/video URLs are handled by the playlist IE
                 (?(1).+)?                                                # if we found the ID, everything can follow
                 $"""

Luckily it's commented...