output template support for %(channel_id)s

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The output template should have a way to reference the YouTube channel id, i.e.

%(channel_id)s

Currently, %(uploader_id)s falls back to rendering the channel id if the uploader doesn’t have a name, but there’s no way to force this behavior if the uploader does have a name.

Note: If this feature is already available somewhere in youtube-dl, then I could not find it in the documentation, so documentation should be added or improved.

[glenn-slayden]

Here is the complete code which fixes this issue if someone would like to propagate it to the master. All changes in ‘extractor/youtube.py’. Line numbers are based on youtube-dl-master as of 12/23/2016, but should be obvious from context. The two changes (listed in reverse order in order to keep original line numbers valid) are as follows:

extractor/youtube.dl, INSERT at line 1716: ________‘channel_id’: video_channel_id,

extractor/youtube.dl, INSERT at line 1413: ________# channel_id ________mobj = re.search(r’itemprop=“channelId” content=“(UC[-_A-Za-z0-9]{21}[AQgw])”', video_webpage) ________if mobj is not None: ____________video_channel_id = mobj.group(1) ________else: ____________self._downloader.report_warning(‘unable to extract channelId’) ____________video_channel_id = None

With this change, you can specify %(channel_id)s in your output template, and you will consistently get the “UC…” identifier for both single item and playlist-oriented downloads.

[glenn-slayden]

It’s useful for downloaded videos because you can use the output template to specify folder names in the full path leading to the target filename.

You say that having the name “makes organization a lot easier,” but as I noted above, I find exactly the opposite to be true for my application. When the downloaded files are managed by a file management program, it is better to have a unique folder identifier which has a well-understood fixed-width format, and is composed from a very limited ASCII character set (many channel names use e.g. Japanese, combining Unicode characters, etc.)