Double underscores in the value of %(uploader_id)s are truncated

[Ectomind1990]

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2017.02.28. If it's not read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2017.02.28

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• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

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• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
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If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output as follows:

Add -v flag to your command line you run youtube-dl with, copy the whole output and insert it here. It should look similar to one below (replace it with your log inserted between triple ```):

$ youtube-dl -v --skip-download -o '%(uploader_id)s/%(id)s.%(ext)s' https://youtu.be/nmeZVeXo7O4
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'--skip-download', u'-o', u'%(uploader_id)s/%(id)s.%(ext)s', u'https://youtu.be/nmeZVeXo7O4']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.02.28
[debug] Python version 2.7.12 - Linux-4.4.0-64-generic-x86_64-with-Ubuntu-16.04-xenial
[debug] exe versions: ffmpeg 2.8.11-0ubuntu0.16.04.1, ffprobe 2.8.11-0ubuntu0.16.04.1
[debug] Proxy map: {}
[youtube] nmeZVeXo7O4: Downloading webpage
[youtube] nmeZVeXo7O4: Downloading video info webpage
[youtube] nmeZVeXo7O4: Extracting video information
[youtube] {22} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {43} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {18} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {36} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {17} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {137} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {248} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {136} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {247} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {135} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {244} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {134} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {243} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {133} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {242} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {160} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {278} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {140} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {171} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {249} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {250} signature length 40.44, html5 player en_US-vflVlxFvV
[youtube] {251} signature length 40.44, html5 player en_US-vflVlxFvV
...
<end of log>

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Description of your issue, suggested solution and other information

When using Output Templates, if the value of %(uploader_id)s should contain a double underscore, the actual behaviour is as if there is only a single underscore at that position.

This can be seen in the command I gave above. It creates a directory named...
UC_Bdma6rGmzx9EBtSCQNpQ but it should be named...
UC__Bdma6rGmzx9EBtSCQNpQ

YouTube's Channel IDs are always 24 characters long, so clearly the single underscore version is wrong. You can also demonstrate that it is wrong by trying these two URLs:
https://www.youtube.com/channel/UC_Bdma6rGmzx9EBtSCQNpQ
https://www.youtube.com/channel/UC__Bdma6rGmzx9EBtSCQNpQ
The single underscore URL gives the following error: "This channel does not exist."

Additional Info

If you use --write-info-json, the values of the uploader_id and uploader_url fields in the JSON file are not affected by the bug. The value of the _filename field is affected.

It is not just directory creation that is affected by this bug. If you use a different Output Template so that the value of %(uploader_id)s gets included in the filename, you still see the bug.

YouTube's Channel IDs always start with "UC". The channel I've given as an example is UC__Bdma6rGmzx9EBtSCQNpQ - in which the double underscore comes immediately after the "UC". But the bug still happens when the double underscore is found at other positions. E.g. channel UCtk1Yv__Jcsw2NWbmZcCqsA

[glenn-slayden]

I can confirm that this bug also affected the %(channel_id)s feature (see #9676)), and that, if you are using the patch that I posted in the comments section of #9676, then commit 40df485 fixes the double-underscore problem for that case as well.

I guess If I get a chance I should figure out how to do a pull request to properly submit my fix for #9676...