Command line credentials are applied for all the extractors in extraction process

[dstftw]

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• I've verified and I assure that I'm running youtube-dl 2016.04.19
• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• 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)
• Question
• Other

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[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'http://vk.com/video356065542_456239041', u'-p', u'PRIVATE', u'-u', u'PRIVATE', u'-v']
[debug] Encodings: locale cp1251, fs mbcs, out cp866, pref cp1251
[debug] youtube-dl version 2016.04.19
[debug] Git HEAD: 494ab6d
[debug] Python version 2.6.6 - Windows-2003Server-5.2.3790-SP2
[debug] exe versions: ffmpeg 3.0, ffprobe N-77883-gd7c75a5, rtmpdump 2.4
[debug] Proxy map: {}
[vk] Downloading login page
[vk] Logging in as <snip>
[vk] 356065542_456239041: Downloading webpage
[youtube] Downloading login page
[youtube] Logging in
ERROR: Unable to login: The email and password you entered don't match.
Traceback (most recent call last):
  File "C:\Dev\git\youtube-dl\master\youtube_dl\YoutubeDL.py", line 671, in extract_info
    ie_result = ie.extract(url)
  File "C:\Dev\git\youtube-dl\master\youtube_dl\extractor\common.py", line 340, in extract
    self.initialize()
  File "C:\Dev\git\youtube-dl\master\youtube_dl\extractor\common.py", line 334, in initialize
    self._real_initialize()
  File "C:\Dev\git\youtube-dl\master\youtube_dl\extractor\youtube.py", line 187, in _real_initialize
    if not self._login():
  File "C:\Dev\git\youtube-dl\master\youtube_dl\extractor\youtube.py", line 132, in _login
    raise ExtractorError('Unable to login: %s' % error_msg, expected=True)
ExtractorError: Unable to login: The email and password you entered don't match.

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Account credentials when specified via command line are used for all the extractors involved in extraction process. For example in aforementioned log VK authentication successfully happens first, then YouTube embed is detected and extraction process is delegated to youtube extractor that tries to login with the same credentials obviously resulting in expected error.
In general authentication error can be worked around with .netrc authentication.
Since it's not feasible to add exclusive --username/--password options for each extractor that supports authentication, command line credentials probably should not be considered for any consequent extractor involved apart from the first one.