youtube-dl python module fails with external downloader set.

[fluver]

• 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
• 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

---

>>> import youtube_dl

>>> ydl = youtube_dl.YoutubeDL({'outtmpl': '/tmp/%(id)s', 'quiet': False, 'verbose': True,'restrictfilenames': True, 'ignoreerrors': True, 'external_downloader': 'axel', 'external_downloader_args': "-n50"}) #dosnt work
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2016.04.19
[debug] Python version 3.4.2 - Linux-3.16.0-4-amd64-x86_64-with-debian-8.4
[debug] exe versions: none
[debug] Proxy map: {}
>>> with ydl:
...     result = ydl.extract_info(
...         'https://www.youtube.com/watch?v=pV8PQ9NH2zo',
...         download=True 
...     )
...
[youtube] pV8PQ9NH2zo: Downloading webpage
[youtube] pV8PQ9NH2zo: Downloading video info webpage
[youtube] pV8PQ9NH2zo: Extracting video information
[youtube] pV8PQ9NH2zo: Downloading MPD manifest
[debug] Invoking downloader on 'https://r11---sn-25ge7n7d.googlevideo.com/videoplayback?ratebypass=yes&itag=22&cnr=14&mime=video%2Fmp4&initcwndbps=7122500&ipbits=0&mt=1461342844&fexp=9407190%2C9408214%2C9416126%2C9416891%2C9419452%2C9420095%2C9422596%2C9424134%2C9426926%2C9428398%2C9431012%2C9432683%2C9433096%2C9433118%2C9433191%2C9433626%2C9433947%2C9434075%2C9434769%2C9435039&key=yt6&upn=OeuoyuiF4Wo&expire=1461364559&sparams=cnr%2Cdur%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cnh%2Cpl%2Cratebypass%2Crequiressl%2Csource%2Cupn%2Cusequic%2Cexpire&ms=au&source=youtube&mv=m&dur=196.673&pl=24&ip=62.210.81.222&lmt=1461338298247924&usequic=no&id=o-ADfemKiqADhwAwl1nR5IZTpJJkmnrRC-T01qiOTwEYkY&mm=31&mn=sn-25ge7n7d&nh=IgpwcjAxLnBhcjAxKgkxMjcuMC4wLjE&sver=3&signature=0F7E86DA0A4D3D86ACC997BCFF26A483824130DE.172268C46CA7F00291FB219664A60A56597BD88B&requiressl=yes'
[download] Destination: /tmp/pV8PQ9NH2zo
ERROR:
Traceback (most recent call last):
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/YoutubeDL.py", line 682, in extract_info
    return self.process_ie_result(ie_result, download, extra_info)
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/YoutubeDL.py", line 727, in process_ie_result
    return self.process_video_result(ie_result, download=download)
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/YoutubeDL.py", line 1376, in process_video_result
    self.process_info(new_info)
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/YoutubeDL.py", line 1638, in process_info
    success = dl(filename, info_dict)
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/YoutubeDL.py", line 1580, in dl
    return fd.download(name, info)
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/downloader/common.py", line 350, in download
    return self.real_download(filename, info_dict)
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/downloader/external.py", line 27, in real_download
    retval = self._call_downloader(tmpfilename, info_dict)
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/downloader/external.py", line 79, in _call_downloader
    cmd = [encodeArgument(a) for a in self._make_cmd(tmpfilename, info_dict)]
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/downloader/external.py", line 113, in _make_cmd
    cmd += self._configuration_args()
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/downloader/external.py", line 75, in _configuration_args
    return cli_configuration_args(self.params, 'external_downloader_args', default)
  File "/usr/local/lib/python3.4/dist-packages/youtube_dl/utils.py", line 2209, in cli_configuration_args
    assert isinstance(ex_args, list)
AssertionError

Description of your issue, suggested solution and other information

When using youtube-dl as a python module with an external downloader download fails.
OS: Debian Jessie(up-to date)

[yan12125]

It should be a list. FYI, the following codes show how youtube-dl transform command line arguments into a list:

    external_downloader_args = None
    if opts.external_downloader_args:
        external_downloader_args = compat_shlex_split(opts.external_downloader_args)

From: https://github.com/rg3/youtube-dl/blob/52bb437/youtube_dl/__init__.py#L269-L271