Add documentation about supported sites (Fixes #4503)

Author: phihag

Makefile

@@ -1,4 +1,4 @@
-all: youtube-dl README.md CONTRIBUTING.md README.txt youtube-dl.1 youtube-dl.bash-completion youtube-dl.zsh youtube-dl.fish
+all: youtube-dl README.md CONTRIBUTING.md README.txt youtube-dl.1 youtube-dl.bash-completion youtube-dl.zsh youtube-dl.fish supportedsites
 
 clean:
 	rm -rf youtube-dl.1.temp.md youtube-dl.1 youtube-dl.bash-completion README.txt MANIFEST build/ dist/ .coverage cover/ youtube-dl.tar.gz youtube-dl.zsh youtube-dl.fish *.dump *.part *.info.json CONTRIBUTING.md.tmp
@@ -50,7 +50,7 @@ offlinetest: codetest
 
 tar: youtube-dl.tar.gz
 
-.PHONY: all clean install test tar bash-completion pypi-files zsh-completion fish-completion ot offlinetest codetest
+.PHONY: all clean install test tar bash-completion pypi-files zsh-completion fish-completion ot offlinetest codetest supportedsites
 
 pypi-files: youtube-dl.bash-completion README.txt youtube-dl.1 youtube-dl.fish
 
@@ -68,6 +68,9 @@ README.md: youtube_dl/*.py youtube_dl/*/*.py
 CONTRIBUTING.md: README.md
 	python devscripts/make_contributing.py README.md CONTRIBUTING.md
 
+supportedsites:
+	python devscripts/make_supportedsites.py docs/supportedsites.md
+
 README.txt: README.md
 	pandoc -f markdown -t plain README.md -o README.txt
 

README.md

@@ -449,6 +449,14 @@ Since June 2012 (#342) youtube-dl is packed as an executable zipfile, simply unz
 
 To run the exe you need to install first the [Microsoft Visual C++ 2008 Redistributable Package](http://www.microsoft.com/en-us/download/details.aspx?id=29).
 
+### How can I detect whether a given URL is supported by youtube-dl?
+
+For one, have a look at the [list of supported sites](docs/supportedsites). Note that it can sometimes happen that the site changes its URL scheme (say, from http://example.com/v/1234567 to http://example.com/v/1234567 ) and youtube-dl reports an URL of a service in that list as unsupported. In that case, simply report a bug.
+
+It is *not* possible to detect whether a URL is supported or not. That's because youtube-dl contains a generic extractor which maches **all** URLs. You may be tempted to disable, exclude, or remove the generic extractor, but the generic extractor not only allows users to extract videos from lots of websites that embed a video from another service, but may also be used to extract video from a service that it's hosting itself. Therefore, we neither recommend nor support disabling, excluding, or removing the generic extractor.
+
+If you want to find out, simply call youtube-dl. If you get no videos back, chances are the URL is either not referring to a video or unsupported. You can find out which by examining the output (if you run youtube-dl on the console) or catching an `UnsupportedError` exception if you run it from a Python program.
+
 # DEVELOPER INSTRUCTIONS
 
 Most users do not need to build youtube-dl and can [download the builds](http://rg3.github.io/youtube-dl/download.html) or get them from their distribution.

devscripts/make_supportedsites.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python
+from __future__ import unicode_literals
+
+import io
+import optparse
+import os
+import sys
+
+
+# Import youtube_dl
+ROOT_DIR = os.path.join(os.path.dirname(__file__), '..')
+sys.path.append(ROOT_DIR)
+import youtube_dl
+
+
+def main():
+    parser = optparse.OptionParser(usage='%prog OUTFILE.md')
+    options, args = parser.parse_args()
+    if len(args) != 1:
+        parser.error('Expected an output filename')
+
+    outfile, = args
+
+    def gen_ies_md(ies):
+        for ie in ies:
+            ie_md = '**{}**'.format(ie.IE_NAME)
+            ie_desc = getattr(ie, 'IE_DESC', None)
+            if ie_desc is False:
+                continue
+            if ie_desc is not None:
+                ie_md += ': {}'.format(ie.IE_DESC)
+            if not ie.working():
+                ie_md += ' (Currently broken)'
+            yield ie_md
+
+    ies = sorted(youtube_dl.gen_extractors(), key=lambda i: i.IE_NAME.lower())
+    out = '# Supported sites\n' + ''.join(
+        ' - ' + md + '\n'
+        for md in gen_ies_md(ies))
+
+    with io.open(outfile, 'w', encoding='utf-8') as outf:
+        outf.write(out)
+
+if __name__ == '__main__':
+    main()