yt-dlc/youtube_dl/YoutubeDL.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from __future__ import absolute_import, unicode_literals

import collections
import datetime
import errno
import io
import json
import locale
import os
import platform
import re
import shutil
import subprocess
import socket
import sys
import time
import traceback

if os.name == 'nt':
    import ctypes

from .compat import (
    compat_cookiejar,
    compat_expanduser,
    compat_http_client,
    compat_str,
    compat_urllib_error,
    compat_urllib_request,
)
from .utils import (
    escape_url,
    ContentTooShortError,
    date_from_str,
    DateRange,
    DEFAULT_OUTTMPL,
    determine_ext,
    DownloadError,
    encodeFilename,
    ExtractorError,
    format_bytes,
    formatSeconds,
    get_term_width,
    locked_file,
    make_HTTPS_handler,
    MaxDownloadsReached,
    PagedList,
    PostProcessingError,
    platform_name,
    preferredencoding,
    SameFileError,
    sanitize_filename,
    subtitles_filename,
    takewhile_inclusive,
    UnavailableVideoError,
    url_basename,
    write_json_file,
    write_string,
    YoutubeDLHandler,
    prepend_extension,
)
from .cache import Cache
from .extractor import get_info_extractor, gen_extractors
from .downloader import get_suitable_downloader
from .downloader.rtmp import rtmpdump_version
from .postprocessor import FFmpegMergerPP, FFmpegPostProcessor
from .version import __version__


class YoutubeDL(object):
    """YoutubeDL class.

    YoutubeDL objects are the ones responsible of downloading the
    actual video file and writing it to disk if the user has requested
    it, among some other tasks. In most cases there should be one per
    program. As, given a video URL, the downloader doesn't know how to
    extract all the needed information, task that InfoExtractors do, it
    has to pass the URL to one of them.

    For this, YoutubeDL objects have a method that allows
    InfoExtractors to be registered in a given order. When it is passed
    a URL, the YoutubeDL object handles it to the first InfoExtractor it
    finds that reports being able to handle it. The InfoExtractor extracts
    all the information about the video or videos the URL refers to, and
    YoutubeDL process the extracted information, possibly using a File
    Downloader to download the video.

    YoutubeDL objects accept a lot of parameters. In order not to saturate
    the object constructor with arguments, it receives a dictionary of
    options instead. These options are available through the params
    attribute for the InfoExtractors to use. The YoutubeDL also
    registers itself as the downloader in charge for the InfoExtractors
    that are added to it, so this is a "mutual registration".

    Available options:

    username:          Username for authentication purposes.
    password:          Password for authentication purposes.
    videopassword:     Password for acces a video.
    usenetrc:          Use netrc for authentication instead.
    verbose:           Print additional info to stdout.
    quiet:             Do not print messages to stdout.
    no_warnings:       Do not print out anything for warnings.
    forceurl:          Force printing final URL.
    forcetitle:        Force printing title.
    forceid:           Force printing ID.
    forcethumbnail:    Force printing thumbnail URL.
    forcedescription:  Force printing description.
    forcefilename:     Force printing final filename.
    forceduration:     Force printing duration.
    forcejson:         Force printing info_dict as JSON.
    dump_single_json:  Force printing the info_dict of the whole playlist
                       (or video) as a single JSON line.
    simulate:          Do not download the video files.
    format:            Video format code.
    format_limit:      Highest quality format to try.
    outtmpl:           Template for output names.
    restrictfilenames: Do not allow "&" and spaces in file names
    ignoreerrors:      Do not stop on download errors.
    nooverwrites:      Prevent overwriting files.
    playliststart:     Playlist item to start at.
    playlistend:       Playlist item to end at.
    matchtitle:        Download only matching titles.
    rejecttitle:       Reject downloads for matching titles.
    logger:            Log messages to a logging.Logger instance.
    logtostderr:       Log messages to stderr instead of stdout.
    writedescription:  Write the video description to a .description file
    writeinfojson:     Write the video description to a .info.json file
    writeannotations:  Write the video annotations to a .annotations.xml file
    writethumbnail:    Write the thumbnail image to a file
    writesubtitles:    Write the video subtitles to a file
    writeautomaticsub: Write the automatic subtitles to a file
    allsubtitles:      Downloads all the subtitles of the video
                       (requires writesubtitles or writeautomaticsub)
    listsubtitles:     Lists all available subtitles for the video
    subtitlesformat:   Subtitle format [srt/sbv/vtt] (default=srt)
    subtitleslangs:    List of languages of the subtitles to download
    keepvideo:         Keep the video file after post-processing
    daterange:         A DateRange object, download only if the upload_date is in the range.
    skip_download:     Skip the actual download of the video file
    cachedir:          Location of the cache files in the filesystem.
                       False to disable filesystem cache.
    noplaylist:        Download single video instead of a playlist if in doubt.
    age_limit:         An integer representing the user's age in years.
                       Unsuitable videos for the given age are skipped.
    min_views:         An integer representing the minimum view count the video
                       must have in order to not be skipped.
                       Videos without view count information are always
                       downloaded. None for no limit.
    max_views:         An integer representing the maximum view count.
                       Videos that are more popular than that are not
                       downloaded.
                       Videos without view count information are always
                       downloaded. None for no limit.
    download_archive:  File name of a file where all downloads are recorded.
                       Videos already present in the file are not downloaded
                       again.
    cookiefile:        File name where cookies should be read from and dumped to.
    nocheckcertificate:Do not verify SSL certificates
    prefer_insecure:   Use HTTP instead of HTTPS to retrieve information.
                       At the moment, this is only supported by YouTube.
    proxy:             URL of the proxy server to use
    socket_timeout:    Time to wait for unresponsive hosts, in seconds
    bidi_workaround:   Work around buggy terminals without bidirectional text
                       support, using fridibi
    debug_printtraffic:Print out sent and received HTTP traffic
    include_ads:       Download ads as well
    default_search:    Prepend this string if an input url is not valid.
                       'auto' for elaborate guessing
    encoding:          Use this encoding instead of the system-specified.
    extract_flat:      Do not resolve URLs, return the immediate result.
                       Pass in 'in_playlist' to only show this behavior for
                       playlist items.

    The following parameters are not used by YoutubeDL itself, they are used by
    the FileDownloader:
    nopart, updatetime, buffersize, ratelimit, min_filesize, max_filesize, test,
    noresizebuffer, retries, continuedl, noprogress, consoletitle

    The following options are used by the post processors:
    prefer_ffmpeg:     If True, use ffmpeg instead of avconv if both are available,
                       otherwise prefer avconv.
    exec_cmd:          Arbitrary command to run after downloading
    """

    params = None
    _ies = []
    _pps = []
    _download_retcode = None
    _num_downloads = None
    _screen_file = None

    def __init__(self, params=None, auto_init=True):
        """Create a FileDownloader object with the given options."""
        if params is None:
            params = {}
        self._ies = []
        self._ies_instances = {}
        self._pps = []
        self._progress_hooks = []
        self._download_retcode = 0
        self._num_downloads = 0
        self._screen_file = [sys.stdout, sys.stderr][params.get('logtostderr', False)]
        self._err_file = sys.stderr
        self.params = params
        self.cache = Cache(self)

        if params.get('bidi_workaround', False):
            try:
                import pty
                master, slave = pty.openpty()
                width = get_term_width()
                if width is None:
                    width_args = []
                else:
                    width_args = ['-w', str(width)]
                sp_kwargs = dict(
                    stdin=subprocess.PIPE,
                    stdout=slave,
                    stderr=self._err_file)
                try:
                    self._output_process = subprocess.Popen(
                        ['bidiv'] + width_args, **sp_kwargs
                    )
                except OSError:
                    self._output_process = subprocess.Popen(
                        ['fribidi', '-c', 'UTF-8'] + width_args, **sp_kwargs)
                self._output_channel = os.fdopen(master, 'rb')
            except OSError as ose:
                if ose.errno == 2:
                    self.report_warning('Could not find fribidi executable, ignoring --bidi-workaround . Make sure that  fribidi  is an executable file in one of the directories in your $PATH.')
                else:
                    raise

        if (sys.version_info >= (3,) and sys.platform != 'win32' and
                sys.getfilesystemencoding() in ['ascii', 'ANSI_X3.4-1968']
                and not params.get('restrictfilenames', False)):
            # On Python 3, the Unicode filesystem API will throw errors (#1474)
            self.report_warning(
                'Assuming --restrict-filenames since file system encoding '
                'cannot encode all characters. '
                'Set the LC_ALL environment variable to fix this.')
            self.params['restrictfilenames'] = True

        if '%(stitle)s' in self.params.get('outtmpl', ''):
            self.report_warning('%(stitle)s is deprecated. Use the %(title)s and the --restrict-filenames flag(which also secures %(uploader)s et al) instead.')

        self._setup_opener()

        if auto_init:
            self.print_debug_header()
            self.add_default_info_extractors()

    def add_info_extractor(self, ie):
        """Add an InfoExtractor object to the end of the list."""
        self._ies.append(ie)
        self._ies_instances[ie.ie_key()] = ie
        ie.set_downloader(self)

    def get_info_extractor(self, ie_key):
        """
        Get an instance of an IE with name ie_key, it will try to get one from
        the _ies list, if there's no instance it will create a new one and add
        it to the extractor list.
        """
        ie = self._ies_instances.get(ie_key)
        if ie is None:
            ie = get_info_extractor(ie_key)()
            self.add_info_extractor(ie)
        return ie

    def add_default_info_extractors(self):
        """
        Add the InfoExtractors returned by gen_extractors to the end of the list
        """
        for ie in gen_extractors():
            self.add_info_extractor(ie)

    def add_post_processor(self, pp):
        """Add a PostProcessor object to the end of the chain."""
        self._pps.append(pp)
        pp.set_downloader(self)

    def add_progress_hook(self, ph):
        """Add the progress hook (currently only for the file downloader)"""
        self._progress_hooks.append(ph)

    def _bidi_workaround(self, message):
        if not hasattr(self, '_output_channel'):
            return message

        assert hasattr(self, '_output_process')
        assert isinstance(message, compat_str)
        line_count = message.count('\n') + 1
        self._output_process.stdin.write((message + '\n').encode('utf-8'))
        self._output_process.stdin.flush()
        res = ''.join(self._output_channel.readline().decode('utf-8')
                       for _ in range(line_count))
        return res[:-len('\n')]

    def to_screen(self, message, skip_eol=False):
        """Print message to stdout if not in quiet mode."""
        return self.to_stdout(message, skip_eol, check_quiet=True)

    def _write_string(self, s, out=None):
        write_string(s, out=out, encoding=self.params.get('encoding'))

    def to_stdout(self, message, skip_eol=False, check_quiet=False):
        """Print message to stdout if not in quiet mode."""
        if self.params.get('logger'):
            self.params['logger'].debug(message)
        elif not check_quiet or not self.params.get('quiet', False):
            message = self._bidi_workaround(message)
            terminator = ['\n', ''][skip_eol]
            output = message + terminator

            self._write_string(output, self._screen_file)

    def to_stderr(self, message):
        """Print message to stderr."""
        assert isinstance(message, compat_str)
        if self.params.get('logger'):
            self.params['logger'].error(message)
        else:
            message = self._bidi_workaround(message)
            output = message + '\n'
            self._write_string(output, self._err_file)

    def to_console_title(self, message):
        if not self.params.get('consoletitle', False):
            return
        if os.name == 'nt' and ctypes.windll.kernel32.GetConsoleWindow():
            # c_wchar_p() might not be necessary if `message` is
            # already of type unicode()
            ctypes.windll.kernel32.SetConsoleTitleW(ctypes.c_wchar_p(message))
        elif 'TERM' in os.environ:
            self._write_string('\033]0;%s\007' % message, self._screen_file)

    def save_console_title(self):
        if not self.params.get('consoletitle', False):
            return
        if 'TERM' in os.environ:
            # Save the title on stack
            self._write_string('\033[22;0t', self._screen_file)

    def restore_console_title(self):
        if not self.params.get('consoletitle', False):
            return
        if 'TERM' in os.environ:
            # Restore the title from stack
            self._write_string('\033[23;0t', self._screen_file)

    def __enter__(self):
        self.save_console_title()
        return self

    def __exit__(self, *args):
        self.restore_console_title()

        if self.params.get('cookiefile') is not None:
            self.cookiejar.save()

    def trouble(self, message=None, tb=None):
        """Determine action to take when a download problem appears.

        Depending on if the downloader has been configured to ignore
        download errors or not, this method may throw an exception or
        not when errors are found, after printing the message.

        tb, if given, is additional traceback information.
        """
        if message is not None:
            self.to_stderr(message)
        if self.params.get('verbose'):
            if tb is None:
                if sys.exc_info()[0]:  # if .trouble has been called from an except block
                    tb = ''
                    if hasattr(sys.exc_info()[1], 'exc_info') and sys.exc_info()[1].exc_info[0]:
                        tb += ''.join(traceback.format_exception(*sys.exc_info()[1].exc_info))
                    tb += compat_str(traceback.format_exc())
                else:
                    tb_data = traceback.format_list(traceback.extract_stack())
                    tb = ''.join(tb_data)
            self.to_stderr(tb)
        if not self.params.get('ignoreerrors', False):
            if sys.exc_info()[0] and hasattr(sys.exc_info()[1], 'exc_info') and sys.exc_info()[1].exc_info[0]:
                exc_info = sys.exc_info()[1].exc_info
            else:
                exc_info = sys.exc_info()
            raise DownloadError(message, exc_info)
        self._download_retcode = 1

    def report_warning(self, message):
        '''
        Print the message to stderr, it will be prefixed with 'WARNING:'
        If stderr is a tty file the 'WARNING:' will be colored
        '''
        if self.params.get('logger') is not None:
            self.params['logger'].warning(message)
        else:
            if self.params.get('no_warnings'):
                return
            if self._err_file.isatty() and os.name != 'nt':
                _msg_header = '\033[0;33mWARNING:\033[0m'
            else:
                _msg_header = 'WARNING:'
            warning_message = '%s %s' % (_msg_header, message)
            self.to_stderr(warning_message)

    def report_error(self, message, tb=None):
        '''
        Do the same as trouble, but prefixes the message with 'ERROR:', colored
        in red if stderr is a tty file.
        '''
        if self._err_file.isatty() and os.name != 'nt':
            _msg_header = '\033[0;31mERROR:\033[0m'
        else:
            _msg_header = 'ERROR:'
        error_message = '%s %s' % (_msg_header, message)
        self.trouble(error_message, tb)

    def report_file_already_downloaded(self, file_name):
        """Report file has already been fully downloaded."""
        try:
            self.to_screen('[download] %s has already been downloaded' % file_name)
        except UnicodeEncodeError:
            self.to_screen('[download] The file has already been downloaded')

    def prepare_filename(self, info_dict):
        """Generate the output filename."""
        try:
            template_dict = dict(info_dict)

            template_dict['epoch'] = int(time.time())
            autonumber_size = self.params.get('autonumber_size')
            if autonumber_size is None:
                autonumber_size = 5
            autonumber_templ = '%0' + str(autonumber_size) + 'd'
            template_dict['autonumber'] = autonumber_templ % self._num_downloads
            if template_dict.get('playlist_index') is not None:
                template_dict['playlist_index'] = '%0*d' % (len(str(template_dict['n_entries'])), template_dict['playlist_index'])
            if template_dict.get('resolution') is None:
                if template_dict.get('width') and template_dict.get('height'):
                    template_dict['resolution'] = '%dx%d' % (template_dict['width'], template_dict['height'])
                elif template_dict.get('height'):
                    template_dict['resolution'] = '%sp' % template_dict['height']
                elif template_dict.get('width'):
                    template_dict['resolution'] = '?x%d' % template_dict['width']

            sanitize = lambda k, v: sanitize_filename(
                compat_str(v),
                restricted=self.params.get('restrictfilenames'),
                is_id=(k == 'id'))
            template_dict = dict((k, sanitize(k, v))
                                 for k, v in template_dict.items()
                                 if v is not None)
            template_dict = collections.defaultdict(lambda: 'NA', template_dict)

            outtmpl = self.params.get('outtmpl', DEFAULT_OUTTMPL)
            tmpl = compat_expanduser(outtmpl)
            filename = tmpl % template_dict
            return filename
        except ValueError as err:
            self.report_error('Error in output template: ' + str(err) + ' (encoding: ' + repr(preferredencoding()) + ')')
            return None

    def _match_entry(self, info_dict):
        """ Returns None iff the file should be downloaded """

        video_title = info_dict.get('title', info_dict.get('id', 'video'))
        if 'title' in info_dict:
            # This can happen when we're just evaluating the playlist
            title = info_dict['title']
            matchtitle = self.params.get('matchtitle', False)
            if matchtitle:
                if not re.search(matchtitle, title, re.IGNORECASE):
                    return '"' + title + '" title did not match pattern "' + matchtitle + '"'
            rejecttitle = self.params.get('rejecttitle', False)
            if rejecttitle:
                if re.search(rejecttitle, title, re.IGNORECASE):
                    return '"' + title + '" title matched reject pattern "' + rejecttitle + '"'
        date = info_dict.get('upload_date', None)
        if date is not None:
            dateRange = self.params.get('daterange', DateRange())
            if date not in dateRange:
                return '%s upload date is not in range %s' % (date_from_str(date).isoformat(), dateRange)
        view_count = info_dict.get('view_count', None)
        if view_count is not None:
            min_views = self.params.get('min_views')
            if min_views is not None and view_count < min_views:
                return 'Skipping %s, because it has not reached minimum view count (%d/%d)' % (video_title, view_count, min_views)
            max_views = self.params.get('max_views')
            if max_views is not None and view_count > max_views:
                return 'Skipping %s, because it has exceeded the maximum view count (%d/%d)' % (video_title, view_count, max_views)
        age_limit = self.params.get('age_limit')
        if age_limit is not None:
            actual_age_limit = info_dict.get('age_limit')
            if actual_age_limit is None:
                actual_age_limit = 0
            if age_limit < actual_age_limit:
                return 'Skipping "' + title + '" because it is age restricted'
        if self.in_download_archive(info_dict):
            return '%s has already been recorded in archive' % video_title
        return None

    @staticmethod
    def add_extra_info(info_dict, extra_info):
        '''Set the keys from extra_info in info dict if they are missing'''
        for key, value in extra_info.items():
            info_dict.setdefault(key, value)

    def extract_info(self, url, download=True, ie_key=None, extra_info={},
                     process=True):
        '''
        Returns a list with a dictionary for each video we find.
        If 'download', also downloads the videos.
        extra_info is a dict containing the extra values to add to each result
         '''

        if ie_key:
            ies = [self.get_info_extractor(ie_key)]
        else:
            ies = self._ies

        for ie in ies:
            if not ie.suitable(url):
                continue

            if not ie.working():
                self.report_warning('The program functionality for this site has been marked as broken, '
                                    'and will probably not work.')

            try:
                ie_result = ie.extract(url)
                if ie_result is None: # Finished already (backwards compatibility; listformats and friends should be moved here)
                    break
                if isinstance(ie_result, list):
                    # Backwards compatibility: old IE result format
                    ie_result = {
                        '_type': 'compat_list',
                        'entries': ie_result,
                    }
                self.add_default_extra_info(ie_result, ie, url)
                if process:
                    return self.process_ie_result(ie_result, download, extra_info)
                else:
                    return ie_result
            except ExtractorError as de: # An error we somewhat expected
                self.report_error(compat_str(de), de.format_traceback())
                break
            except MaxDownloadsReached:
                raise
            except Exception as e:
                if self.params.get('ignoreerrors', False):
                    self.report_error(compat_str(e), tb=compat_str(traceback.format_exc()))
                    break
                else:
                    raise
        else:
            self.report_error('no suitable InfoExtractor for URL %s' % url)

    def add_default_extra_info(self, ie_result, ie, url):
        self.add_extra_info(ie_result, {
            'extractor': ie.IE_NAME,
            'webpage_url': url,
            'webpage_url_basename': url_basename(url),
            'extractor_key': ie.ie_key(),
        })

    def process_ie_result(self, ie_result, download=True, extra_info={}):
        """
        Take the result of the ie(may be modified) and resolve all unresolved
        references (URLs, playlist items).

        It will also download the videos if 'download'.
        Returns the resolved ie_result.
        """

        result_type = ie_result.get('_type', 'video')

        if result_type in ('url', 'url_transparent'):
            extract_flat = self.params.get('extract_flat', False)
            if ((extract_flat == 'in_playlist' and 'playlist' in extra_info) or
                    extract_flat is True):
                if self.params.get('forcejson', False):
                    self.to_stdout(json.dumps(ie_result))
                return ie_result

        if result_type == 'video':
            self.add_extra_info(ie_result, extra_info)
            return self.process_video_result(ie_result, download=download)
        elif result_type == 'url':
            # We have to add extra_info to the results because it may be
            # contained in a playlist
            return self.extract_info(ie_result['url'],
                                     download,
                                     ie_key=ie_result.get('ie_key'),
                                     extra_info=extra_info)
        elif result_type == 'url_transparent':
            # Use the information from the embedding page
            info = self.extract_info(
                ie_result['url'], ie_key=ie_result.get('ie_key'),
                extra_info=extra_info, download=False, process=False)

            def make_result(embedded_info):
                new_result = ie_result.copy()
                for f in ('_type', 'url', 'ext', 'player_url', 'formats',
                          'entries', 'ie_key', 'duration',
                          'subtitles', 'annotations', 'format',
                          'thumbnail', 'thumbnails'):
                    if f in new_result:
                        del new_result[f]
                    if f in embedded_info:
                        new_result[f] = embedded_info[f]
                return new_result
            new_result = make_result(info)

            assert new_result.get('_type') != 'url_transparent'
            if new_result.get('_type') == 'compat_list':
                new_result['entries'] = [
                    make_result(e) for e in new_result['entries']]

            return self.process_ie_result(
                new_result, download=download, extra_info=extra_info)
        elif result_type == 'playlist':
            # We process each entry in the playlist
            playlist = ie_result.get('title', None) or ie_result.get('id', None)
            self.to_screen('[download] Downloading playlist: %s' % playlist)

            playlist_results = []

            playliststart = self.params.get('playliststart', 1) - 1
            playlistend = self.params.get('playlistend', None)
            # For backwards compatibility, interpret -1 as whole list
            if playlistend == -1:
                playlistend = None

            if isinstance(ie_result['entries'], list):
                n_all_entries = len(ie_result['entries'])
                entries = ie_result['entries'][playliststart:playlistend]
                n_entries = len(entries)
                self.to_screen(
                    "[%s] playlist %s: Collected %d video ids (downloading %d of them)" %
                    (ie_result['extractor'], playlist, n_all_entries, n_entries))
            else:
                assert isinstance(ie_result['entries'], PagedList)
                entries = ie_result['entries'].getslice(
                    playliststart, playlistend)
                n_entries = len(entries)
                self.to_screen(
                    "[%s] playlist %s: Downloading %d videos" %
                    (ie_result['extractor'], playlist, n_entries))

            for i, entry in enumerate(entries, 1):
                self.to_screen('[download] Downloading video #%s of %s' % (i, n_entries))
                extra = {
                    'n_entries': n_entries,
                    'playlist': playlist,
                    'playlist_id': ie_result.get('id'),
                    'playlist_title': ie_result.get('title'),
                    'playlist_index': i + playliststart,
                    'extractor': ie_result['extractor'],
                    'webpage_url': ie_result['webpage_url'],
                    'webpage_url_basename': url_basename(ie_result['webpage_url']),
                    'extractor_key': ie_result['extractor_key'],
                }

                reason = self._match_entry(entry)
                if reason is not None:
                    self.to_screen('[download] ' + reason)
                    continue

                entry_result = self.process_ie_result(entry,
                                                      download=download,
                                                      extra_info=extra)
                playlist_results.append(entry_result)
            ie_result['entries'] = playlist_results
            return ie_result
        elif result_type == 'compat_list':
            def _fixup(r):
                self.add_extra_info(r,
                    {
                        'extractor': ie_result['extractor'],
                        'webpage_url': ie_result['webpage_url'],
                        'webpage_url_basename': url_basename(ie_result['webpage_url']),
                        'extractor_key': ie_result['extractor_key'],
                    })
                return r
            ie_result['entries'] = [
                self.process_ie_result(_fixup(r), download, extra_info)
                for r in ie_result['entries']
            ]
            return ie_result
        else:
            raise Exception('Invalid result type: %s' % result_type)

    def select_format(self, format_spec, available_formats):
        if format_spec == 'best' or format_spec is None:
            return available_formats[-1]
        elif format_spec == 'worst':
            return available_formats[0]
        elif format_spec == 'bestaudio':
            audio_formats = [
                f for f in available_formats
                if f.get('vcodec') == 'none']
            if audio_formats:
                return audio_formats[-1]
        elif format_spec == 'worstaudio':
            audio_formats = [
                f for f in available_formats
                if f.get('vcodec') == 'none']
            if audio_formats:
                return audio_formats[0]
        elif format_spec == 'bestvideo':
            video_formats = [
                f for f in available_formats
                if f.get('acodec') == 'none']
            if video_formats:
                return video_formats[-1]
        elif format_spec == 'worstvideo':
            video_formats = [
                f for f in available_formats
                if f.get('acodec') == 'none']
            if video_formats:
                return video_formats[0]
        else:
            extensions = ['mp4', 'flv', 'webm', '3gp', 'm4a']
            if format_spec in extensions:
                filter_f = lambda f: f['ext'] == format_spec
            else:
                filter_f = lambda f: f['format_id'] == format_spec
            matches = list(filter(filter_f, available_formats))
            if matches:
                return matches[-1]
        return None

    def process_video_result(self, info_dict, download=True):
        assert info_dict.get('_type', 'video') == 'video'

        if 'id' not in info_dict:
            raise ExtractorError('Missing "id" field in extractor result')
        if 'title' not in info_dict:
            raise ExtractorError('Missing "title" field in extractor result')

        if 'playlist' not in info_dict:
            # It isn't part of a playlist
            info_dict['playlist'] = None
            info_dict['playlist_index'] = None

        thumbnails = info_dict.get('thumbnails')
        if thumbnails:
            thumbnails.sort(key=lambda t: (
                t.get('width'), t.get('height'), t.get('url')))
            for t in thumbnails:
                if 'width' in t and 'height' in t:
                    t['resolution'] = '%dx%d' % (t['width'], t['height'])

        if thumbnails and 'thumbnail' not in info_dict:
            info_dict['thumbnail'] = thumbnails[-1]['url']

        if 'display_id' not in info_dict and 'id' in info_dict:
            info_dict['display_id'] = info_dict['id']

        if info_dict.get('upload_date') is None and info_dict.get('timestamp') is not None:
            upload_date = datetime.datetime.utcfromtimestamp(
                info_dict['timestamp'])
            info_dict['upload_date'] = upload_date.strftime('%Y%m%d')

        # This extractors handle format selection themselves
        if info_dict['extractor'] in ['Youku']:
            if download:
                self.process_info(info_dict)
            return info_dict

        # We now pick which formats have to be downloaded
        if info_dict.get('formats') is None:
            # There's only one format available
            formats = [info_dict]
        else:
            formats = info_dict['formats']

        if not formats:
            raise ExtractorError('No video formats found!')

        # We check that all the formats have the format and format_id fields
        for i, format in enumerate(formats):
            if 'url' not in format:
                raise ExtractorError('Missing "url" key in result (index %d)' % i)

            if format.get('format_id') is None:
                format['format_id'] = compat_str(i)
            if format.get('format') is None:
                format['format'] = '{id} - {res}{note}'.format(
                    id=format['format_id'],
                    res=self.format_resolution(format),
                    note=' ({0})'.format(format['format_note']) if format.get('format_note') is not None else '',
                )
            # Automatically determine file extension if missing
            if 'ext' not in format:
                format['ext'] = determine_ext(format['url']).lower()

        format_limit = self.params.get('format_limit', None)
        if format_limit:
            formats = list(takewhile_inclusive(
                lambda f: f['format_id'] != format_limit, formats
            ))

        # TODO Central sorting goes here

        if formats[0] is not info_dict:
            # only set the 'formats' fields if the original info_dict list them
            # otherwise we end up with a circular reference, the first (and unique)
            # element in the 'formats' field in info_dict is info_dict itself,
            # wich can't be exported to json
            info_dict['formats'] = formats
        if self.params.get('listformats', None):
            self.list_formats(info_dict)
            return

        req_format = self.params.get('format')
        if req_format is None:
            req_format = 'best'
        formats_to_download = []
        # The -1 is for supporting YoutubeIE
        if req_format in ('-1', 'all'):
            formats_to_download = formats
        else:
            for rfstr in req_format.split(','):
                # We can accept formats requested in the format: 34/5/best, we pick
                # the first that is available, starting from left
                req_formats = rfstr.split('/')
                for rf in req_formats:
                    if re.match(r'.+?\+.+?', rf) is not None:
                        # Two formats have been requested like '137+139'
                        format_1, format_2 = rf.split('+')
                        formats_info = (self.select_format(format_1, formats),
                            self.select_format(format_2, formats))
                        if all(formats_info):
                            # The first format must contain the video and the
                            # second the audio
                            if formats_info[0].get('vcodec') == 'none':
                                self.report_error('The first format must '
                                    'contain the video, try using '
                                    '"-f %s+%s"' % (format_2, format_1))
                                return
                            selected_format = {
                                'requested_formats': formats_info,
                                'format': rf,
                                'ext': formats_info[0]['ext'],
                            }
                        else:
                            selected_format = None
                    else:
                        selected_format = self.select_format(rf, formats)
                    if selected_format is not None:
                        formats_to_download.append(selected_format)
                        break
        if not formats_to_download:
            raise ExtractorError('requested format not available',
                                 expected=True)

        if download:
            if len(formats_to_download) > 1:
                self.to_screen('[info] %s: downloading video in %s formats' % (info_dict['id'], len(formats_to_download)))
            for format in formats_to_download:
                new_info = dict(info_dict)
                new_info.update(format)
                self.process_info(new_info)
        # We update the info dict with the best quality format (backwards compatibility)
        info_dict.update(formats_to_download[-1])
        return info_dict

    def process_info(self, info_dict):
        """Process a single resolved IE result."""

        assert info_dict.get('_type', 'video') == 'video'

        max_downloads = self.params.get('max_downloads')
        if max_downloads is not None:
            if self._num_downloads >= int(max_downloads):
                raise MaxDownloadsReached()

        info_dict['fulltitle'] = info_dict['title']
        if len(info_dict['title']) > 200:
            info_dict['title'] = info_dict['title'][:197] + '...'

        # Keep for backwards compatibility
        info_dict['stitle'] = info_dict['title']

        if 'format' not in info_dict:
            info_dict['format'] = info_dict['ext']

        reason = self._match_entry(info_dict)
        if reason is not None:
            self.to_screen('[download] ' + reason)
            return

        self._num_downloads += 1

        filename = self.prepare_filename(info_dict)

        # Forced printings
        if self.params.get('forcetitle', False):
            self.to_stdout(info_dict['fulltitle'])
        if self.params.get('forceid', False):
            self.to_stdout(info_dict['id'])
        if self.params.get('forceurl', False):
            # For RTMP URLs, also include the playpath
            self.to_stdout(info_dict['url'] + info_dict.get('play_path', ''))
        if self.params.get('forcethumbnail', False) and info_dict.get('thumbnail') is not None:
            self.to_stdout(info_dict['thumbnail'])
        if self.params.get('forcedescription', False) and info_dict.get('description') is not None:
            self.to_stdout(info_dict['description'])
        if self.params.get('forcefilename', False) and filename is not None:
            self.to_stdout(filename)
        if self.params.get('forceduration', False) and info_dict.get('duration') is not None:
            self.to_stdout(formatSeconds(info_dict['duration']))
        if self.params.get('forceformat', False):
            self.to_stdout(info_dict['format'])
        if self.params.get('forcejson', False):
            info_dict['_filename'] = filename
            self.to_stdout(json.dumps(info_dict))
        if self.params.get('dump_single_json', False):
            info_dict['_filename'] = filename

        # Do nothing else if in simulate mode
        if self.params.get('simulate', False):
            return

        if filename is None:
            return

        try:
            dn = os.path.dirname(encodeFilename(filename))
            if dn and not os.path.exists(dn):
                os.makedirs(dn)
        except (OSError, IOError) as err:
            self.report_error('unable to create directory ' + compat_str(err))
            return

        if self.params.get('writedescription', False):
            descfn = filename + '.description'
            if self.params.get('nooverwrites', False) and os.path.exists(encodeFilename(descfn)):
                self.to_screen('[info] Video description is already present')
            else:
                try:
                    self.to_screen('[info] Writing video description to: ' + descfn)
                    with io.open(encodeFilename(descfn), 'w', encoding='utf-8') as descfile:
                        descfile.write(info_dict['description'])
                except (KeyError, TypeError):
                    self.report_warning('There\'s no description to write.')
                except (OSError, IOError):
                    self.report_error('Cannot write description file ' + descfn)
                    return

        if self.params.get('writeannotations', False):
            annofn = filename + '.annotations.xml'
            if self.params.get('nooverwrites', False) and os.path.exists(encodeFilename(annofn)):
                self.to_screen('[info] Video annotations are already present')
            else:
                try:
                    self.to_screen('[info] Writing video annotations to: ' + annofn)
                    with io.open(encodeFilename(annofn), 'w', encoding='utf-8') as annofile:
                        annofile.write(info_dict['annotations'])
                except (KeyError, TypeError):
                    self.report_warning('There are no annotations to write.')
                except (OSError, IOError):
                    self.report_error('Cannot write annotations file: ' + annofn)
                    return

        subtitles_are_requested = any([self.params.get('writesubtitles', False),
                                       self.params.get('writeautomaticsub')])

        if subtitles_are_requested and 'subtitles' in info_dict and info_dict['subtitles']:
            # subtitles download errors are already managed as troubles in relevant IE
            # that way it will silently go on when used with unsupporting IE
            subtitles = info_dict['subtitles']
            sub_format = self.params.get('subtitlesformat', 'srt')
            for sub_lang in subtitles.keys():
                sub = subtitles[sub_lang]
                if sub is None:
                    continue
                try:
                    sub_filename = subtitles_filename(filename, sub_lang, sub_format)
                    if self.params.get('nooverwrites', False) and os.path.exists(encodeFilename(sub_filename)):
                        self.to_screen('[info] Video subtitle %s.%s is already_present' % (sub_lang, sub_format))
                    else:
                        self.to_screen('[info] Writing video subtitles to: ' + sub_filename)
                        with io.open(encodeFilename(sub_filename), 'w', encoding='utf-8') as subfile:
                                subfile.write(sub)
                except (OSError, IOError):
                    self.report_error('Cannot write subtitles file ' + sub_filename)
                    return

        if self.params.get('writeinfojson', False):
            infofn = os.path.splitext(filename)[0] + '.info.json'
            if self.params.get('nooverwrites', False) and os.path.exists(encodeFilename(infofn)):
                self.to_screen('[info] Video description metadata is already present')
            else:
                self.to_screen('[info] Writing video description metadata as JSON to: ' + infofn)
                try:
                    write_json_file(info_dict, encodeFilename(infofn))
                except (OSError, IOError):
                    self.report_error('Cannot write metadata to JSON file ' + infofn)
                    return

        if self.params.get('writethumbnail', False):
            if info_dict.get('thumbnail') is not None:
                thumb_format = determine_ext(info_dict['thumbnail'], 'jpg')
                thumb_filename = os.path.splitext(filename)[0] + '.' + thumb_format
                if self.params.get('nooverwrites', False) and os.path.exists(encodeFilename(thumb_filename)):
                    self.to_screen('[%s] %s: Thumbnail is already present' %
                                   (info_dict['extractor'], info_dict['id']))
                else:
                    self.to_screen('[%s] %s: Downloading thumbnail ...' %
                                   (info_dict['extractor'], info_dict['id']))
                    try:
                        uf = self.urlopen(info_dict['thumbnail'])
                        with open(thumb_filename, 'wb') as thumbf:
                            shutil.copyfileobj(uf, thumbf)
                        self.to_screen('[%s] %s: Writing thumbnail to: %s' %
                            (info_dict['extractor'], info_dict['id'], thumb_filename))
                    except (compat_urllib_error.URLError, compat_http_client.HTTPException, socket.error) as err:
                        self.report_warning('Unable to download thumbnail "%s": %s' %
                            (info_dict['thumbnail'], compat_str(err)))

        if not self.params.get('skip_download', False):
            if self.params.get('nooverwrites', False) and os.path.exists(encodeFilename(filename)):
                success = True
            else:
                try:
                    def dl(name, info):
                        fd = get_suitable_downloader(info)(self, self.params)
                        for ph in self._progress_hooks:
                            fd.add_progress_hook(ph)
                        if self.params.get('verbose'):
                            self.to_stdout('[debug] Invoking downloader on %r' % info.get('url'))
                        return fd.download(name, info)
                    if info_dict.get('requested_formats') is not None:
                        downloaded = []
                        success = True
                        merger = FFmpegMergerPP(self, not self.params.get('keepvideo'))
                        if not merger._executable:
                            postprocessors = []
                            self.report_warning('You have requested multiple '
                                'formats but ffmpeg or avconv are not installed.'
                                ' The formats won\'t be merged')
                        else:
                            postprocessors = [merger]
                        for f in info_dict['requested_formats']:
                            new_info = dict(info_dict)
                            new_info.update(f)
                            fname = self.prepare_filename(new_info)
                            fname = prepend_extension(fname, 'f%s' % f['format_id'])
                            downloaded.append(fname)
                            partial_success = dl(fname, new_info)
                            success = success and partial_success
                        info_dict['__postprocessors'] = postprocessors
                        info_dict['__files_to_merge'] = downloaded
                    else:
                        # Just a single file
                        success = dl(filename, info_dict)
                except (compat_urllib_error.URLError, compat_http_client.HTTPException, socket.error) as err:
                    self.report_error('unable to download video data: %s' % str(err))
                    return
                except (OSError, IOError) as err:
                    raise UnavailableVideoError(err)
                except (ContentTooShortError, ) as err:
                    self.report_error('content too short (expected %s bytes and served %s)' % (err.expected, err.downloaded))
                    return

            if success:
                try:
                    self.post_process(filename, info_dict)
                except (PostProcessingError) as err:
                    self.report_error('postprocessing: %s' % str(err))
                    return

        self.record_download_archive(info_dict)

    def download(self, url_list):
        """Download a given list of URLs."""
        outtmpl = self.params.get('outtmpl', DEFAULT_OUTTMPL)
        if (len(url_list) > 1 and
                '%' not in outtmpl
                and self.params.get('max_downloads') != 1):
            raise SameFileError(outtmpl)

        for url in url_list:
            try:
                #It also downloads the videos
                res = self.extract_info(url)
            except UnavailableVideoError:
                self.report_error('unable to download video')
            except MaxDownloadsReached:
                self.to_screen('[info] Maximum number of downloaded files reached.')
                raise
            else:
                if self.params.get('dump_single_json', False):
                    self.to_stdout(json.dumps(res))

        return self._download_retcode

    def download_with_info_file(self, info_filename):
        with io.open(info_filename, 'r', encoding='utf-8') as f:
            info = json.load(f)
        try:
            self.process_ie_result(info, download=True)
        except DownloadError:
            webpage_url = info.get('webpage_url')
            if webpage_url is not None:
                self.report_warning('The info failed to download, trying with "%s"' % webpage_url)
                return self.download([webpage_url])
            else:
                raise
        return self._download_retcode

    def post_process(self, filename, ie_info):
        """Run all the postprocessors on the given file."""
        info = dict(ie_info)
        info['filepath'] = filename
        keep_video = None
        pps_chain = []
        if ie_info.get('__postprocessors') is not None:
            pps_chain.extend(ie_info['__postprocessors'])
        pps_chain.extend(self._pps)
        for pp in pps_chain:
            try:
                keep_video_wish, new_info = pp.run(info)
                if keep_video_wish is not None:
                    if keep_video_wish:
                        keep_video = keep_video_wish
                    elif keep_video is None:
                        # No clear decision yet, let IE decide
                        keep_video = keep_video_wish
            except PostProcessingError as e:
                self.report_error(e.msg)
        if keep_video is False and not self.params.get('keepvideo', False):
            try:
                self.to_screen('Deleting original file %s (pass -k to keep)' % filename)
                os.remove(encodeFilename(filename))
            except (IOError, OSError):
                self.report_warning('Unable to remove downloaded video file')

    def _make_archive_id(self, info_dict):
        # Future-proof against any change in case
        # and backwards compatibility with prior versions
        extractor = info_dict.get('extractor_key')
        if extractor is None:
            if 'id' in info_dict:
                extractor = info_dict.get('ie_key')  # key in a playlist
        if extractor is None:
            return None  # Incomplete video information
        return extractor.lower() + ' ' + info_dict['id']

    def in_download_archive(self, info_dict):
        fn = self.params.get('download_archive')
        if fn is None:
            return False

        vid_id = self._make_archive_id(info_dict)
        if vid_id is None:
            return False  # Incomplete video information

        try:
            with locked_file(fn, 'r', encoding='utf-8') as archive_file:
                for line in archive_file:
                    if line.strip() == vid_id:
                        return True
        except IOError as ioe:
            if ioe.errno != errno.ENOENT:
                raise
        return False

    def record_download_archive(self, info_dict):
        fn = self.params.get('download_archive')
        if fn is None:
            return
        vid_id = self._make_archive_id(info_dict)
        assert vid_id
        with locked_file(fn, 'a', encoding='utf-8') as archive_file:
            archive_file.write(vid_id + '\n')

    @staticmethod
    def format_resolution(format, default='unknown'):
        if format.get('vcodec') == 'none':
            return 'audio only'
        if format.get('resolution') is not None:
            return format['resolution']
        if format.get('height') is not None:
            if format.get('width') is not None:
                res = '%sx%s' % (format['width'], format['height'])
            else:
                res = '%sp' % format['height']
        elif format.get('width') is not None:
            res = '?x%d' % format['width']
        else:
            res = default
        return res

    def _format_note(self, fdict):
        res = ''
        if fdict.get('ext') in ['f4f', 'f4m']:
            res += '(unsupported) '
        if fdict.get('format_note') is not None:
            res += fdict['format_note'] + ' '
        if fdict.get('tbr') is not None:
            res += '%4dk ' % fdict['tbr']
        if fdict.get('container') is not None:
            if res:
                res += ', '
            res += '%s container' % fdict['container']
        if (fdict.get('vcodec') is not None and
                fdict.get('vcodec') != 'none'):
            if res:
                res += ', '
            res += fdict['vcodec']
            if fdict.get('vbr') is not None:
                res += '@'
        elif fdict.get('vbr') is not None and fdict.get('abr') is not None:
            res += 'video@'
        if fdict.get('vbr') is not None:
            res += '%4dk' % fdict['vbr']
        if fdict.get('fps') is not None:
            res += ', %sfps' % fdict['fps']
        if fdict.get('acodec') is not None:
            if res:
                res += ', '
            if fdict['acodec'] == 'none':
                res += 'video only'
            else:
                res += '%-5s' % fdict['acodec']
        elif fdict.get('abr') is not None:
            if res:
                res += ', '
            res += 'audio'
        if fdict.get('abr') is not None:
            res += '@%3dk' % fdict['abr']
        if fdict.get('asr') is not None:
            res += ' (%5dHz)' % fdict['asr']
        if fdict.get('filesize') is not None:
            if res:
                res += ', '
            res += format_bytes(fdict['filesize'])
        elif fdict.get('filesize_approx') is not None:
            if res:
                res += ', '
            res += '~' + format_bytes(fdict['filesize_approx'])
        return res

    def list_formats(self, info_dict):
        def line(format, idlen=20):
            return (('%-' + compat_str(idlen + 1) + 's%-10s%-12s%s') % (
                format['format_id'],
                format['ext'],
                self.format_resolution(format),
                self._format_note(format),
            ))

        formats = info_dict.get('formats', [info_dict])
        idlen = max(len('format code'),
                    max(len(f['format_id']) for f in formats))
        formats_s = [line(f, idlen) for f in formats]
        if len(formats) > 1:
            formats_s[0] += (' ' if self._format_note(formats[0]) else '') + '(worst)'
            formats_s[-1] += (' ' if self._format_note(formats[-1]) else '') + '(best)'

        header_line = line({
            'format_id': 'format code', 'ext': 'extension',
            'resolution': 'resolution', 'format_note': 'note'}, idlen=idlen)
        self.to_screen('[info] Available formats for %s:\n%s\n%s' %
                       (info_dict['id'], header_line, '\n'.join(formats_s)))

    def urlopen(self, req):
        """ Start an HTTP download """

        # According to RFC 3986, URLs can not contain non-ASCII characters, however this is not
        # always respected by websites, some tend to give out URLs with non percent-encoded
        # non-ASCII characters (see telemb.py, ard.py [#3412])
        # urllib chokes on URLs with non-ASCII characters (see http://bugs.python.org/issue3991)
        # To work around aforementioned issue we will replace request's original URL with
        # percent-encoded one
        req_is_string = isinstance(req, basestring if sys.version_info < (3, 0) else compat_str)
        url = req if req_is_string else req.get_full_url()
        url_escaped = escape_url(url)

        # Substitute URL if any change after escaping
        if url != url_escaped:
            if req_is_string:
                req = url_escaped
            else:
                req = compat_urllib_request.Request(
                    url_escaped, data=req.data, headers=req.headers,
                    origin_req_host=req.origin_req_host, unverifiable=req.unverifiable)

        return self._opener.open(req, timeout=self._socket_timeout)

    def print_debug_header(self):
        if not self.params.get('verbose'):
            return

        if type('') is not compat_str:
            # Python 2.6 on SLES11 SP1 (https://github.com/rg3/youtube-dl/issues/3326)
            self.report_warning(
                'Your Python is broken! Update to a newer and supported version')

        stdout_encoding = getattr(
            sys.stdout, 'encoding', 'missing (%s)' % type(sys.stdout).__name__)
        encoding_str = (
            '[debug] Encodings: locale %s, fs %s, out %s, pref %s\n' % (
                locale.getpreferredencoding(),
                sys.getfilesystemencoding(),
                stdout_encoding,
                self.get_encoding()))
        write_string(encoding_str, encoding=None)

        self._write_string('[debug] youtube-dl version ' + __version__ + '\n')
        try:
            sp = subprocess.Popen(
                ['git', 'rev-parse', '--short', 'HEAD'],
                stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                cwd=os.path.dirname(os.path.abspath(__file__)))
            out, err = sp.communicate()
            out = out.decode().strip()
            if re.match('[0-9a-f]+', out):
                self._write_string('[debug] Git HEAD: ' + out + '\n')
        except:
            try:
                sys.exc_clear()
            except:
                pass
        self._write_string('[debug] Python version %s - %s\n' % (
            platform.python_version(), platform_name()))

        exe_versions = FFmpegPostProcessor.get_versions()
        exe_versions['rtmpdump'] = rtmpdump_version()
        exe_str = ', '.join(
            '%s %s' % (exe, v)
            for exe, v in sorted(exe_versions.items())
            if v
        )
        if not exe_str:
            exe_str = 'none'
        self._write_string('[debug] exe versions: %s\n' % exe_str)

        proxy_map = {}
        for handler in self._opener.handlers:
            if hasattr(handler, 'proxies'):
                proxy_map.update(handler.proxies)
        self._write_string('[debug] Proxy map: ' + compat_str(proxy_map) + '\n')

    def _setup_opener(self):
        timeout_val = self.params.get('socket_timeout')
        self._socket_timeout = 600 if timeout_val is None else float(timeout_val)

        opts_cookiefile = self.params.get('cookiefile')
        opts_proxy = self.params.get('proxy')

        if opts_cookiefile is None:
            self.cookiejar = compat_cookiejar.CookieJar()
        else:
            self.cookiejar = compat_cookiejar.MozillaCookieJar(
                opts_cookiefile)
            if os.access(opts_cookiefile, os.R_OK):
                self.cookiejar.load()

        cookie_processor = compat_urllib_request.HTTPCookieProcessor(
            self.cookiejar)
        if opts_proxy is not None:
            if opts_proxy == '':
                proxies = {}
            else:
                proxies = {'http': opts_proxy, 'https': opts_proxy}
        else:
            proxies = compat_urllib_request.getproxies()
            # Set HTTPS proxy to HTTP one if given (https://github.com/rg3/youtube-dl/issues/805)
            if 'http' in proxies and 'https' not in proxies:
                proxies['https'] = proxies['http']
        proxy_handler = compat_urllib_request.ProxyHandler(proxies)

        debuglevel = 1 if self.params.get('debug_printtraffic') else 0
        https_handler = make_HTTPS_handler(
            self.params.get('nocheckcertificate', False), debuglevel=debuglevel)
        ydlh = YoutubeDLHandler(debuglevel=debuglevel)
        opener = compat_urllib_request.build_opener(
            https_handler, proxy_handler, cookie_processor, ydlh)
        # Delete the default user-agent header, which would otherwise apply in
        # cases where our custom HTTP handler doesn't come into play
        # (See https://github.com/rg3/youtube-dl/issues/1309 for details)
        opener.addheaders = []
        self._opener = opener

    def encode(self, s):
        if isinstance(s, bytes):
            return s  # Already encoded

        try:
            return s.encode(self.get_encoding())
        except UnicodeEncodeError as err:
            err.reason = err.reason + '. Check your system encoding configuration or use the --encoding option.'
            raise

    def get_encoding(self):
        encoding = self.params.get('encoding')
        if encoding is None:
            encoding = preferredencoding()
        return encoding