使用youtube_-dl下载youtube音频，但即使指定，也不总是mp3类型

Question

这是我当前使用youtube的代码：

from __future__ import unicode_literals 
     import youtube_dl 
     import ffmpeg
     ydl_opts = { 
      'format': 'bestaudio/best', 
      'postprocessors': [{ 
         'key': 'FFmpegExtractAudio', 
        'preferredcodec': 'mp3',  
        'preferredquality': '320',   # selected mp3 and 320
    }],
}
with youtube_dl.YoutubeDL(ydl_opts) as ydl:
    ydl.download([input('Youtube link: ')])

它很管用，但给了我不同的类型。我得到这个错误：

^{pr2}$

在处理上述异常时，发生了另一个异常：

Traceback (most recent call last):
  File "C:/Users/Eng-Ahmed Saleh/Desktop/asd/A.py", line 15, in <module>
    ydl.download([input('Youtube link: ')])
  File "C:\Users\Eng-Ahmed Saleh\PycharmProjects\untitled5\venv\lib\site-packages\youtube_dl\YoutubeDL.py", line 2008, in download
    url, force_generic_extractor=self.params.get('force_generic_extractor', False))
  File "C:\Users\Eng-Ahmed Saleh\PycharmProjects\untitled5\venv\lib\site-packages\youtube_dl\YoutubeDL.py", line 807, in extract_info
    return self.process_ie_result(ie_result, download, extra_info)
  File "C:\Users\Eng-Ahmed Saleh\PycharmProjects\untitled5\venv\lib\site-packages\youtube_dl\YoutubeDL.py", line 861, in process_ie_result
    return self.process_video_result(ie_result, download=download)
  File "C:\Users\Eng-Ahmed Saleh\PycharmProjects\untitled5\venv\lib\site-packages\youtube_dl\YoutubeDL.py", line 1642, in process_video_result
    self.process_info(new_info)
  File "C:\Users\Eng-Ahmed Saleh\PycharmProjects\untitled5\venv\lib\site-packages\youtube_dl\YoutubeDL.py", line 1989, in process_info
    self.post_process(filename, info_dict)
  File "C:\Users\Eng-Ahmed Saleh\PycharmProjects\untitled5\venv\lib\site-packages\youtube_dl\YoutubeDL.py", line 2056, in post_process
    self.report_error(e.msg)
  File "C:\Users\Eng-Ahmed Saleh\PycharmProjects\untitled5\venv\lib\site-packages\youtube_dl\YoutubeDL.py", line 624, in report_error
    self.trouble(error_message, tb)
  File "C:\Users\Eng-Ahmed Saleh\PycharmProjects\untitled5\venv\lib\site-packages\youtube_dl\YoutubeDL.py", line 594, in trouble
    raise DownloadError(message, exc_info)
youtube_dl.utils.DownloadError: ERROR: ffprobe/avprobe and ffmpeg/avconv not found. Please install one.