Can't grab filename when using youtube-dl as python module

[kereilly]

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I'm using youtube-dl as a python module in a script. i'm running youtube-dl 2016.05.16 and python 2.7.10 on mac os 10.11. The problem i'm having is getting youtube-dl to report the name of the file it downloads. i'm using the ydl.extract_info function but the file extension will be different from what is reported by youtube-dl and what is actually saved. here is the code

destination = archival_location + ".%(ext)s"
ydl_opts = {
    'logger' : MyLogger(),
    'progress_hooks': [my_hook],
    'outtmpl' : destination,
    'ignoreerrors' : True
}
try:
    with YoutubeDL(ydl_opts) as ydl:
        info = ydl.extract_info(url, download=False)
        download_target = ydl.prepare_filename(info)
        ydl.download([url])

so my download_target variable will have something like "downloaded_file.mp4" but the actually file is "downloaded_file.mkv". I'm guessing since it was a youtube download ffmpeg muxed the audio and video together making it an mkv file? But i need to preserve file extensions when i can so i know its type for further processing. thats why i'm using the ".%(ext)s" in the 'outtmpl' option. anything i'm doing wrong here or is this just what happens when ffmpeg gets used.

[dstftw]

Duplicate of #5710.