近日在django項(xiàng)目中采用了librosa來分析用戶上傳的音頻，主要是對(duì)其做一定間隔的采樣用于繪制波形圖。在本地(MacOS)上工作正常，部署到Ubuntu服務(wù)器后，自己手動(dòng)用python方式運(yùn)行django的server時(shí)，工作都是正常的，然而當(dāng)我用superviosr掛起之后就一直報(bào)NoBackendError的錯(cuò)誤，反復(fù)檢查了FFmpeg的安裝，應(yīng)該是沒有問題的。網(wǎng)上搜了一下沒有看到比較合適的解決辦法，故不得不自己讀源碼來調(diào)試了。

由于我的程序中只采用了librosa.load這個(gè)命令，通過源代碼可以發(fā)現(xiàn)librosa實(shí)際上是用audioread這個(gè)庫的audioread.audio_open來讀取音頻文件的。這個(gè)函數(shù)的源代碼如下：

def audio_open(path):
    """Open an audio file using a library that is available on this
    system.
    """
    # Standard-library WAV and AIFF readers.
    from . import rawread
    try:
        return rawread.RawAudioFile(path)
    except DecodeError:
        pass

    # Core Audio.
    if _ca_available():
        from . import macca
        try:
            return macca.ExtAudioFile(path)
        except DecodeError:
            pass

    # GStreamer.
    if _gst_available():
        from . import gstdec
        try:
            return gstdec.GstAudioFile(path)
        except DecodeError:
            pass

    # MAD.
    if _mad_available():
        from . import maddec
        try:
            return maddec.MadAudioFile(path)
        except DecodeError:
            pass

    # FFmpeg.
    from . import ffdec
    try:
        return ffdec.FFmpegAudioFile(path)
    except DecodeError:
        pass

    # All backends failed!
    raise NoBackendError()

可見之前我們遇到的NoBackendError就是這里的最后一行拋出的了，由于我安裝的FFmpeg，進(jìn)一步進(jìn)入ffdec.py這個(gè)文件中。不難發(fā)現(xiàn)實(shí)際打開文件的是下面這個(gè)函數(shù)：

def popen_multiple(commands, command_args, *args, **kwargs):
    """Like `subprocess.Popen`, but can try multiple commands in case
    some are not available.

    `commands` is an iterable of command names and `command_args` are
    the rest of the arguments that, when appended to the command name,
    make up the full first argument to `subprocess.Popen`. The
    other positional and keyword arguments are passed through.
    """
    for i, command in enumerate(commands):
        cmd = [command] + command_args
        try:
            return subprocess.Popen(cmd, *args, **kwargs)
        except OSError:
            if i == len(commands) - 1:
                # No more commands to try.
                raise

這里的commands是直接傳入的第33行的COMMANDS變量

COMMANDS = ('ffmpeg', 'avconv')

我這里倒騰了好幾下，最終發(fā)現(xiàn)實(shí)際是錯(cuò)誤的原因是沒有找到ffmpeg這個(gè)命令。我們?cè)谶@里將ffmpeg替換成ffmpeg的絕對(duì)路徑。你可以通過下面這行命令找到。

which ffmpeg

最終我改成了

COMMANDS = ('/usr/bin/ffmpeg', 'avconv')

Boom！一切就工作正常了。這么想起來應(yīng)該是安裝ffmpeg的時(shí)候的環(huán)境變量有問題，導(dǎo)致在command line方式下調(diào)用ffmpeg命令出錯(cuò)吧。