How to use the essentia.standard.MonoLoader function in essentia

def read_audio(audio_file, sample_rate):
    """Reads the audio file using Essentia."""
    audio = ES.MonoLoader(filename=audio_file,
                          sampleRate=sample_rate)()
    return audio

"""

    # Makes sure the output features folder exists
    utils.ensure_dir(OUTPUT_FEATURES)
    features_file = os.path.join(OUTPUT_FEATURES,
                                 os.path.basename(audio_file) + ".json")

    # If already precomputed, read and return
    if os.path.exists(features_file):
        with open(features_file, "r") as f:
            features = json.load(f)
        return list_to_array(features)

    # Load Audio
    logging.info("Loading audio file %s" % os.path.basename(audio_file))
    audio = ES.MonoLoader(filename=audio_file, sampleRate=SAMPLE_RATE)()
    duration = len(audio) / float(SAMPLE_RATE)

    # Estimate Beats
    features = {}
    ticks, conf = compute_beats(audio)
    ticks = np.concatenate(([0], ticks, [duration]))  # Add first and last time
    ticks = essentia.array(np.unique(ticks))
    features["beats"] = ticks.tolist()

    # Compute Beat-sync features
    features["mfcc"], features["hpcp"], features["tonnetz"] = \
        compute_beatsync_features(ticks, audio)

    # Save output as audio file
    if audio_beats:
        logging.info("Saving Beats as an audio file")

def __init__(self, audio_file, mono=True, sample_rate=44100, normalize_gain=False):
        """"""
        self.fs = sample_rate
        if normalize_gain:
            self.audio_vector = estd.EasyLoader(filename=audio_file, sampleRate=self.fs, replayGain=-9)()
        else:
            self.audio_vector = estd.MonoLoader(filename=audio_file, sampleRate=self.fs)()
        print "== Audio vector of %s loaded with shape %s and sample rate %s ==" % (audio_file, self.audio_vector.shape, self.fs)
        return