Python Vamp Plugin JSON Encoder

Question

I'm using in Python a VAMP plugin:

import vamp
import librosa
data, rate = librosa.load("example.mp3")
chroma = vamp.collect(data, rate, "nnls-chroma:nnls-chroma")

I have to decode results in a json object. Since the Vamp plugin uses a specific object vampy.RealTime object to wrap numbers I wrote an Encoder for that, otherwise you will get a TypeError: (integer) is not JSON serializing - see here for this specific error: "TypeError: (Integer) is not JSON serializable" when serializing JSON in Python?

For this reason I wrote a custom JSON Encoder that uses the conversion to number from the vampy.RealTime object, taken from here

import json

class VampJSONEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, vamp.vampyhost.RealTime):
            r = vamp.vampyhost.RealTime('seconds', obj)
             return r
        return super(VampJSONEncoder, self).default(obj)

but when I run it like

json.dumps(chroma, sort_keys=True, indent=4, cls=VampJSONEncoder)

I get a

RuntimeError: maximum recursion depth exceeded while calling a Python object