Why does json.dumps(list(np.arange(5))) fail while json.dumps(np.arange(5).tolist()) works

Question

I noticed this problem when a computer running Ubuntu was updated recently and the default version of Python changed to 2.7.

import json
import numpy as np

         


        

Answer 1

Because the elements of a NumPy array are not native ints, but of NUmPy's own types:

>>> type(np.arange(5)[0])

You can use a custom JSONEncoder to support the ndarray type returned by arange:

import numpy as np
import json

class NumPyArangeEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, np.ndarray):
            return obj.tolist() # or map(int, obj)
        return json.JSONEncoder.default(self, obj)

print(json.dumps(np.arange(5), cls=NumPyArangeEncoder))