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

json.dumps(list(np.arange(5))) # Fails, throws a "TypeError: 0 is not JSON serializable"
json.dumps(np.arange(5).tolist()) # Works 

Is there a difference between list() and the tolist() methd of a numpy array?

Answer 1

It looks like the tolist() method turns the numpy int32 (or whatever size you have) back into an int, which JSON knows what to do with:

>>> list(np.arange(5))
[0, 1, 2, 3, 4]
>>> type(list(np.arange(5)))
<type 'list'>
>>> type(list(np.arange(5))[0])
<type 'numpy.int32'>
>>> np.arange(5).tolist()
[0, 1, 2, 3, 4]
>>> type(np.arange(5).tolist())
<type 'list'>
>>> type(np.arange(5).tolist()[0])
<type 'int'>

As the docs say for tolist():

Return the array as a (possibly nested) list.

Return a copy of the array data as a (nested) Python list. Data items
are converted to the nearest compatible Python type.

The last line makes the difference here.