NumPy array is not JSON serializable

Question

After creating a NumPy array, and saving it as a Django context variable, I receive the following error when loading the webpage:

array([   0,  239,  479,  717,  952, 1192, 1432, 1667], dtype=int64) is not JSON serializable 

What does this mean?

Answer 1

I regularly "jsonify" np.arrays. Try using the ".tolist()" method on the arrays first, like this:

import numpy as np import codecs, json   a = np.arange(10).reshape(2,5) # a 2 by 5 array b = a.tolist() # nested lists with same data, indices file_path = "/path.json" ## your path variable json.dump(b, codecs.open(file_path, 'w', encoding='utf-8'),            separators=(',', ':'),            sort_keys=True,            indent=4) ### this saves the array in .json format 

In order to "unjsonify" the array use:

obj_text = codecs.open(file_path, 'r', encoding='utf-8').read() b_new = json.loads(obj_text) a_new = np.array(b_new) 

Answer 2

Store as JSON a numpy.ndarray or any nested-list composition.

class NumpyEncoder(json.JSONEncoder):     def default(self, obj):         if isinstance(obj, np.ndarray):             return obj.tolist()         return json.JSONEncoder.default(self, obj)  a = np.array([[1, 2, 3], [4, 5, 6]]) print(a.shape) json_dump = json.dumps({'a': a, 'aa': [2, (2, 3, 4), a], 'bb': [2]},                         cls=NumpyEncoder) print(json_dump) 

Will output:

(2, 3) {"a": [[1, 2, 3], [4, 5, 6]], "aa": [2, [2, 3, 4], [[1, 2, 3], [4, 5, 6]]], "bb": [2]} 

To restore from JSON:

json_load = json.loads(json_dump) a_restored = np.asarray(json_load["a"]) print(a_restored) print(a_restored.shape) 

Will output:

[[1 2 3]  [4 5 6]] (2, 3)