TypeError: Object of type numpy.int64 is not JSON serializable

TypeError: Object of type numpy.int64 is not JSON serializable

$ python - <<'PY'
import numpy as np
import json
json.dumps(np.int64(123))
PY
Traceback (most recent call last):
  File "<string>", line 3, in <module>
TypeError: Object of type int64 is not JSON serializable

Why this happens

Python’s json module works with built-in types (int, float, dict, list). NumPy scalars like numpy.int64 and numpy.float64, and ndarray objects, are not plain Python types and thus aren’t serializable by json.dumps out of the box.

Fix

Convert NumPy scalars to Python scalars with .item() or int()/float(), and convert arrays to lists with .tolist() before serialization. Alternatively, provide a custom serializer by passing default= to json.dumps.

Wrong code

import numpy as np
import json
arr = np.array([1, 2, 3])
json.dumps(arr)  # TypeError

Fixed code

import numpy as np
import json
arr = np.array([1, 2, 3])
# Convert to native list
json_str = json.dumps(arr.tolist())
print(json_str)

# Convert a single numpy scalar
val = np.int64(123)
print(json.dumps(int(val)))

# Or use default serializer
json.dumps(arr, default=lambda o: o.tolist() if hasattr(o, 'tolist') else int(o))