Why are some numpy datatypes JSON serializable and others not?

The data types that are JSON serializable are all Python built-ins:

>>> np.int is intTrue>>> np.float is floatTrue>>> np.bool is boolTrue

So all NumPy data types that you show are not JSON serializable. At least is consistent.

np.float_ is the same as np.float64 (tested on MacOS):

>>> np.float_ is np.float64True

The help says:

np.float64

64-bit floating-point number. Character code 'd'. Python float compatible.

Whereas:

np.float32

32-bit floating-point number. Character code 'f'. C float compatible.

So, Python float compatible types work with json.dumps(), but C compatible ones don't.

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