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pythonarrayspython-3.xlistnumpy

Is there any difference between list(array) and array.tolist() in Python?

The following variables seem to be similar but they aren't and I don't understand why:

import ujson
import numpy as np

arr = np.array([1, 2, 3, 4])
arr_1 = arr.tolist()
arr_2 = list(arr)

arr_1 == arr_2  # True

ujson.dumps({'arr': arr_1})  # it works
ujson.dumps({'arr': arr_2})  # it does not work (OverflowError: Maximum recursion level reached)

I am using Python-3.5.6, ujson-1.35 and numpy-1.16.4.

Thank you a lot for your help!!

Solution

  • numpy has its own numeric data types for different levels of precision.

    These are created in ways that allow easy comparison with regular Python integers.

    np.int32(3) == 3   # True
[np.int32(1), 4] == [1, np.int32(4)]    # True

    (Lists are equal in Python if all elements at the same index are equal)

    That is why your arr_1 == arr_2.

    They cannot readily be serialized to json, but the tolist method converts them to regular Python numeric types which do allow serialization.