# This code, which assumes Python 3.6+, provides a flatten() function and can be
# tested from the command line with 'python flatten.py'.

import unittest

def flatten(arg):
    """
    Flatten an array of arbitrarily nested arrays of integers into a flat array of integers: [[1,2,[3]],4] -> [1,2,3,4].
    Edge case behavior: Invoking on a single integer returns an array containing that integer, flatten(4) -> [4]
    Raise an error if the input contains an unsupported data type (dict, set, tuple, decimal, etc.).
    """
    # If arg is integer, wrap it in an array
    if (type(arg) == type(123)):
        return [arg]
    if type(arg) != type([]):
        raise ValueError(f'Unexpected argument {arg}')
    if not arg:
        return []
    else:
        # Traverse the array depth-first
        return flatten(arg[0]) + flatten(arg[1 : len(arg)])

class TestFlatten(unittest.TestCase):
    def test_success(self):
        self.assertEqual(flatten([5, 6, 7, 8]), [5, 6, 7, 8])
        self.assertEqual(flatten([[0, 1, 1], [2, [3, 4]], 5]), [0, 1, 1, 2, 3, 4, 5])
        self.assertEqual(flatten([]), [])
        self.assertEqual(flatten(55), [55])
        self.assertEqual(flatten([[[[4]]]]), [4])

    def test_exceptions(self):
        self.assertRaises(ValueError, flatten, [1, 2, 3, 4.5])
        self.assertRaises(ValueError, flatten, {"a": 33})
        self.assertRaises(ValueError, flatten, [1, [2, 3], {"a": [100, 101], "b": 23}, [4, 5]])

# Run the unit tests
if __name__ == '__main__':
    unittest.main()