#!/usr/bin/python3
"""
This module provides an example of how to flatten an array of arbitrary
nest integers using recursion.
"""
import unittest


def flatten(array: list) -> list:
    """
    Accepts an arbitrary list of integers and returns a flattened version
    of the list.
    >>>flatten([[1, 2, [3]], 4])
    [1,2,3,4]


    :param array: arbitrary list of nested integers
    :type array: list
    :return: flatten list of integers
    :rtype: list
    :raise: TypeError
    """
    if type(array) is not list:
        raise TypeError

    flat = []

    for item in array:

        if type(item) is int:
            flat.append(item)

        # If the item encountered is a list rather than an integer
        # recursively concatenate the list.
        elif type(item) is list:
            flat = flat + flatten(item)

        else:
            raise TypeError

    return flat


class TestFlatten(unittest.TestCase):
    def test_flatten(self):
        """
        Verifies that the flatten() function correctly returns
        a flatten list of integers
        """
        correctly_flatten_list = [1, 2, 3, 4]

        self.assertListEqual(flatten([[1, 2, [3]], 4]), correctly_flatten_list)
        self.assertListEqual(flatten([[1, [2], [3]], 4]), correctly_flatten_list)
        self.assertListEqual(flatten([[1, [2], [3]], 4]), correctly_flatten_list)
        self.assertListEqual(flatten([[1, [2, [3]]], 4]), correctly_flatten_list)

    def test_exception_raising(self):
        """
        Verifies that the flatten() function raises a TypeError when a type
        other than int or list is encountered.
        """
        bad_types = ['r', '3', [1, 2, 'three']]

        with self.assertRaises(TypeError):

            for bad_type in bad_types:
                flatten(bad_type)


if __name__ == "__main__":
    print("Running tests!")
    unittest.main()