array pancakes (testable with python3 -m unittest test)

flatten.py

def flatten(array):
    """
    Drop in an array of a arrays (ex: [[1,2,[3]],4]) and get back a flattened array (ex: [1,2,3,4]).
    """
    new_array = []
    for i in array:
        if not isinstance(i, list):
            new_array.append(i)
        else:
            [new_array.append(j) for j in flatten(i)]
    return new_array

test.py

import unittest
import random

from flatten import flatten


class FlattenUnitTest(unittest.TestCase):

    def test_a_basic_array(self):
        test_arr = [1, 2, 3, 4]
        correct_arr = [1, 2, 3, 4]

        pancake = flatten(test_arr)
        self.assertSequenceEqual(pancake, correct_arr)

    def test_a_complex_array(self):
        test_arr = [[1, 2, [3]], 4]
        correct_arr = [1, 2, 3, 4]

        pancake = flatten(test_arr)
        self.assertSequenceEqual(pancake, correct_arr)

    def test_a_ridiculous_array(self):
        start = random.randint(0, 10)
        test_arr = []
        correct_arr = []
        for i in range(start, start + 10):
            spot_array = list(range(i, i + random.randint(4, 40)))
            use_array = random.randint(0, 1) == 0
            if use_array:
                test_arr.append(spot_array)
            for j in spot_array:
                correct_arr.append(j)
                if not use_array:
                    test_arr.append(j)

        pancake = flatten(test_arr)
        self.assertSequenceEqual(pancake, correct_arr)