Traverse a 2D Array in a circular clockwise path.

arrtraverse.py

import unittest

def traverse(board, start):
    '''
        Traverse a 2d array in an anti-clockwise manner
        array: board
        start[r,c] : start position
    '''
    r, c = start
    size = len(board[0])
    readout = []
    
    # traverse the outer cells layer by layer, clockwise
    for i in range(1, size):  # layer control

        # traverse right column
        if c+i < size:
            for j in range(-i, i+1):
                if r+j >= 0 and r+j < size:
                    readout.append(board[r+j][c+i])
                else: break

        # traverse bottom row
        if r+i < size:
            for j in range(i-1, -i-1, -1):
                if c+j >= 0 and c+j < size:
                    readout.append(board[r+i][c+j])
                else: break

        # traverse left column
        if c-i >= 0:
            for j in range(i-1, -i-1, -1):
                if r+j >= 0 and r+j < size:
                    readout.append(board[r+j][c-i])
                else: break

        # traverse top row
        if r-i >= 0:
            for j in range(-i+1, i):
                if c+j >= 0 and c+j < size:
                    readout.append(board[r-i][c+j])
                else: break

    return readout


class TestTraversal(unittest.TestCase):
    arr = [
            [1,   2,  3,  4,  5],
            [6,   7,  8,  9, 10],
            [11, 12, 13, 14, 15],
            [16, 17, 18, 19, 20],
            [21, 22, 23, 24, 25],
        ]
    def test_traverse_from_center_point(self):
        self.assertEqual(traverse(self.arr, [2, 2]), [9, 14, 19, 18, 17, 12, 7, 8, 5, 10, 15, 20, 25,24,23,22,21,16,11,6,1,2,3,4])
    
    def test_traverse_from_random_point(self):
        # Fails
        self.assertEqual(traverse(self.arr, [2, 4]), [])

if __name__ == '__main__':
    unittest.main()