flatten.py

def flatten(arg):                                                                     
  if isinstance(arg, list):                                                           
    if len(arg) == 0:                                                                 
      return arg                                                                      
    else:                                                                             
      return flatten(arg[0]) + flatten(arg[1:])                                       
  else:                                                                               
    return [arg]                                                                      
                                                                                      
import unittest                                                                       
                                                                                      
class MyTest(unittest.TestCase):                                                      
    def testBasic(self):                                                              
        self.assertEqual(flatten([[1,2,[3]],4]), [1,2,3,4])                           
                                                                                      
    def testArrMatrix(self):                                                          
        self.assertEqual(flatten([[1,2,3],[4,5,6],[7,8,9]]), [1,2,3,4,5,6,7,8,9])     
                                                                                      
    def testEmptyList(self):                                                          
        self.assertEqual(flatten([]), [])                                             
                                                                                      
    def testSingleElement(self):                                                      
        self.assertEqual(flatten([1]), [1])                                           
                                                                                      
    def testDeepArrayComposition(self):                                               
        self.assertEqual(flatten([[[[[4]]]]]), [4])                                   
                                                                                      
if __name__ == '__main__':                                                            
    unittest.main()