class Matrix: 
 def __init__(self,n):
  self.matrix = [[None] * n for j in [[None]*n for i in range(n)] ]
 def __str__(self):
  self.printable = []
  for row in self.matrix:
   for element in row:
    if element is None: element = '.' 
    self.printable.append(element + ' ')  
   self.printable.append('\n')
  return ''.join(self.printable) 

a = Matrix(4)
a.matrix[1][2] = 'a'
a.matrix[3][3] = 'x'
print a