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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def fill_n_todo(self):
 """ Calculate and record the number of edge pixels left to do on each tile """

        left = self.left
        right = self.right
        top = self.top
        bottom = self.bottom
        for i in xrange(self.n_chunks):
            self.n_todo.ravel()[i] = np.sum([left.ravel()[i].n_todo,
                                            right.ravel()[i].n_todo,
                                            top.ravel()[i].n_todo,
                                            bottom.ravel()[i].n_todo])