The most elegant way to walk a matrix in spiral order

spiral_order.py

def spiral_order(matrix):
    output = []
    if matrix is None:
        return output
    # Copy the matrix to avoid modifying the input
    matrix = [line[:] for line in matrix]
    while matrix:
        output.extend(matrix[0])
        del matrix[0]
        # Transpose the matrix, and then reverse its rows 
        matrix = list(zip(*matrix))[::-1]
    return output