把字体显示在屏幕的中间
sentence = raw_input("Sentence:")
screen_width = 80text_width = len(sentence)box_width = text_width + 6left_margin = (screen_width - box_width) // 2
printprint ' '*left_margin + '+' + '-'*(box_width) + '+'print ' '*(left_margin+2) + '| ' + ' '*text_width + ' |'print ' '*(left_margin+2) + '| ' + sentence + ' |'print ' '*(left_margin+2) + '| ' + ' '*text_width + ' |'print ' '*left_margin + '+' + '-'*(box_width) + '+'简单地定义了屏幕的宽度，没有去取实际值，后面在更新
