看下面的例子就会明白了：
print '|','*'.ljust(10),'|'
print '|','*'.ljust(10,'-'),'|'
print '|','*'.rjust(10,'-'),'|'
print '|','*'.center(10,'-'),'|'
for a in range(1, 6):
print 'a = '.ljust(5), repr(a).ljust(10), 'b = '.ljust(5), repr(a * 2)
输出结果:
| * |
| *--------- |
| ---------* |
| ----*----- |
a = 1 b = 2
a = 2 b = 4
a = 3 b = 6
a = 4 b = 8
a = 5 b = 10
