字符串在输出时的对齐:
S.ljust(width,[fillchar]) 
#输出width个字符，S左对齐，不足部分用fillchar填充，默认的为空格。 
S.rjust(width,[fillchar]) #右对齐 
S.center(width, [fillchar]) #中间对齐 
S.zfill(width) #把S变成width长，并在右对齐，不足部分用0补足 

1 >>> str = "this is string example....wow!!!";
2 >>> str.ljust(50,'0')
3 'this is string example....wow!!!000000000000000000'
4 >>> str.ljust(50)
5 'this is string example....wow!!!                  '
6 >>> str.rjust(50)
7 '                  this is string example....wow!!!'
8 >>> str.rjust(50,'0')
9 '000000000000000000this is string example....wow!!!'
10 >>> str.center(50,'0')
11 '000000000this is string example....wow!!!000000000'
12 >>> str.center(50)
13 '         this is string example....wow!!!         '
14 >>> str.zfill(50)
15 '000000000000000000this is string example....wow!!!'