这个叫格式化字符串。

>>> uid = "sa"
>>> pwd = "secret"
>>> print pwd + " is not a good password for " + uid
secret is not a good password for sa
>>> print "%s is not a good password for %s" % (pwd, uid) 
secret is not a good password for sa
>>> userCount = 6
>>> print "Users connected: %d" % (userCount, )            
Users connected: 6
>>> print "Users connected: " + userCount                 
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
TypeError: cannot concatenate 'str' and 'int' objects

更多可以看 pe Into Python - 格式化字符串

其他语言的printf

>>>x = "a %d c" % 10  
>>>print x
a 10 c