One particularly useful string method is format. The format method is used to construct strings by inserting values into template strings. Consider this example for generating log messages for a hypothetical web server.
log_message = "IP address {} accessed {} at {}".format(user_ip, url, now)
If the variables user_ip, url and now are defined then they will be substituted for the {} placeholder values:
> print(log_message) IP address 208.94.117.90 accessed /bears/koala at 16:20
原文：http://www.cnblogs.com/Answer1215/p/7827928.html