Programming Python, 3rd Edition

Saving State Information in CGI Scripts

One of the most unusual aspects of the basic CGI model, and one of its starkest contrasts to the GUI programming techniques we studied in the prior part of this book, is that CGI scripts are stateless—each is a standalone program, normally run autonomously, with no knowledge of any other scripts that may run before or after. There is no notion of things such as global variables or objects that outlive a single step of interaction and retain context. Each script begins from scratch, with no memory of where the prior left off.

This makes web servers simple and robust—a buggy CGI script won’t interfere with the server process. In fact, a flaw in a CGI script generally affects only the single page it implements, not the entire web-based application. But this is a very different model from callback-handler functions in a single process GUI, and it requires extra work to remember things longer than a single script’s execution.

Lack of state retention hasn’t mattered in our simple examples so far, but larger systems are usually composed of multiple user interaction steps and many scripts, and they need a way to keep track of information gathered along the way. As suggested in the last two sections, generating query parameters on URL links and hidden form fields in reply pages are two simple ways for a CGI script to pass data to the next script in the application. When clicked or submitted, such parameters send preprogrammed selection or session information ...