The previous page showed the following diagram of web-based programming.
Let's look at this as application developers and focus on the software components:
You can see that I have now labelled the arrows with the protocols of immediate interest to us. HyperText Transport Protocol (HTTP) used to communicate from client to server and CGI used both to communicate between client and server and to communicate between programs on the server.
CGI is commonly used as a shorthand for "web programming". It actually only refers to the Common Gateway Interface which isn't a language or a platform but simply a specification that describes how data from a browser should be encoded to be sent to the server. Because that is a necessary component of web programming, and also perhaps because it was the first component to be favoured with an acronym, it is often used as an umbrella term for much of web programming.
A decade ago one had to write one's own code to parse CGI encoded messages, but the standard has been around long enough that standard libraries have evolved to take care of it and it has become almost transparent to the programmer. Transparent enough that we can further simplify this diagram and write our programs (mostly) as though the situation was this:
which is not so different from the console-based programs we had been writing:
Conceptually all we have to do is replace our text print statements with
ones that output HTML instead of plain text (since this is being sent to a
web browser), and to replace raw_input statements with ones that read in
information from a web form.
The first could be done just by using a lot more print statements, since HTML is a verbose format compared to plain text, but we will use string interpolation into triple-quoted strings to make it simpler.
The second is easier than you might think since Python provides a module that will take the encoded data from an HTML input form and decode it into a dictionary whose keys are the names of the form elements.
Before we can explore those however we need to set up a web server to use.