Suppose, not entirely hypothetically, that you have a tool that prints out records (one per line) and each record has a bunch of information associated with it, which you print out in columns. You'd like to provide a way for people to control which columns of information are printed for the records. If there's only a few options, maybe you can do this with a few different format strings using the traditional "%s %s %s" approach of positional formatting (because you're old fashioned and haven't really updated to the modern world of string formatting ), but this doesn't really scale up very well; you rapidly get into a massive explosion of options and formatting.

As I was contemplating exactly this issue for a tool of mine, it belatedly occurred to me that the solution I wanted was named format arguments, instead of positional ones. Named format arguments have two great advantages here. First, you can shuffle the order that they occur in within the format string without having to change the arguments. Second, you don't have to use all of them; Python is perfectly happy if you supply extra named arguments to your string formatting that aren't used.

This means that you can simply build up a big dictionary of all of your available information for a given record (perhaps even in multiple formats, for example if you have an option to print numbers precisely or abbreviate them to K, M, G, and so on), and then either pick a formatting string or assemble it from pieces based on what columns you want to print (and how). Then you can just do the actual formatting with:

outstr = fmtstr.format_map(datadict)

It doesn't matter that you supplied (way) more information in your datadict than your assembled or chosen format string uses, or what order your format string puts things. Everything just works.

(You can use ' fmtstr % datadict ' instead if you want to. I'm not sure which I'll use, but a bit of me feels that I should switch to modern Python string formatting instead of sticking with the old printf style of '%', even if it allows named arguments too.)

This feels like something that I should have realized long ago, back when named ('keyword') format arguments were added to Python, but for some reason it never clicked until now. Several of my programs are probably going to start providing a lot more options for formatting their output.