File extensions

It is a convention to indicate a file’s type by putting a dot followed by a code at the end of its name.

The code is called the file’s extension and is often — but not always — three letters long.

This is only a convention but it’s a very useful one, that’s helpful to humans and computers: it allows you to reliably guess what kind of file something is.

For example, assignment-3.pdf has the extension “pdf”. This suggests it is a PDF document (“Portable Document Format”), and not a spreadsheet or a Python program.

However, it is only a convention: the extension is only advisory, and really it’s just part of the name. Most graphical user interfaces will warn you if you try to change the extension on a filename, because it often causes confusion later.

You put a tiger in a box, and put a label on the box that says TIGER. That might be helpful to the next person who opens the box. If you then change the label (that’s like renaming the file) to FOOTBALLS, obviously the box still contains a tiger.

Changing the name doesn’t affect its contents. But it will probably affect how people treat that box, and what happens when they open it.

It’s very common for your computer to choose which application or program to use to open a file based on its extension — so be very careful about deviating from this convention. There are other ways to determine what type of thing is really inside a file, but it’s not as straightforward as you might expect so the file extension convention is very widely used.

However, in computer science you are very often dealing directly with files, so you can and should call them what you want. From your point of view, the filename, including the extension, is often just a name. Be disciplined about how you use extensions.

A filename does not need to have an extension. Directories — which in many ways are really just another kind of file — typically do not.

A filename can contain more than one dot. The extension is comprised of the characters after the last dot.

In earlier versions of some operating systems, filenames needed to have a three-letter extension. That is no longer the case, but it’s where the convention of three-letters comes from.