* To unit-test a script that does a lot of printing, by redirecting sys.stdout to a StringIO instance for easy analysis;
* To create a guaranteed well-formed XML document (a custom API request) using ElementTree and then write it for sending via a HTTP connection.
* Not that you need StringIO often, but sometimes it's pretty useful.
* StringIO gives you file-like access to strings, so you can use an existing module that deals with a file and change almost nothing and make it work with strings.
* For example, say you have a logger that writes things to a file and you want to instead send the log output over the network. You can read the file and write its contents to the network, or you can write the log to a StringIO object and ship it off to its network destination without touching the filesystem. StringIO makes it easy to do it the first way then switch to the second way.
In cases where you want a file-like object that ACTS like a file, but is writing to an in-memory string buffer: StringIO is the tool. If you're building large strings, such as plain-text documents, and doing a lot of string concatenation, you might find it easier to just use StringIO instead of a bunch of mystr += 'more stuff\n' type of operations.