Let me tell you, since I was there.

StringIO was created in order to fit code designed to a file, where all you want to do is capture its output and process it further, in the same process. (Or vice versa for the reading case of course.) IOW its *primary* feature is that it is a duck type for a file, and that is what it's optimized for. Also note that it only applies to use cases where the data does, indeed, fit in the process's memory somewhat easily -- else you should probably use a temporary file. If the filesystem were fast enough and temporary files were easier to use we wouldn't have needed it.