Is there a Python equivalent / pseudo-equivalent to java's OutputStream or PrintWriter?
I want to be able to have a handle that represents either a stream like stdout/sterr, or a file, or something else (a pipe or a socket or a dummy sink) and abstract away what kind of thing it is, so I can just send output to it.
How can I do this?
"Abstracting away what type it is" happens automatically in Python - it's called 'duck typing'. Just pass any file-like object to the function, and have it use the interface of file-like objects.
FWIW, the standard input/output/error streams are represented by stdin, stdout and stderr in the sys module. To get file-like objects that read and write strings, use the StringIO module.