By the way Filip, you were told on the Stackoverflow page that the output was correct and that you were not using doctest correctly. Serhiy also hinted to you that you should check the output in the REPL and you falsely claimed that it gave the expected output, but it doesn't:

    py> multiline_output()
    'First line\nSecond line\n'

which is nothing like the output you put in your doctest.

This is not a bug in doctest. The very first line of the doctest documentation says:

    The doctest module searches for pieces of text that look like
    interactive Python sessions

so this is expected and documented behaviour, not a bug.