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io.StrinIO.getvalue() correctly decodes bytes from the underlying
buffer, but does not translate \r\n to \n.
Python 3.0x (py3k, Aug 26 2007, 14:39:16) [MSC v.1400 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import io
>>> a = io.StringIO()
>>> a.write('\n')
2
>>> a.getvalue()
'\r\n'
The attached patch corrects this and adds a test.

As far as I know, StringIO should not do any string transformations.
From PEP-3116 "New I/O", last paragraph of the section "Text I/O":
> Another implementation, StringIO, creates a file-like TextIO
> implementation without an underlying Buffered I/O object. [...] It
> does not support encodings or newline translations; you always read
> back exactly the characters you wrote.

> As far as I know, StringIO should not do any string transformations.
(Not sure if you agree with the patch or not)
That's why the current behaviour is not correct: When I write('\n'),
getvalue() currently returns '\r\n'.

> That's why the current behaviour is not correct: When I write('\n'),
> getvalue() currently returns '\r\n'.
Oh, I missed your example in your initial message. So yes, I agree that
StringIO shouldn't translate newlines like that. I attached a patch that
should fix the bug.
However, I would favor removing the "newline" keyword argument, instead.

Here is a new version of the class, which removes the 'newline' argument
from the constructor.
I also removed the 'encoding' argument, since this is really an
implementation detail of the underlying buffer.
Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 57564)
+++ Lib/io.py	(working copy)
@@ -1390,10 +1390,10 @@
 # XXX This is really slow, but fully functional
- def __init__(self, initial_value="", encoding="utf-8", newline=None):
+ def __init__(self, initial_value=""):
 super(StringIO, self).__init__(BytesIO(),
- encoding=encoding,
- newline=newline)
+ encoding="utf-8",
+ newline='\n')
 if initial_value:
 if not isinstance(initial_value, basestring):
 initial_value = str(initial_value)

I'ev fixed this slightly differently, by simply changing the *default*
of the newline argument to StringIO to be "\n". This is a good default;
but I see no reason to prevent users from messing with it if they have a
need.