I noticed that io.StringIO is inconsistent with open on how newlines are 
handled. The default approach open uses is universal newlines:

>>> with open('foo', 'w') as f:
...   f.write('hello hi\r\nla la\r\n')
... 
17
>>> open('foo').readlines()
['hello hi\n', 'la la\n']

io.StringIO, however, defaults to just treating \n as newlines:

>>> io.StringIO('hello hi\r\nla la \r\n').readlines()
['hello hi\r\n', 'la la \r\n']

The attached patch changes this so that if the newline keyword isn't 
specified, then StringIO will act just like open with respect to 
keywords. It will then produce this:

>>> io.StringIO('hello hi\r\nla la \r\n').readlines()
['hello hi\n', 'la la \n']

This is the intended behavior. It is consist with 2.x's
StringIO.StringIO behavior.

Thanks Benjamin. Could the documentation for StringIO be updated then? The 
docs for io.StringIO [1] says that io.TextIOWrapper implements the 
constructor, but TextIOWrapper defaults newline to None [2], and there's 
nothing that states that io.StringIO changes the default to newline='\n'.

[1]: http://docs.python.org/3.0/library/io.html#io.StringIO
[2]: http://docs.python.org/3.0/library/io.html#io.TextIOWrapper

Moving this to a documentation bug here: http://bugs.python.org/issue5452