The topic of avoiding string copies in certain string methods came up in 
the
ChiPy list:

  http://mail.python.org/pipermail/chicago/2007-December/002975.html.

The attached patch modifies the split and rsplit implementations to 
avoid
making a copy of self when the split fails to find anything to split on:

    >>> s = "abc def"
    >>> x = s.split(';')
    >>> x[0] is s
    True
    >>> y = s.rsplit('-')
    >>> y[0] is s
    True
    >>> t = "abcdef"
    >>> x = t.split()
    >>> x[0] is t
    True
    >>> y = t.rsplit()
    >>> y[0] is t
    True

All tests pass.  Given that this is just a small optimization I
don't believe any changes to the docs or the existing tests are
necessary.