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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.

I think this is not quite right; you shouldn't return self unless it is
an exact string instance. If it is a subclass of PyString should make a
copy.

I'm not sure why a string subclass shouldn't work, but I've attached a new 
version of the patch that calls PyString_CheckExact() to prevent using a 
string subclass.

In the absence of any more feedback, I checked this in as r59420.