When toying with the "with" statement, I fell on this:

Python 2.6.4

>>> with open('abc.txt', 'r') as f:
        for line in f:
            print line.rstrip()
            
abc
def
>>> 
>>> import StringIO
>>> fo = StringIO.StringIO('abc\ndef\n')
>>> fo.seek(0)
>>> with fo as f2:
        for line in f2:
            print line.rstrip()
            
Traceback (most recent call last):
  File "<psi last command>", line 2, in <module>
AttributeError: StringIO instance has no attribute '__exit__'
>>> 
>>> 

Same result with cStringIO

-----

Python 3.1.1

>>> fo = io.StringIO('abc\ndef\n')
>>> fo.seek(0)
0
>>> with fo as f:
	for line in f:
	    print(line.rstrip())

	    
abc
def
>>>

#1286 looks related