>>> import sys, copy
>>> myerr = copy.copy(sys.stderr)
>>> def write2(str):
...     print 'hoo ' + str
...     
>>> myerr.write = write2
>>> sys.stderr = myerr
>>> 1/0
Traceback (most recent call last):
  File "<input>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero
>>> myerr.write('waa')
hoo waa
>>> sys.stderr.write('waa')
waa