Some of my tests use io.StringIO and assert that captured print output equals expected output. Until now, I reused one buffer by truncating between tests. I recently replaced 3.1.1 with 3.1.2 (WinXP) and the *second* test of each run started failing. The following minimal code shows the problem (and should suggest a new unit test):

from io import StringIO; s = StringIO(); print(repr(s.getvalue()))
print('abc', file=s); print(repr(s.getvalue()))
s.truncate(0); print(repr(s.getvalue()))
print('abc', file=s); print(repr(s.getvalue()))

prints (both command window and IDLE)

''
'abc\n'
''
'\x00\x00\x00\x00abc\n' 
# should be and previously would have been 'abc\n'

s.truncate(0) zeros the buffer and appears to set the length to 0, but a subsequent print sees the length as what it was before the truncate and appends after the zeroed characters. Ugh.

I presume the problem is StringIO-emulation specific but have not tested 'real' files to be sure.

---
also...

>>> help(s.truncate)
Help on built-in function truncate:

truncate(...)
    Truncate size to pos.
...

should be, for greater clarity, something like

truncate([pos])
    Truncate the size of the file or buffer to pos
...