s.decode("utf-8")

sometimes silently truncates the result if s has more than 2E9 Bytes,
sometimes raises a fairly incomprehensible exception:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/lib64/python2.5/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
TypeError: utf_8_decode() argument 1 must be (unspecified), not str