The doc says that StringIO.truncate should not change the current position.
Consider this code:

  try:
    import StringIO
  except ImportError:
    import io as StringIO
  buf = StringIO.StringIO()
  assert_equal(buf.getvalue(), "")
  print("buf: %r" % buf.getvalue())

  buf.write("hello")
  print("buf: %r" % buf.getvalue())
  assert_equal(buf.getvalue(), "hello")
  buf.truncate(0)
  print("buf: %r" % buf.getvalue())
  assert_equal(buf.getvalue(), "")

  buf.write("hello")
  print("buf: %r" % buf.getvalue())
  assert_equal(buf.getvalue(), "\x00\x00\x00\x00\x00hello")
  buf.truncate(0)
  print("buf: %r" % buf.getvalue())
  assert_equal(buf.getvalue(), "")


On Python 3.6, I get the output:

buf: ''
buf: 'hello'
buf: ''
buf: '\x00\x00\x00\x00\x00hello'

On Python 2.7, I get the output:

buf: ''
buf: 'hello'
buf: ''
buf: 'hello'


Thus it seems that Python 2.7 StringIO.truncate does actually resets the position for this case or there is some other bug in Python 2.7. At least from the doc, it seems that the Python 3.6 behavior is the expected behavior.

For the record, the StringIO module is Python 2 only (replaced by io.StringIO and io.BytesIO for Py3, which are also available on 2.6 and higher if you need consistency), and only documents truncate by reference to the generic docs for the built-in file object. Admittedly, the built-in file object docs say the position shouldn't change on truncate, but given:

1. StringIO is deprecated and no longer maintained
2. Code using it may rely on this incorrect behavior

I can't see fixing this beyond possibly mentioning the quirk in the 2.x StringIO docs. Since io.StringIO is correct, there is nothing to fix in 3.x.

Python 2 is EOL.