@pitrou: You're right, but the state have to be changed for the 
encoder, not the decoder. I added the following code to TextIOWrapper 
constructor (for the C and the Python version of io library):

        if self._seekable and self.writable():
            position = self.buffer.tell()
            if position != 0:
                self._encoder = self._get_encoder()
                self._encoder.setstate(0)