The "cursor" attribute in a QTextEdit.ExtraSelection() object cannot be changed
or replaced. Example:

---
from PySide.QtGui import *
from PySide.QtCore import *

import sys

if __name__ == '__main__':
  app = QApplication(sys.argv)

  textEdit = QPlainTextEdit()
  textEdit.insertPlainText("test 123")
  selection = QTextEdit.ExtraSelection()

  print "Text cursor in ExtraSelection:"

  print selection.cursor
  selection.cursor = textEdit.textCursor()
  print selection.cursor
  #selection.cursor still points to the same object as before...

  print selection.cursor.position()
  #8
  selection.cursor.setPosition(0)
  print selection.cursor.position()
  #8  

  print "Normal text cursor:"

  cursor = textEdit.textCursor()
  print cursor.position()
  #8
  cursor.setPosition(0)
  print cursor.position()
  #0
---