XML : AttributeError: 'str' object has no attribute 'compute_mark_list' Python

I got a error message in Python:

  AttributeError: 'str' object has no attribute 'compute_mark_list'    

in my code.

This code suppose to read though a XML file and get the content inside its children:

  import libxml2  import sys    '''  purpose      store the information from an answer element  '''  class Answer:      def __init__(self, index, path, result, answer, time):          self.index = index          self.path = path          self.result = result          self.answer = answer          self.time = time    '''  purpose      Store the information from a display element.  '''  class Display:      def __init__(self, index, path, time):          self.index = index          self.path = path          self.time = time    '''  purpose      Extract the information from log_file and return it as a list      of answer and display objects.  preconditions      log_file is the name of a legal, readable quiz log XML file  '''  def load_quiz_log(log_file):      # parse the XML file in argv[2] and load it into memory      parse_tree = libxml2.parseFile(sys.argv[1])        # create a context for tree traversal      context = parse_tree.xpathNewContext()        root = parse_tree.getRootElement()        log_file = root.children        #create a list to hold the objects        L = [ ]        while log_file is not None:          if log_file.name == 'answer':              answer = log_file.content          if log_file.name == 'display':              display = log_file.content          log_file = log_file.next        L = [answer, display]        return L    '''  purpose      Return the number of distinct questions in log_list.  preconditions      log_list was returned by load_quiz_log  '''  def compute_question_count(log_list):      while log_list is not None:          if log_list.name == 'answer':              count +=  1          else:              break         log_list = log_list.next      return count    '''  purpose      Extract the list of marks.      For each index value, use the result from the last non-empty answer,      or 0 if there are no non-empty results.  preconditions      log_list was returned by load_quiz_log  '''  def compute_mark_list(log_list):      marks = [ ]      while log_list is not None:          if log_list.name == 'answer':              log_list_answer = log_list.children              while log_list_answer is not None:                  if log_list_answer.name == 'index':                      index = 0                      if log_list_answer.content == index:                          result = log_list_answer.next.next.content                          index = log_list_answer.content                           marks.append(result)                                          else:                          continue              log_list_answer = log_list_answer.next      log_list = log_list.next      return marks    

When I run this code I get the error message in the title

  import libxml2  import sys  import quiz_library    print '-------------------- test load_quiz_log'    log_list = quiz_library.load_quiz_log(sys.argv[1])    for x in log_list:      if isinstance(x, quiz_library.Answer):          print 'Answer:', x.index, x.path, x.result, x.answer, x.time      else:          print 'Display:', x.index, x.path, x.time    print '-------------------- test compute_question_count'    question_count = quiz_library.compute_question_count(log_list)  print question_count    print '-------------------- test compute_mark_list'    mark_list = quiz_library.compute_mark_list(log_list)  print mark_list    

The XML file looks like: