Accessing `self.x` vs accessing a locally scoped equivalent?

does_thread_issue_exist.py

class MySharedObject(object):
  """
  If using a locally scoped dictionary to check for duplicates,
  do the same thread issues arise?
  Note that I don't care about what gets stored -- it should be 
  the same end-result regardless of which threads do what, the
  ValueError is mainly a developer-awareness issue.
  """
  
  def do_stuff(self):
    self.var = {}
    seen = {} # the duplicate detector
    iterable = self.collect_some_stuff()
    for x in iterable:
      if x in seen:
        raise ValueError('Duplicate detected')
      seen[x] = 1
      self.var[x] = 1
    self._built = True

thread_issue.py

class MySharedObject(object):
  """
  If two threads hit do_stuff in tandem, ValueError may get raised 
  because the other thread may have already put `x` into `self.var`
  """
  
  def do_stuff(self):
    self._built = False
    self.var = {}
    iterable = self.collect_some_stuff()
    for x in iterable:
      if x in self.var:
        raise ValueError('Duplicate detected')
      self.var[x] = 1
    self._built = True