question about mutex.py

[m_palmer45 at yahoo.ca]

Hi, I was looking at the code in the standard lib's mutex.py, which is
used for queuing function calls. Here is how it lets you acquire a
lock:


    def testandset(self):
        """Atomic test-and-set -- grab the lock if it is not set,
        return True if it succeeded."""
        if not self.locked:
            self.locked = 1
            return True
        else:
            return False

Question: Is that really atomic? With multiple threads, couldn't the
state of self.locked change between the 'if' clause and the
'self.locked=1' ?
If so, wouldn't a possible fix be to delete and set an 'unlocked' flag:

   def testandset(self):
       try:
            del self.unlocked    # this should be atomic, shouldn't it
            return True
       except AttributeError:
            return False

? Any advice welcome. M.