[Python-Dev] A few lessons from the tempfile.py rewrite

[Martin Sjögren]

lör 2002-08-17 klockan 00.30 skrev Zack Weinberg:
> 2) pthread_once equivalent.
> 
> pthread_once is a handy function in the C pthreads library which
> can be used to guarantee that some data object is initialized exactly
> once, and no thread sees it in a partially initialized state.  I had
> to implement a fake version in tempfile.py.
> 
> | _once_lock = _allocate_lock()
> | 
> | def _once(var, initializer):
> |     """Wrapper to execute an initialization operation just once,
> |     even if multiple threads reach the same point at the same time.
> | 
> |     var is the name (as a string) of the variable to be entered into
> |     the current global namespace.
> | 
> |     initializer is a callable which will return the appropriate initial
> |     value for variable.  It will be called only if variable is not
> |     present in the global namespace, or its current value is None.
> | 
> |     Do not call _once from inside an initializer routine, it will deadl=
ock.
> |     """
> | 
> |     vars = globals()
> |     # Check first outside the lock.
> |     if vars.get(var) is not None:
> |         return

Wouldn't it make more sense to use has_key (or 'in')? If var is assigned
to None before _once is called, that value is overridden...

> |     try:
> |         _once_lock.acquire()
> |         # Check again inside the lock.
> |         if vars.get(var) is not None:
> |             return
> |         vars[var] = initializer()
> |     finally:
> |         _once_lock.release()


I agree that pthread_once is useful, and it would be nice to have
something like this in the standard library.


Regards,
Martin