Are Lists thread safe?

Gabriel Genellina gagsl-py2 at
Sat Mar 10 02:22:14 CET 2007

En Fri, 09 Mar 2007 16:50:04 -0300, abcd <codecraig at> escribió:

> I guess this might be overkill then...

That depends on your target. For the *current* CPython implementation,  
yes, because it has an internal lock. But other versions (like Jython or  
IronPython) may not behave that way.

> class MyList(list):
>     def __init__(self):
>         self.l = threading.Lock()

Better to use an RLock, and another name instead of l:
	self.lock = threading.RLock()
(A method may call another, and a Lock() won't allow that)

>     def append(self, val):
>         try:
>             self.l.acquire()
>             list.append(self, val)
>         finally:
>             if self.l.locked():
>                 self.l.release()

I'd write it as:

      def append(self, val):
              list.append(self, val)
> ....performing the same locking/unlocking for the other methods (i.e.
> remove, extend, etc).

Note that even if you wrap *all* methods, operations like mylist += other  
are still unsafe.

py> def f(self): self.mylist += other
py> import dis; dis.dis(f)
   1           0 LOAD_FAST                0 (self)
               3 DUP_TOP
               4 LOAD_ATTR                0 (mylist)
               7 LOAD_GLOBAL              1 (other)
              10 INPLACE_ADD
              11 ROT_TWO
              12 STORE_ATTR               0 (mylist)
              15 LOAD_CONST               0 (None)
              18 RETURN_VALUE

INPLACE_ADD would call MyList.__iadd__ which you have wrapped. But you  
have a race condition between that moment and the following STORE_ATTR, a  
context switch may happen in the middle.

It may not be possible to create an absolutely thread-safe list without  
some help on the client side. (Comments, someone?)

Gabriel Genellina

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