thread lock object.

[Peter Hansen]

Irmen de Jong wrote:
> ajikoe at gmail.com wrote:
>> How can we make only one thread at a time be able to access and
>> increment i ?
> 
> Use a synchronization primitive such as a lock
> (threading.Lock, threading.RLock)
> 
> But for simply incrementing a number (i+=1) this is not needed
> because that operation cannot be interrupted by another thread,
> as far as I know.

Most assuredly it can:

 >>> def f():
...   global x
...   x += 1
...
 >>> dis.dis(f)
   3           0 LOAD_GLOBAL              0 (x)
               3 LOAD_CONST               1 (1)
               6 INPLACE_ADD
...a context-change here can lead to incorrect results...
               7 STORE_GLOBAL             0 (x)
              10 LOAD_CONST               0 (None)
              13 RETURN_VALUE

-Peter