How safe is modifying locals()?

Paul Paterson paulpaterson at
Sat Jul 26 04:01:33 CEST 2003

Steven Taschuk wrote:

> Quoth Paul Paterson:
>   [...]
>>Does anyone have any alternative strategies for trying to achieve the 
>>objective? A "Pythonic" approach such as,
>>def change(x, y):
>>    x = x + 1
>>    y = y + 1
>>    return y
>>x = 0; y = 0
>>y = change(x, y)
>>Works in a single threaded environment but, because the value of 'y' 
>>changes at the wrong time, with multiple threads there is a possibility 
>>that two users of 'y' could dissagree on its value.
> Even if the simple
>     y = y + 1
> could change the caller's binding, there would be a race condition
> -- you'll need a lock anyway.  (I'd be a bit surprised if this
> were not so in VB as well.  Are statements atomic in VB?)

AFAIK in VB, if 'y' is a global (or in some other way accessible via two 
threads) then concurrent changes are managed behind the scenes by VB - 
presumably either some locking is occuring or the assignment statement 
is atomic. To be honest though, I'm not sure. Threading usually occurs 
either by using timers or COM. I have seen issues where you have to 
manually implement semaphores when using timers and COM together.

That asside, you make an excellent point. The thread safety and ByRef 
issues are probably orthogonal and I should probably look for two 
complementary solutions.

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