Immutable object thread-safety

Alcari The Mad AlcariTheMad at
Mon Oct 27 02:53:50 CET 2008

Laszlo Nagy wrote:
>> Also, the other question is the operation st = 'ThreadBWasHere' is
>> atomic? 
> I think this is the same question. And I believe it is not atomic,
> because it is actually rebinding a name. Consider this:
> a,b = b,a
> This will rebind both a and b. In order to be correct, it MUST happen in
> two phases: first calculate the right side, then do the rebind to the
> names on the left side. The expression on the right side can be anything
> that executes for a long time, and can even rebind 'a' and 'b' several
> times, and will probably be paused by other threads etc. So the
> assignment above cannot be atomic.

You are correct.
a,b = b,a
yields the bytecode:

  1           0 LOAD_NAME                0 (b)
              3 LOAD_NAME                1 (a)
              6 ROT_TWO
              7 STORE_NAME               1 (a)
             10 STORE_NAME               0 (b)

Which is most definitely not atomic.

> I strongly feel that if such an assignment is not atomic for "a,b = b,a"
> then int wont be atomic for "a+=b" or eveb "a=b" or any other
> assignment. However, I can be completely wrong. :-)

'a = b' requires two bytecodes:

  1           0 LOAD_NAME                0 (b)
              3 STORE_NAME               1 (a)

>> I mean, if Python does not guarantee if an immutable object
>> assignment is atomic, then how can we say that the object is thread-
>> safe? 
> An assigment rebinds name to an object. It is assignment to a name.
> Assigning immutable and mutable objects to names are not specially
> distinguished.
>> So, if it is an atomic operation, which operators are atomic,
>> means only assignment'='? 
> I don't think that '=' operator is atomic, see above.
>> I am confused about which data structure to rely on thread-safety, or
>> operator in Python?
> The immutable object itself will never change state, so it is thread
> safe. The problem is not with the object, but the dictionary which holds
> the name 'b' and 'a' and 'st' in your example. It is mutable, and so
> must be protected in a threaded application.

I would suggest reading up on using the 'with' statement and

> Best,
>  Laszlo

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