Are python objects thread-safe?

Duncan Booth duncan.booth at invalid.invalid
Tue Dec 23 08:30:25 EST 2008

Aaron Brady <castironpi at> wrote:

> Th.1   Th.2
> a=X
>        a=Y
> a=Z
> You are saying that if 'a=Z' interrupts 'a=Y' at the wrong time, the
> destructor for 'X' or 'Y' might not get called.  Correct?  In serial
> flow, the destructor for X is called, then Y.

No, the destructors will be called, but the destructors can do pretty much 
anything they want so you can't say the assignment is atomic. This isn't 
actually a threading issue: you don't need multiple threads to experience 
werid issues here. If you do strange things in a destructor then you can 
come up with confusing code even with a single thread.

>> Other nasty things can happen if you use dictionaries from multiple
>> threads. You cannot add or remove a dictionary key while iterating over
>> a dictionary. This isn't normally a big issue, but as soon as you try to
>> share the dictionary between threads you'll have to be careful never to
>> iterate through it.
> These aren't documented, IIRC.  Did you just discover them by trial
> and error?
It is documented, but I can't remember where for Python 2.x. For Python 3, 
PEP 3106 says: "As in Python 2.x, mutating a dict while iterating over it 
using an iterator has an undefined effect and will in most cases raise a 
RuntimeError exception. (This is similar to the guarantees made by the Java 
Collections Framework.)"

Duncan Booth

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