RajNewbie <raj.indian...@gmail.com> wrote:
> Say, I have two threads, updating the same dictionary object - but for
> different parameters:
> Please find an example below:
> a = {file1Data : '',
> file2Data : ''}
>
> Now, I send it to two different threads, both of which are looping
> infinitely:
> In thread1:
> a['file1Data'] = open(filename1).read
> and
> in thread2:
> a['file2Data'] = open(filename2).read
>
> My question is - is this object threadsafe? - since we are working on
> two different parameters in the object. Or should I have to block the
> whole object?
>
It depends exactly what you mean by 'threadsafe'. The GIL will guarantee
that you can't screw up Python's internal data structures: so your
dictionary always remains a valid dictionary rather than a pile of bits.
However, when you dig a bit deeper, it makes very few guarantees at the
Python level. Individual bytecode instructions are not guaranteed
atomic: for example, any assignment (including setting a new value into
the dictionary) could overwrite an existing value and the value which is
overwritten may have a destructor written in Python. If that happens you
can get context switches within the assignment.
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.
You will probably find it less error prone in the long run if you get
your threads to write (key,value) tuples into a queue which the
consuming thread can read and use to update the dictionary.
--
Duncan Booth http://kupuguy.blogspot.com
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