What kind of "thread safe" are deque's actually?

[Jack Dangler]

On 3/29/23 02:08, Chris Angelico wrote:
> On Wed, 29 Mar 2023 at 16:56, Greg Ewing via Python-list
> <python-list at python.org> wrote:
>> On 28/03/23 2:25 pm, Travis Griggs wrote:
>>> Interestingly the error also only started showing up when I switched from running a statistics.mean() on one of these, instead of what I had been using, a statistics.median(). Apparently the kind of iteration done in a mean, is more conflict prone than a median?
>> It may be a matter of whether the GIL is held or not. I had a look
>> at the source for deque, and it doesn't seem to explicitly do
>> anything about locking, it just relies on the GIL.
>>
>> So maybe statistics.median() is implemented in C and statistics.mean()
>> in Python, or something like that?
>>
> Both functions are implemented in Python, but median() starts out with
> this notable line:
>
>      data = sorted(data)
>
> which gives back a copy, iterated over rapidly in C. All subsequent
> work is done on that copy.
>
> The same effect could be had with mean() by taking a snapshot using
> list(q) and, I believe, would have the same effect (the source code
> for the sorted() function begins by calling PySequence_List).
>
> In any case, it makes *conceptual* sense to do your analysis on a copy
> of the queue, thus ensuring that your stats are stable. The other
> threads can keep going while you do your calculations, even if that
> means changing the queue.
>
> ChrisA
Sorry for any injected confusion here, but that line "data = 
sorted(data)" appears as though it takes the value of the variable named 
_data_, sorts it and returns it to the same variable store, so no copy 
would be created. Am I missing something there?