[Python-Dev] [RFC] Removing pure Python implementation of OrderedDict

Paul Moore p.f.moore at gmail.com
Wed Sep 6 06:40:33 EDT 2017

On 6 September 2017 at 11:09, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Wed, 6 Sep 2017 11:26:52 +0900
> INADA Naoki <songofacandy at gmail.com> wrote:
>> Like that, should we say "atomic & threadsafe __setitem__ for simple
>> key is implementation detail of CPython and PyPy.  We recommend
>> using mutex when using OrderedDict from multiple thread."?
> I think you may be overstating the importance of making OrderedDict
> thread-safe.  It's quite rare to be able to rely on the thread safety
> of a single structure, since most often your state is more complex than
> that and you have to use a lock anyway.
> The statu quo is that only experts rely on the thread-safety of list
> and dict, and they should be ready to reconsider if some day the
> guarantees change.

Agreed. I wasn't even aware that list and dict were guaranteed
threadsafe (in the language reference). And even if they are, there's
going to be a lot of provisos that mean in practice you need to know
what you're doing to rely on that. Simple example:

    mydict[the_value()] += 1

isn't thread safe, no matter how thread safe dictionaries are.

I don't have a strong opinion on making OrderedDict "guaranteed thread
safe" according to the language definition. But I'm pretty certain
that whether we do or not will make very little practical difference
to the vast majority of Python users.


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