Safely add a key to a dict only if it does not already exist?
Roy Smith
roy at panix.com
Sat Jan 19 08:04:15 EST 2013
In article <50fa1bf1$0$30003$c3e8da3$5496439d at news.astraweb.com>,
Steven D'Aprano <steve+comp.lang.python at pearwood.info> wrote:
> I wish to add a key to a dict only if it doesn't already exist, but do it
> in a thread-safe manner.
>
> The naive code is:
>
> if key not in dict:
> dict[key] = value
>
>
> but of course there is a race condition there: it is possible that
> another thread may have added the same key between the check and the
> store.
>
> How can I add a key in a thread-safe manner?
You want something along the lines of:
from threading import Lock
lock = Lock()
[...]
lock.acquire()
if key not in dict:
dict[key] = value
lock.release()
You probably want to wrap that up in a context manager to ensure the
lock is released if you get an exception. You don't want your entire
program to hang just because somebody handed you a key which wasn't
hashable, for example.
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