Oh, I like that. It does feel like a property of the variable. (But can you efficiently enumerate all context vars when creating a thread?)
I imagine that thread pools add some complication, because you don’t want to inherit these accidentally between tasks run by the same worker. On Thu, Aug 19, 2021 at 14:28 Paul Prescod <pres...@gmail.com> wrote: > On Thu, Aug 19, 2021 at 8:43 AM Guido van Rossum <gu...@python.org> wrote: > >> Perhaps we need a library for creating/managing threads that inherits all >> current context values? >> > > Or is it a "kind of context variable that is shared among threads?" That > was more the direction my mind was going. > > Context variables that hold explicitly or implicitly immutable values > are safe to share. Context variables that hold values intended to be > mutated: seldom. > > What if it were some kind of ContextVar subclass or flag where you would > say you did or didn't want something shared? > > And for legacy modules like decimal, you could turn on the flag somehow to > get the sharing behaviour. > > Sometimes threads are made by third-party libraries and those libraries > don't know anything about the context. > > In my case, the pattern was: > > 1. my code set context variable > 2. my code configured a third party library with a callback > 3. the third party library used threading for perf reasons > 4. it called my callback, which failed due to the context variable failing > to be inherited > > I believe (after only thinking about it for a day or so) that one can > generally determine the right "sharing rule" at the point of definition of > a ContextVar, based on the semantics and type of the Var. > > On Thu, Aug 19, 2021 at 8:43 AM Guido van Rossum <gu...@python.org> wrote: > >> Perhaps we need a library for creating/managing threads that inherits all >> current context values? >> >> On Thu, Aug 19, 2021 at 7:48 AM Paul Prescod <pres...@gmail.com> wrote: >> >>> There are certain values that are managed independent of any specific >>> code-accessible object. The decimal precision is a good example: >>> >>> https://docs.python.org/3/library/decimal.html >>> >>> We can call these context variables. As your code shows, the true "home" >>> of the "prec" value is not some object you manage in your code, but rather >>> some background object called the Context. >>> >>> The Context is not inherited by sub-threads. >>> >>> Your code is fine, because you did not use threads. Replace the map with >>> concurrent.futures code (as in my example). >>> >>> Your code will not work properly anymore. This is the issue. >>> _______________________________________________ >>> Python-ideas mailing list -- python-ideas@python.org >>> To unsubscribe send an email to python-ideas-le...@python.org >>> https://mail.python.org/mailman3/lists/python-ideas.python.org/ >>> Message archived at >>> https://mail.python.org/archives/list/python-ideas@python.org/message/6YU3RZ6WDWVJI7QEJKICJZN72B5FYOEV/ >>> Code of Conduct: http://python.org/psf/codeofconduct/ >>> >> >> >> -- >> --Guido van Rossum (python.org/~guido) >> *Pronouns: he/him **(why is my pronoun here?)* >> <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/> >> > -- --Guido (mobile)
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