Trying to use threading.local()

[Chris Angelico]

On Fri, Sep 14, 2018 at 5:25 PM, Antoon Pardon <antoon.pardon at vub.be> wrote:
> On 13-09-18 14:29, Chris Angelico wrote:
>> "Preferred" doesn't exclude the possibility that alternatives are
>> needed, though. For example, good luck making decimal.Decimal contexts
>> work correctly without the help of thread-locals - there MIGHT be a
>> way to do it, but even if there is, it sure won't be pretty.
>
> Can you elaborate on that. Suppose I have two threads, one in which I need
> a precision of 3 and the other in which I need a precision of 7. In what
> circumstances is it needed to use threads-locals to accomplish this and
> how do I accomplish this?
>
> If I want my decimal code be usable with threads, how should I write it?

Let's say both of those threads call the same function, which does this:

def f(x, y):
    return x * 3 + y / 4

You can't pass a context as a parameter, so you have to set the
context externally. Which is done thus:

https://docs.python.org/3/library/decimal.html#decimal.setcontext

Note that this sets it *for the current thread*.

ChrisA