[Python-Dev] PEP 567 pre v3 (original) (raw)

Ethan Furman ethan at stoneleaf.us
Thu Jan 11 01:39:05 EST 2018

On 01/10/2018 10:23 PM, Yury Selivanov wrote:

On Thu, Jan 11, 2018 at 4:44 AM, Nathaniel Smith wrote:

It may have gotten lost in that email, but my actual favorite approach is that we make the signatures:

ContextVar(name, , initialvalue) # or even (, name, initialvalue) ContextVar.get() ContextVar.set(value) so that when you create a ContextVar you always state the initial value, whatever makes sense in a particular case. (Obviously None will be a very popular choice, but this way it won't be implicit, and no-one will be surprised to see it returned from get().) Alright, you've shown that most of the time when we use threading.local in the standard library we subclass it in order to provide a default value (and avoid AttributeError being thrown). This is a solid argument in favour of keeping the 'default' parameter for the ContextVar constructor. Let's keep it.

[...]

I think that when you have an int stored in a context variable it would usually make sense to give it a 0 default (or some other number). However, for a complex object (like current request object) there is no sensible default value sometimes. Forcing the user to set it to None feels like a badly designed API that forces the user to work around it.

Therefore I'm still in favour of keeping the current PEP 567 behaviour.

To be clear: We'll now be able to specify a default when we create the variable, but we can also leave it out so a LookupError can be raised later?

-- Ethan

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