[Python-Dev] PEP 567 -- Context Variables (original) (raw)

Guido van Rossum guido at python.org
Tue Dec 12 22:36:37 EST 2017

Some more feedback:

This proposal builds directly upon concepts originally introduced in :pep:550.

The phrase "builds upon" typically implies that the other resource must be read and understood first. I don't think that we should require PEP 550 for understanding of PEP 567. Maybe "This proposal is a simplified version of :pep:550." ?

The notion of "current value" deserves special consideration: different asynchronous tasks that exist and execute concurrently may have different values. This idea is well-known from thread-local storage but in this case the locality of the value is not always necessarily to a thread. Instead, there is the notion of the "current Context" which is stored in thread-local storage, and is accessed via contextvars.getcontext() function. Manipulation of the current Context is the responsibility of the task framework, e.g. asyncio.

This begs two (related) questions:

I realize the answers are clear, but at this point in the exposition you haven't given the reader enough information to answer them, so this paragraph may confuse readers.

Specification ============= [points 1, 2, 3]

Shouldn't this also list Token? (It must be a class defined here so users can declare the type of variables/arguments in their code representing these tokens.)

The ContextVar class has the following constructor signature: ContextVar(name, *, default=nodefault).

I think a word or two about the provenance of no_default would be good. (I think it's an internal singleton right?) Ditto for NO_DEFAULT in the C implementation sketch.

class Task: def init(self, coro):

Do we need a keyword arg 'context=None' here too? (I'm not sure what would be the use case, but somehow it stands out in comparison to call_later() etc.)

CPython C API ------------- TBD

Yeah, what about it? :-)

The internal immutable dictionary for Context is implemented using Hash Array Mapped Tries (HAMT). They allow for O(log N) set operation, and for O(1) getcontext() function. [...]

I wonder if we can keep the HAMT out of the discussion at this point. I have nothing against it, but given that you already say you're leaving out optimizations and nothing in the pseudo code given here depends on them I wonder if they shouldn't be mentioned later. (Also the appendix with the perf analysis is the one thing that I think we can safely leave out, just reference PEP 550 for this.)

class ContextData

Since this isn't a real class anyway I think the __mapping attribute might as well be named _mapping. Ditto for other __variables later.

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