<div dir="ltr"><div class="gmail_default"><div class="gmail_default"><font face="monospace, monospace">Hi all,</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">as promised, here is a draft PEP for context variable semantics and implementation. Apologies for the slight delay; I had a not-so-minor autosave accident and had to retype the majority of this first draft.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">During the past years, there has been growing interest in something like task-local storage or async-local storage. This PEP proposes an alternative approach to solving the problems that are typically stated as motivation for such concepts.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">This proposal is based on sketches of solutions since spring 2015, with some minor influences from the recent discussion related to PEP 550. I can also see some potential implementation synergy between this PEP and PEP 550, even if the proposed semantics are quite different.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">So, here it is. This is the first draft and some things are still missing, but the essential things should be there.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">-- Koos</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">||||||||||||||||||||||||||||||||||||||||||||||||||||||||||</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">PEP: 999</font></div><div class="gmail_default"><font face="monospace, monospace">Title: Context-local variables (contextvars)</font></div><div class="gmail_default"><font face="monospace, monospace">Version: $Revision$</font></div><div class="gmail_default"><font face="monospace, monospace">Last-Modified: $Date$</font></div><div class="gmail_default"><font face="monospace, monospace">Author: Koos Zevenhoven</font></div><div class="gmail_default"><font face="monospace, monospace">Status: Draft</font></div><div class="gmail_default"><font face="monospace, monospace">Type: Standards Track</font></div><div class="gmail_default"><font face="monospace, monospace">Content-Type: text/x-rst</font></div><div class="gmail_default"><font face="monospace, monospace">Created: DD-Mmm-YYYY</font></div><div class="gmail_default"><font face="monospace, monospace">Post-History: DD-Mmm-YYYY</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Abstract</font></div><div class="gmail_default"><font face="monospace, monospace">========</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Sometimes, in special cases, it is desired that code can pass information down the function call chain to the callees without having to explicitly pass the information as arguments to each function in the call chain. This proposal describes a construct which allows code to explicitly switch in and out of a context where a certain context variable has a given value assigned to it. This is a modern alternative to some uses of things like global variables in traditional single-threaded (or thread-unsafe) code and of thread-local storage in traditional *concurrency-unsafe* code (single- or multi-threaded). In particular, the proposed mechanism can also be used with more modern concurrent execution mechanisms such as asynchronously executed coroutines, without the concurrently executed call chains interfering with each other's contexts. </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">The "call chain" can consist of normal functions, awaited coroutines, or generators. The semantics of context variable scope are equivalent in all cases, allowing code to be refactored freely into *subroutines* (which here refers to functions, sub-generators or sub-coroutines) without affecting the semantics of context variables. Regarding implementation, this proposal aims at simplicity and minimum changes to the CPython interpreter and to other Python interpreters.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Rationale</font></div><div class="gmail_default"><font face="monospace, monospace">=========</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Consider a modern Python *call chain* (or call tree), which in this proposal refers to any chained (nested) execution of *subroutines*, using any possible combinations of normal function calls, or expressions using ``await`` or ``yield from``. In some cases, passing necessary *information* down the call chain as arguments can substantially complicate the required function signatures, or it can even be impossible to achieve in practice. In these cases, one may search for another place to store this information. Let us look at some historical examples. </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">The most naive option is to assign the value to a global variable or similar, where the code down the call chain can access it. However, this immediately makes the code thread-unsafe, because with multiple threads, all threads assign to the same global variable, and another thread can interfere at any point in the call chain.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">A somewhat less naive option is to store the information as per-thread information in thread-local storage, where each thread has its own "copy" of the variable which other threads cannot interfere with. Although non-ideal, this has been the best solution in many cases. However, thanks to generators and coroutines, the execution of the call chain can be suspended and resumed, allowing code in other contexts to run concurrently. Therefore, using thread-local storage is *concurrency-unsafe*, because other call chains in other contexts may interfere with the thread-local variable.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Note that in the above two historical approaches, the stored information has the *widest* available scope without causing problems. For a third solution along the same path, one would first define an equivalent of a "thread" for asynchronous execution and concurrency. This could be seen as the largest amount of code and nested calls that is guaranteed to be executed sequentially without ambiguity in execution order. This might be referred to as concurrency-local or task-local storage. In this meaning of "task", there is no ambiguity in the order of execution of the code within one task. (This concept of a task is close to equivalent to a ``Task`` in ``asyncio``, but not exactly.) In such concurrency-locals, it is possible to pass information down the call chain to callees without another code path interfering with the value in the background.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Common to the above approaches is that they indeed use variables with a wide but just-narrow-enough scope. Thread-locals could also be called thread-wide globals---in single-threaded code, they are indeed truly global. And task-locals could be called task-wide globals, because tasks can be very big. </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">The issue here is that neither global variables, thread-locals nor task-locals are really meant to be used for this purpose of passing information of the execution context down the call chain. Instead of the widest possible variable scope, the scope of the variables should be controlled by the programmer, typically of a library, to have the desired scope---not wider. In other words, task-local variables (and globals and thread-locals) have nothing to do with the kind of context-bound information passing that this proposal intends to enable, even if task-locals can be used to emulate the desired semantics. Therefore, in the following, this proposal describes the semantics and the outlines of an implementation for *context-local variables* (or context variables, contextvars). In fact, as a side effect of this PEP, an async framework can use the proposed feature to implement task-local variables.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Proposal</font></div><div class="gmail_default"><font face="monospace, monospace">========</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Because the proposed semantics are not a direct extension to anything already available in Python, this proposal is first described in terms of semantics and API at a fairly high level. In particular, Python ``with`` statements are heavily used in the description, as they are a good match with the proposed semantics. However, the underlying ``__enter__`` and ``__exit__`` methods correspond to functions in the lower-level speed-optimized (C) API. For clarity of this document, the lower-level functions are not explicitly named in the definition of the semantics. After describing the semantics and high-level API, the implementation is described, going to a lower level.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Semantics and higher-level API</font></div><div class="gmail_default"><font face="monospace, monospace">------------------------------</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">Core concept</font></div><div class="gmail_default"><font face="monospace, monospace">''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">A context-local variable is represented by a single instance of ``contextvars.Var``, say ``cvar``. Any code that has access to the ``cvar`` object can ask for its value with respect to the current context. In the high-level API, this value is given by the ``cvar.value`` property::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> cvar = contextvars.Var(default="the default value", </font></div><div class="gmail_default"><font face="monospace, monospace"> description="example context variable")</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value" # default still applies</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> # In code examples, all ``assert`` statements should</font></div><div class="gmail_default"><font face="monospace, monospace"> # succeed according to the proposed semantics. </font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">No assignments to ``cvar`` have been applied for this context, so ``cvar.value`` gives the default value. Assigning new values to contextvars is done in a highly scope-aware manner::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign(new_value):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is new_value</font></div><div class="gmail_default"><font face="monospace, monospace"> # Any code here, or down the call chain from here, sees:</font></div><div class="gmail_default"><font face="monospace, monospace"> # cvar.value is new_value</font></div><div class="gmail_default"><font face="monospace, monospace"> # unless another value has been assigned in a </font></div><div class="gmail_default"><font face="monospace, monospace"> # nested context</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is new_value</font></div><div class="gmail_default"><font face="monospace, monospace"> # the assignment of ``cvar`` to ``new_value`` is no longer visible </font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">Here, ``cvar.assign(value)`` returns another object, namely ``contextvars.Assignment(cvar, new_value)``. The essential part here is that applying a context variable assignment (``Assignment.__enter__``) is paired with a de-assignment (``Assignment.__exit__``). These operations set the bounds for the scope of the assigned value.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Assignments to the same context variable can be nested to override the outer assignment in a narrower context::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign("outer"):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "outer"</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign("inner"):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "inner"</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "outer"</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">Also multiple variables can be assigned to in a nested manner without affecting each other::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> cvar1 = contextvars.Var() </font></div><div class="gmail_default"><font face="monospace, monospace"> cvar2 = contextvars.Var() </font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is None # default is None by default</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar1.assign(value1):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is value1</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar2.assign(value2):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is value1</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is value2</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is value1</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Or with more convenient Python syntax:: </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar1.assign(value1), cvar2.assign(value2):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is value1</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is value2</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">In another *context*, in another thread or otherwise concurrently executed task or code path, the context variables can have a completely different state. The programmer thus only needs to worry about the context at hand.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Refactoring into subroutines</font></div><div class="gmail_default"><font face="monospace, monospace">''''''''''''''''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Code using contextvars can be refactored into subroutines without affecting the semantics. For instance::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> assi = cvar.assign(new_value)</font></div><div class="gmail_default"><font face="monospace, monospace"> def apply():</font></div><div class="gmail_default"><font face="monospace, monospace"> assi.__enter__()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> apply()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is new_value</font></div><div class="gmail_default"><font face="monospace, monospace"> assi.__exit__()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">Or similarly in an asynchronous context where ``await`` expressions are used. The subroutine can now be a coroutine::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> assi = cvar.assign(new_value)</font></div><div class="gmail_default"><font face="monospace, monospace"> async def apply():</font></div><div class="gmail_default"><font face="monospace, monospace"> assi.__enter__()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> await apply()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is new_value</font></div><div class="gmail_default"><font face="monospace, monospace"> assi.__exit__()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">Or when the subroutine is a generator::</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> def apply():</font></div><div class="gmail_default"><font face="monospace, monospace"> yield</font></div><div class="gmail_default"><font face="monospace, monospace"> assi.__enter__()</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">which is called using ``yield from apply()`` or with calls to ``next`` or ``.send``. This is discussed further in later sections.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Semantics for generators and generator-based coroutines</font></div><div class="gmail_default"><font face="monospace, monospace">'''''''''''''''''''''''''''''''''''''''''''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Generators, coroutines and async generators act as subroutines in much the same way that normal functions do. However, they have the additional possibility of being suspended by ``yield`` expressions. Assignment contexts entered inside a generator are normally preserved across yields::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> def genfunc():</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign(new_value):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is new_value</font></div><div class="gmail_default"><font face="monospace, monospace"> yield</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is new_value</font></div><div class="gmail_default"><font face="monospace, monospace"> g = genfunc() </font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign(another_value):</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">However, the outer context visible to the generator may change state across yields::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> def genfunc():</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is value2</font></div><div class="gmail_default"><font face="monospace, monospace"> yield</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is value1</font></div><div class="gmail_default"><font face="monospace, monospace"> yield</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign(value3):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is value3</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign(value1):</font></div><div class="gmail_default"><font face="monospace, monospace"> g = genfunc()</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign(value2):</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is value1</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Similar semantics apply to async generators defined by ``async def ... yield ...`` ). </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">By default, values assigned inside a generator do not leak through yields to the code that drives the generator. However, the assignment contexts entered and left open inside the generator *do* become visible outside the generator after the generator has finished with a ``StopIteration`` or another exception::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> assi = cvar.assign(new_value)</font></div><div class="gmail_default"><font face="monospace, monospace"> def genfunc():</font></div><div class="gmail_default"><font face="monospace, monospace"> yield</font></div><div class="gmail_default"><font face="monospace, monospace"> assi.__enter__():</font></div><div class="gmail_default"><font face="monospace, monospace"> yield</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> g = genfunc()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g) # assi.__enter__() is called here</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "the default value"</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value is new_value</font></div><div class="gmail_default"><font face="monospace, monospace"> assi.__exit__()</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Special functionality for framework authors</font></div><div class="gmail_default"><font face="monospace, monospace">-------------------------------------------</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Frameworks, such as ``asyncio`` or third-party libraries, can use additional functionality in ``contextvars`` to achieve the desired semantics in cases which are not determined by the Python interpreter. Some of the semantics described in this section are also afterwards used to describe the internal implementation.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Leaking yields</font></div><div class="gmail_default"><font face="monospace, monospace">''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Using the ``contextvars.leaking_yields`` decorator, one can choose to leak the context through ``yield`` expressions into the outer context that drives the generator::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> @contextvars.leaking_yields</font></div><div class="gmail_default"><font face="monospace, monospace"> def genfunc():</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "outer"</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign("inner"):</font></div><div class="gmail_default"><font face="monospace, monospace"> yield</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "inner"</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "outer"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> g = genfunc():</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar.assign("outer"):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "outer"</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "inner"</font></div><div class="gmail_default"><font face="monospace, monospace"> next(g)</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar.value == "outer"</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Capturing contextvar assignments</font></div><div class="gmail_default"><font face="monospace, monospace">''''''''''''''''''''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Using ``contextvars.capture()``, one can capture the assignment contexts that are entered by a block of code. The changes applied by the block of code can then be reverted and subsequently reapplied, even in another context::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is None # default</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None # default</font></div><div class="gmail_default"><font face="monospace, monospace"> assi1 = cvar1.assign(value1)</font></div><div class="gmail_default"><font face="monospace, monospace"> assi2 = cvar1.assign(value2)</font></div><div class="gmail_default"><font face="monospace, monospace"> with contextvars.capture() as delta:</font></div><div class="gmail_default"><font face="monospace, monospace"> assi1.__enter__()</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar2.assign("not captured"):</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is "not captured"</font></div><div class="gmail_default"><font face="monospace, monospace"> assi2.__enter__()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is value2</font></div><div class="gmail_default"><font face="monospace, monospace"> delta.revert()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> ...</font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar1.assign(1), cvar2.assign(2):</font></div><div class="gmail_default"><font face="monospace, monospace"> delta.reapply()</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is value2</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value == 2</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace">However, reapplying the "delta" if its net contents include deassignments may not be possible (see also Implementation and Open Issues).</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Getting a snapshot of context state</font></div><div class="gmail_default"><font face="monospace, monospace">'''''''''''''''''''''''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">The function ``contextvars.get_local_state()`` returns an object representing the applied assignments to all context-local variables in the context where the function is called. This can be seen as equivalent to using ``contextvars.capture()`` to capture all context changes from the beginning of execution. The returned object supports methods ``.revert()`` and ``reapply()`` as above.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Running code in a clean state</font></div><div class="gmail_default"><font face="monospace, monospace">'''''''''''''''''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Although it is possible to revert all applied context changes using the above primitives, a more convenient way to run a block of code in a clean context is provided::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> with context_vars.clean_context():</font></div><div class="gmail_default"><font face="monospace, monospace"> # here, all context vars start off with their default values</font></div><div class="gmail_default"><font face="monospace, monospace"> # here, the state is back to what it was before the with block.</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Implementation</font></div><div class="gmail_default"><font face="monospace, monospace">--------------</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">This section describes to a variable level of detail how the described semantics can be implemented. At present, an implementation aimed at simplicity but sufficient features is described. More details will be added later. </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Alternatively, a somewhat more complicated implementation offers minor additional features while adding some performance overhead and requiring more code in the implementation.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Data structures and implementation of the core concept</font></div><div class="gmail_default"><font face="monospace, monospace">''''''''''''''''''''''''''''''''''''''''''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Each thread of the Python interpreter keeps its on stack of ``contextvars.Assignment`` objects, each having a pointer to the previous (outer) assignment like in a linked list. The local state (also returned by ``contextvars.get_local_state()``) then consists of a reference to the top of the stack and a pointer/weak reference to the bottom of the stack. This allows efficient stack manipulations. An object produced by ``contextvars.capture()`` is similar, but refers to only a part of the stack with the bottom reference pointing to the top of the stack as it was in the beginning of the capture block. </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Now, the stack evolves according to the assignment ``__enter__`` and ``__exit__`` methods. For example::</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> cvar1 = contextvars.Var()</font></div><div class="gmail_default"><font face="monospace, monospace"> cvar2 = contextvars.Var()</font></div><div class="gmail_default"><font face="monospace, monospace"> # stack: []</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar1.assign("outer"):</font></div><div class="gmail_default"><font face="monospace, monospace"> # stack: [Assignment(cvar1, "outer")]</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value == "outer"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar1.assign("inner"):</font></div><div class="gmail_default"><font face="monospace, monospace"> # stack: [Assignment(cvar1, "outer"), </font></div><div class="gmail_default"><font face="monospace, monospace"> # Assignment(cvar1, "inner")]</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value == "inner"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> with cvar2.assign("hello"):</font></div><div class="gmail_default"><font face="monospace, monospace"> # stack: [Assignment(cvar1, "outer"), </font></div><div class="gmail_default"><font face="monospace, monospace"> # Assignment(cvar1, "inner"),</font></div><div class="gmail_default"><font face="monospace, monospace"> # Assignment(cvar2, "hello")]</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value == "hello"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> # stack: [Assignment(cvar1, "outer"), </font></div><div class="gmail_default"><font face="monospace, monospace"> # Assignment(cvar1, "inner")]</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value == "inner"</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> # stack: [Assignment(cvar1, "outer")]</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value == "outer"</font></div><div class="gmail_default"><font face="monospace, monospace"> </font></div><div class="gmail_default"><font face="monospace, monospace"> # stack: []</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar1.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"> assert cvar2.value is None</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Getting a value from the context using ``cvar1.value`` can be implemented as finding the topmost occurrence of a ``cvar1`` assignment on the stack and returning the value there, or the default value if no assignment is found on the stack. However, this can be optimized to instead be an O(1) operation in most cases. Still, even searching through the stack may be reasonably fast since these stacks are not intended to grow very large.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">The above description is already sufficient for implementing the core concept. Suspendable frames require some additional attention, as explained in the following.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Implementation of generator and coroutine semantics</font></div><div class="gmail_default"><font face="monospace, monospace">'''''''''''''''''''''''''''''''''''''''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Within generators, coroutines and async generators, assignments and deassignments are handled in exactly the same way as anywhere else. However, some changes are needed in the builtin generator methods ``send``, ``__next__``, ``throw`` and ``close``. Here is the Python equivalent of the changes needed in ``send`` for a generator (here ``_old_send`` refers to the behavior in Python 3.6)::</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"> def send(self, value):</font></div><div class="gmail_default"><font face="monospace, monospace"> # if decorated with contextvars.leaking_yields</font></div><div class="gmail_default"><font face="monospace, monospace"> if self.gi_contextvars is LEAK:</font></div><div class="gmail_default"><font face="monospace, monospace"> # nothing needs to be done to leak context through yields :)</font></div><div class="gmail_default"><font face="monospace, monospace"> return self._old_send(value)</font></div><div class="gmail_default"><font face="monospace, monospace"> try:</font></div><div class="gmail_default"><font face="monospace, monospace"> with contextvars.capture() as delta:</font></div><div class="gmail_default"><font face="monospace, monospace"> if self.gi_contextvars: </font></div><div class="gmail_default"><font face="monospace, monospace"> # non-zero captured content from previous iteration</font></div><div class="gmail_default"><font face="monospace, monospace"> self.gi_contextvars.reapply()</font></div><div class="gmail_default"><font face="monospace, monospace"> ret = self._old_send(value)</font></div><div class="gmail_default"><font face="monospace, monospace"> except Exception:</font></div><div class="gmail_default"><font face="monospace, monospace"> raise</font></div><div class="gmail_default"><font face="monospace, monospace"> else:</font></div><div class="gmail_default"><font face="monospace, monospace"> # suspending, revert context changes but </font></div><div class="gmail_default"><font face="monospace, monospace"> delta.revert()</font></div><div class="gmail_default"><font face="monospace, monospace"> self.gi_contextvars = delta</font></div><div class="gmail_default"><font face="monospace, monospace"> return ret</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">The corresponding modifications to the other methods is essentially identical. The same applies to coroutines and async generators. </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">For code that does not use ``contextvars``, the additions are O(1) and essentially reduce to a couple of pointer comparisons. For code that does use ``contextvars``, the additions are still O(1) in most cases.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">More on implementation</font></div><div class="gmail_default"><font face="monospace, monospace">''''''''''''''''''''''</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">The rest of the functionality, including ``contextvars.leaking_yields``, contextvars.capture()``, ``contextvars.get_local_state()`` and ``contextvars.clean_context()`` are in fact quite straightforward to implement, but their implementation will be discussed further in later versions of this proposal. Caching of assigned values is somewhat more complicated, and will be discussed later, but it seems that most cases should achieve O(1) complexity.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Backwards compatibility</font></div><div class="gmail_default"><font face="monospace, monospace">=======================</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">There are no *direct* backwards-compatibility concerns, since a completely new feature is proposed. </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">However, various traditional uses of thread-local storage may need a smooth transition to ``contextvars`` so they can be concurrency-safe. There are several approaches to this, including emulating task-local storage with a little bit of help from async frameworks. A fully general implementation cannot be provided, because the desired semantics may depend on the design of the framework. </font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Another way to deal with the transition is for code to first look for a context created using ``contextvars``. If that fails because a new-style context has not been set or because the code runs on an older Python version, a fallback to thread-local storage is used.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Open Issues</font></div><div class="gmail_default"><font face="monospace, monospace">===========</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Out-of-order de-assignments</font></div><div class="gmail_default"><font face="monospace, monospace">---------------------------</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">In this proposal, all variable deassignments are made in the opposite order compared to the preceding assignments. This has two useful properties: it encourages using ``with`` statements to define assignment scope and has a tendency to catch errors early (forgetting a ``.__exit__()`` call often results in a meaningful error. To have this as a requirement requirement is beneficial also in terms of implementation simplicity and performance. Nevertheless, allowing out-of-order context exits is not completely out of the question, and reasonable implementation strategies for that do exist.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Rejected Ideas</font></div><div class="gmail_default"><font face="monospace, monospace">==============</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Dynamic scoping linked to subroutine scopes</font></div><div class="gmail_default"><font face="monospace, monospace">-------------------------------------------</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">The scope of value visibility should not be determined by the way the code is refactored into subroutines. It is necessary to have per-variable control of the assignment scope.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">Acknowledgements</font></div><div class="gmail_default"><font face="monospace, monospace">================</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">To be added.</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">References</font></div><div class="gmail_default"><font face="monospace, monospace">==========</font></div><div class="gmail_default"><font face="monospace, monospace"><br></font></div><div class="gmail_default"><font face="monospace, monospace">To be added.</font></div><div style="font-family:monospace,monospace"><br></div></div><div><br></div>-- <br><div class="gmail_signature">+ Koos Zevenhoven + <a href="http://twitter.com/k7hoven" target="_blank">http://twitter.com/k7hoven</a> +</div>
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