PEP: 568
Title: Generator-sensitivity for Context Variables
Author: Nathaniel J. Smith <njs@pobox.com>
Status: Deferred
Type: Standards Track
Content-Type: text/x-rst
Created: 04-Jan-2018
Python-Version: 3.8
Post-History:


Abstract
========

Context variables provide a generic mechanism for tracking dynamic,
context-local state, similar to thread-local storage but generalized
to cope work with other kinds of thread-like contexts, such as asyncio
Tasks. :pep:`550` proposed a mechanism for context-local state that was
also sensitive to generator context, but this was pretty complicated,
so the BDFL requested it be simplified. The result was :pep:`567`, which
is targeted for inclusion in 3.7. This PEP then extends :pep:`567`'s
machinery to add generator context sensitivity.

This PEP is starting out in the "deferred" status, because there isn't
enough time to give it proper consideration before the 3.7 feature
freeze. The only goal *right now* is to understand what would be
required to add generator context sensitivity in 3.8, so that we can
avoid shipping something in 3.7 that would rule it out by accident.
(Ruling it out on purpose can wait until 3.8 ;-).)


Rationale
=========

[Currently the point of this PEP is just to understand *how* this
would work, with discussion of *whether* it's a good idea deferred
until after the 3.7 feature freeze. So rationale is TBD.]


High-level summary
==================

Instead of holding a single ``Context``, the threadstate now holds a
``ChainMap`` of ``Context``\s. ``ContextVar.get`` and
``ContextVar.set`` are backed by the ``ChainMap``. Generators and
async generators each have an associated ``Context`` that they push
onto the ``ChainMap`` while they're running to isolate their
context-local changes from their callers, though this can be
overridden in cases like ``@contextlib.contextmanager`` where
"leaking" context changes from the generator into its caller is
desirable.


Specification
=============

Review of PEP 567
-----------------

Let's start by reviewing how :pep:`567` works, and then in the next
section we'll describe the differences.

In :pep:`567`, a ``Context`` is a ``Mapping`` from ``ContextVar`` objects
to arbitrary values. In our pseudo-code here we'll pretend that it
uses a ``dict`` for backing storage. (The real implementation uses a
HAMT, which is semantically equivalent to a ``dict`` but with
different performance trade-offs.)::

   class Context(collections.abc.Mapping):
       def __init__(self):
           self._data = {}
           self._in_use = False

       def __getitem__(self, key):
           return self._data[key]

       def __iter__(self):
           return iter(self._data)

       def __len__(self):
           return len(self._data)

At any given moment, the threadstate holds a current ``Context``
(initialized to an empty ``Context`` when the threadstate is created);
we can use ``Context.run`` to temporarily switch the current
``Context``::

   # Context.run
   def run(self, fn, *args, **kwargs):
       if self._in_use:
           raise RuntimeError("Context already in use")
       tstate = get_thread_state()
       old_context = tstate.current_context
       tstate.current_context = self
       self._in_use = True
       try:
           return fn(*args, **kwargs)
       finally:
           state.current_context = old_context
           self._in_use = False

We can fetch a shallow copy of the current ``Context`` by calling
``copy_context``; this is commonly used when spawning a new task, so
that the child task can inherit context from its parent::

   def copy_context():
       tstate = get_thread_state()
       new_context = Context()
       new_context._data = dict(tstate.current_context)
       return new_context

In practice, what end users generally work with is ``ContextVar``
objects, which also provide the only way to mutate a ``Context``. They
work with a utility class ``Token``, which can be used to restore a
``ContextVar`` to its previous value::

   class Token:
       MISSING = sentinel_value()

       # Note: constructor is private
       def __init__(self, context, var, old_value):
           self._context = context
           self.var = var
           self.old_value = old_value

       # XX: PEP 567 currently makes this a method on ContextVar, but
       # I'm going to propose it switch to this API because it's simpler.
       def reset(self):
           # XX: should we allow token reuse?
           # XX: should we allow tokens to be used if the saved
           # context is no longer active?
           if self.old_value is self.MISSING:
               del self._context._data[self.context_var]
           else:
               self._context._data[self.context_var] = self.old_value

   # XX: the handling of defaults here uses the simplified proposal from
   # https://mail.python.org/pipermail/python-dev/2018-January/151596.html
   # This can be updated to whatever we settle on, it was just less
   # typing this way :-)
   class ContextVar:
       def __init__(self, name, *, default=None):
           self.name = name
           self.default = default

       def get(self):
           context = get_thread_state().current_context
           return context.get(self, self.default)

       def set(self, new_value):
           context = get_thread_state().current_context
           token = Token(context, self, context.get(self, Token.MISSING))
           context._data[self] = new_value
           return token


Changes from PEP 567 to this PEP
--------------------------------

In general, ``Context`` remains the same. However, now instead of
holding a single ``Context`` object, the threadstate stores a stack of
them. This stack acts just like a ``collections.ChainMap``, so we'll
use that in our pseudocode. ``Context.run`` then becomes::

   # Context.run
   def run(self, fn, *args, **kwargs):
       if self._in_use:
           raise RuntimeError("Context already in use")
       tstate = get_thread_state()
       old_context_stack = tstate.current_context_stack
       tstate.current_context_stack = ChainMap([self])     # changed
       self._in_use = True
       try:
           return fn(*args, **kwargs)
       finally:
           state.current_context_stack = old_context_stack
           self._in_use = False

Aside from some updated variables names (e.g.,
``tstate.current_context`` → ``tstate.current_context_stack``), the
only change here is on the marked line, which now wraps the context in
a ``ChainMap`` before stashing it in the threadstate.

We also add a ``Context.push`` method, which is almost exactly like
``Context.run``, except that it temporarily pushes the ``Context``
onto the existing stack, instead of temporarily replacing the whole
stack::

   # Context.push
   def push(self, fn, *args, **kwargs):
       if self._in_use:
           raise RuntimeError("Context already in use")
       tstate = get_thread_state()
       tstate.current_context_stack.maps.insert(0, self)  # different from run
       self._in_use = True
       try:
           return fn(*args, **kwargs)
       finally:
           tstate.current_context_stack.maps.pop(0)       # different from run
           self._in_use = False

In most cases, we don't expect ``push`` to be used directly; instead,
it will be used implicitly by generators. Specifically, every
generator object and async generator object gains a new attribute
``.context``. When an (async) generator object is created, this
attribute is initialized to an empty ``Context`` (``self.context =
Context()``). This is a mutable attribute; it can be changed by user
code. But trying to set it to anything that isn't a ``Context`` object
or ``None`` will raise an error.

Whenever we enter an generator via ``__next__``, ``send``, ``throw``,
or ``close``, or enter an async generator by calling one of those
methods on its ``__anext__``, ``asend``, ``athrow``, or ``aclose``
coroutines, then its ``.context`` attribute is checked, and if
non-``None``, is automatically pushed::

   # GeneratorType.__next__
   def __next__(self):
       if self.context is not None:
           return self.context.push(self.__real_next__)
       else:
           return self.__real_next__()

While we don't expect people to use ``Context.push`` often, making it
a public API preserves the principle that a generator can always be
rewritten as an explicit iterator class with equivalent semantics.

Also, we modify ``contextlib.(async)contextmanager`` to always set its
(async) generator objects' ``.context`` attribute to ``None``::

   # contextlib._GeneratorContextManagerBase.__init__
   def __init__(self, func, args, kwds):
       self.gen = func(*args, **kwds)
       self.gen.context = None                  # added
       ...

This makes sure that code like this continues to work as expected::

   @contextmanager
   def decimal_precision(prec):
       with decimal.localcontext() as ctx:
           ctx.prec = prec
           yield

   with decimal_precision(2):
       ...

The general idea here is that by default, every generator object gets
its own local context, but if users want to explicitly get some other
behavior then they can do that.

Otherwise, things mostly work as before, except that we go through and
swap everything to use the threadstate ``ChainMap`` instead of the
threadstate ``Context``. In full detail:

The ``copy_context`` function now returns a flattened copy of the
"effective" context. (As an optimization, the implementation might
choose to do this flattening lazily, but if so this will be made
invisible to the user.) Compared to our previous implementation above,
the only change here is that ``tstate.current_context`` has been
replaced with ``tstate.current_context_stack``::

   def copy_context() -> Context:
       tstate = get_thread_state()
       new_context = Context()
       new_context._data = dict(tstate.current_context_stack)
       return new_context

``Token`` is unchanged, and the changes to ``ContextVar.get`` are
trivial::

   # ContextVar.get
   def get(self):
       context_stack = get_thread_state().current_context_stack
       return context_stack.get(self, self.default)

``ContextVar.set`` is a little more interesting: instead of going
through the ``ChainMap`` machinery like everything else, it always
mutates the top ``Context`` in the stack, and – crucially! – sets up
the returned ``Token`` to restore *its* state later. This allows us to
avoid accidentally "promoting" values between different levels in the
stack, as would happen if we did ``old = var.get(); ...;
var.set(old)``::

   # ContextVar.set
   def set(self, new_value):
       top_context = get_thread_state().current_context_stack.maps[0]
       token = Token(top_context, self, top_context.get(self, Token.MISSING))
       top_context._data[self] = new_value
       return token

And finally, to allow for introspection of the full context stack, we
provide a new function ``contextvars.get_context_stack``::

   def get_context_stack() -> List[Context]:
       return list(get_thread_state().current_context_stack.maps)

That's all.


Comparison to PEP 550
=====================

The main difference from :pep:`550` is that it reified what we're calling
"contexts" and "context stacks" as two different concrete types
(``LocalContext`` and ``ExecutionContext`` respectively). This led to
lots of confusion about what the differences were, and which object
should be used in which places. This proposal simplifies things by
only reifying the ``Context``, which is "just a dict", and makes the
"context stack" an unnamed feature of the interpreter's runtime state
– though it is still possible to introspect it using
``get_context_stack``, for debugging and other purposes.


Implementation notes
====================

``Context`` will continue to use a HAMT-based mapping structure under
the hood instead of ``dict``, since we expect that calls to
``copy_context`` are much more common than ``ContextVar.set``. In
almost all cases, ``copy_context`` will find that there's only one
``Context`` in the stack (because it's rare for generators to spawn
new tasks), and can simply re-use it directly; in other cases HAMTs
are cheap to merge and this can be done lazily.

Rather than using an actual ``ChainMap`` object, we'll represent the
context stack using some appropriate structure – the most appropriate
options are probably either a bare ``list`` with the "top" of the
stack being the end of the list so we can use ``push``\/``pop``, or
else an intrusive linked list (``PyThreadState`` → ``Context`` →
``Context`` → ...), with the "top" of the stack at the beginning of
the list to allow efficient push/pop.

A critical optimization in :pep:`567` is the caching of values inside
``ContextVar``. Switching from a single context to a context stack
makes this a little bit more complicated, but not too much. Currently,
we invalidate the cache whenever the threadstate's current ``Context``
changes (on thread switch, and when entering/exiting ``Context.run``).
The simplest approach here would be to invalidate the cache whenever
stack changes (on thread switch, when entering/exiting
``Context.run``, and when entering/leaving ``Context.push``). The main
effect of this is that iterating a generator will invalidate the
cache. It seems unlikely that this will cause serious problems, but if
it does, then I think it can be avoided with a cleverer cache key that
recognizes that pushing and then popping a ``Context`` returns the
threadstate to its previous state. (Idea: store the cache key for a
particular stack configuration in the topmost ``Context``.)

It seems unavoidable in this design that uncached ``get`` will be
O(n), where n is the size of the context stack. However, n will
generally be very small – it's roughly the number of nested
generators, so usually n=1, and it will be extremely rare to see n
greater than, say, 5. At worst, n is bounded by the recursion limit.
In addition, we can expect that in most cases of deep generator
recursion, most of the ``Context``\s in the stack will be empty, and
thus can be skipped extremely quickly during lookup. And for repeated
lookups the caching mechanism will kick in. So it's probably possible
to construct some extreme case where this causes performance problems,
but ordinary code should be essentially unaffected.


Copyright
=========

This document has been placed in the public domain.

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