PEP: 568 Title: Generator-sensitivity for Context Variables Author: Nathaniel J. Smith 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. .. Local Variables: indent-tabs-mode: nil coding: utf-8 End: