aiocontextvars
IMPORTANT: This package will be deprecated after
contextvars asyncio backport is fixed. Before then, this library
experimentally provides the missing asyncio support for the
contextvars backport library. Please read more in Python 3.7 contextvars
documentation.
Compatibility
In Python 3.7 this package is 100% contextvars.
In Python 3.5 and 3.6, this package added asyncio support to the PEP-567
backport package also named contextvars, in a very different way than
Python 3.7 contextvars implementation:
-
call_soon()and family methods.
Python 3.7 added keyword argument context to call_soon() and its family
methods. By default those methods will copy (inherit) the current context and
run the given method in that context. But aiocontextvars won't touch the
loop, so in order to achieve the same effect, you'll need to:
loop.call_soon(copy_context().run, my_meth)
- Task local.
Python 3.7 used above keyword argument context in Task to make sure
that each step of a coroutine is ran in the same context inherited at the time
its driving task was created. Meanwhile, aiocontextvars uses
Task.current_task() to achieve similar effect: it hacks asyncio and
attaches a copied context to the task on its creation, and replaces thread
local with current task instance to share the context. This behaves identically
to Python 3.7 in most times. What you need to do is to import
aiocontextvars before creating loops.
- Custom tasks and loops.
Because above hack is done by replacing asyncio.get_event_loop and
loop.create_task, therefore tasks and loops created by custom/private API
won't behave correctly as expected, e.g. uvloop.new_event_loop() or
asyncio.Task(). Also, event loops created before importing
aiocontextvars are not patched either. So over all, you should import
aiocontextvars at the beginning before creating event loops, and always use
asyncio.* to operate loops/policies, and public asyncio API to create
tasks.
Credits
Fantix King is the author and maintainer of this library. This library is open source software under BSD license.
