Asyncio support for PEP-567 contextvars backport.
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README.rst

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:

  1. 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)
  1. 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.

  1. 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.