Access to contextvars in asyncio add_done_callback callback

Question

In Python async function I'am creating ContextVar, task and attaching callback to it:

bbb = contextvars.ContextVar('aaa')
bbb.set(3)
task = self.loop.create_task(self.someFunc())
task.add_done_callback(self.commonCallback)
bbb.set(4)

In callback I first start debugger:

def commonCallback(self, result):
 pdb.set_trace()
 try:
  r = result.result()
  print(r)
 except:
  self.log.exception('commonCallback')

And in debugger:

-> try:
(Pdb) bbb.get()
*** NameError: name 'bbb' is not defined
(Pdb) ctx = contextvars.copy_context()
(Pdb) print(list(ctx.items()))
[(<ContextVar name='aaa' at 0xa8245df0>, 3)]
(Pdb)

ContextVar is there, but I can't access it. So, I am missing something, but can't find what?

Why do you expect the bbb local variable defined in one function to be accessible in the commonCallback function defined elsewhere in the code? The documentation states that "Context Variables should be created at the top module level". — user4815162342
– user4815162342, Commented Apr 11, 2021 at 7:24
@user4815162342 Thanks, that was it. I knew it was trivial, and it was, as usual. I just couldn't see it. Sadly, I do not have enough reputation to upvote comment. — ljweko
– ljweko, Commented Apr 11, 2021 at 18:50
No problem, I've now posted a similar suggestion as an answer which you can accept. — user4815162342
– user4815162342, Commented Apr 11, 2021 at 20:36

user4815162342 · Accepted Answer · 2021-04-11 20:35:30Z

2

The bbb local variable is defined in one place, so it won't be automatically accessible in another, such as the commonCallback function defined elsewhere in the code. The documentation states that "Context Variables should be created at the top module level", so you should try that first.

answered Apr 11, 2021 at 20:35

user4815162342

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Alexander Nikitin · Accepted Answer · 2021-06-07 07:56:28Z

0

You can get a value from context without importing top level module. contextvars.Context has __iter__ method. And you can use a for loop to get the value:

def get_ctx_var_value(ctx, var_name, default_value=None):
    for var in ctx:
        if var.name == var_name:
            return ctx[var]
    return default_value

ctx = contextvars.copy_context()
var_value = get_ctx_var_value(ctx, 'aaa')

answered Jun 7, 2021 at 7:56

Alexander Nikitin

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