pythonpython-asyncioasynccallbackpython-contextvars

Access to contextvars in asyncio add_done_callback callback

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?

Solution

  • 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.