Access to contextvars in asyncio add_done_callback callback

user_6174301 0
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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?

asynccallback python-contextvars python-asyncio python
2 Answers

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.

user_16008980

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')
user_134665100
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