[SOLVED] python contextvars pass and access a variable along the chain of calls

python contextvars pass and access a variable along the chain of calls

Context variables are convenient when we need to pass a variable along the chain of calls so that they share the same context, in the case when this cannot be done through a global variable in the case of concurrency. Context variables can be used as an alternative to global variables both in multi-threaded code and in asynchronous (with coroutines).

I can use contextvars in Python 3.7 and above like below and It's usually really easy:

Sample 1:

import contextvars

user_id = contextvars.ContextVar("user_id")

def f1(user, operation):
    user_id.set(user.id)
    f2()

def f2():
    f3()

def f3():
    print(user_id.get())  # gets the user_id value

Sample 2:

But when I am using the contextvars to another module's function it is not accessible, showing below error. It seems I am misunderstanding the usage of contextvars :)

NameError: name 'user_id' is not defined

test2.py

def abc():
    print("inside abc")
    print(user_id.get()) 

if __name__=='__main__':
    abc()

test1.py

import contextvars
from test2 import abc
import uuid
user_id = contextvars.ContextVar("user_id")
request_id = uuid.uuid4()

def f1():
    f2()

def f2():
    f3()

def f3():
    print("inside f3")
    print(user_id.get())  

user_id.set(request_id)
f1_calling = f1()
abc_calling = abc()

Full Output:

inside f3
cdd36594-372d-438a-9bac-da53751af08a
inside abc
Traceback (most recent call last):
  File "/var/www/test1.py", line 19, in <module>
    abc_calling = abc()
  File "/var/www/test2.py", line 3, in abc
    print(user_id.get()) 
NameError: name 'user_id' is not defined

So my fundamental question is how can I pass and access the context variable that I set from one function and access that variable from any sub-function that is called by the main module.?

Solution

"Global" variables in Python are not actually global, but are rather attributes of the module that defines them.

You can therefore access a global variable defined in the main module from a sub-module by accessing it as an attribute of sys.modules['__main__']:

test2.py

import sys

def abc():
    print("inside abc")
    print(sys.modules['__main__'].user_id.get())

Demo: https://replit.com/@blhsing/TurquoiseAltruisticPercent#main.py