How can I use context vars in other file in python 3.7 or above?

I have a context var in in file a.py and I want to use it in b.py.

a.py:

import contextvars

cntx = contextvars.ContextVar("abcd")

b.py:

from .a import cntx

print(cntx.get())

Error:

Traceback (most recent call last):
  File "/home/user/Desktop/b.py", line 1, in <module>
    from .a import cntx
ImportError: attempted relative import with no known parent package

Isn't this how context variables supposed to work? I'm using python 3.9

The ImportError that you are getting is because of the invalid file name. .a is a valid file name and would work if you had a file with filename being .a.py

The reason you are getting the LookupError: <ContextVar name='abcd' at 0x7f7d6209c5e0> is because you are trying to get() the context which hasn't been set yet.

The get() method raises a LookupError if no context was set before.

Try with following -

a.py:

import contextvars

cntx = contextvars.ContextVar("abcd")
cntx.set("example")

b.py:

from a import cntx
print(cntx.get())

When you run b.py -

Output:

example