contextvars_quickstart_declare_set_get_with_async_isolation.py

1import asyncio
2import contextvars
3
4# Declare a context variable
5var = contextvars.ContextVar('var', default=42)
6
7async def main():
8    print(f"Initial value: {var.get()}")
9
10    # Set a new value for the context variable
11    token = var.set(100)
12    try:
13        print(f"Value after set: {var.get()}")
14        
15        # Run another task to see that context is isolated
16        await asyncio.create_task(other_task())
17    finally:
18        # Reset the variable to the value before 'set()' was called
19        var.reset(token)
20
21    print(f"Value after reset: {var.get()}")
22
23async def other_task():
24    # Changes in this task won't affect the parent context if handled correctly
25    print(f"Value in other_task: {var.get()}")
26    var.set(200)
27    print(f"Changed value in other_task: {var.get()}")
28
29asyncio.run(main())