What is a contextvars and how is it used with asyncio?

Question

Accepted Answer

**contextvars** stores **context-local state**. Works correctly with asyncio and threads.

```python
import asyncio
from contextvars import ContextVar

# Define context variable
user_var: ContextVar[str] = ContextVar("user", default="anonymous")

async def process_request(user_id):
    user_var.set(user_id)  # Set for this context
    await asyncio.sleep(0.1)
    print(f"Processing for {user_var.get()}")

async def main():
    # Each task has its own context
    await asyncio.gather(
        process_request("alice"),
        process_request("bob"),
    )
    # Output: Processing for alice, Processing for bob

asyncio.run(main())
```

**vs threading.local:**
- `contextvars` works with asyncio.
- `threading.local` only with threads.
- Context is copied to child tasks.

Python: What is a contextvars and how is it used with asyncio?