Using ContextVars for Request Objects

Using contextvars to store request objects is a good approach for making request data globally accessible in asynchronous web applications. This is safer than using thread-local storage (like threading.local()) in asynchronous code.

Why ContextVars is Appropriate

Context variables are specifically designed for this use case - they maintain context-local state that works properly with asynchronous code. The documentation states:

Context managers that have state should use Context Variables instead of threading.local() to prevent their state from bleeding to other code unexpectedly, when used in concurrent code.

Implementation Example

Here's how you might implement this pattern:

PYTHONCopied
import contextvars

# Create at module level (important)
request_var = contextvars.ContextVar('request', default=None)

# In your middleware
class RequestMiddleware:
    async def __call__(self, request, call_next):
        # Store request in context
        token = request_var.set(request)
        try:
            # Process the request
            response = await call_next(request)
            return response
        finally:
            # Clean up when request is done
            request_var.reset(token)

# In any function that needs the request
def get_current_request():
    request = request_var.get()
    if request is None:
        raise RuntimeError("No request found in context")
    return request

Safety Considerations

1. Proper cleanup: Always reset the context variable when the request is complete (use try/finally as shown)
2. Declaration location: Create context variables at the module level, not in closures (as noted in the docs)
3. Default values: Consider providing a default value or handling the case when no request exists
4. Isolation: Each request will have its own isolated context, preventing data leakage between requests

This approach is safe and is actually the recommended pattern for this exact use case in asynchronous web frameworks.