# Logging Cookbook — Use of contextvars

> Since Python 3.7, the contextvars module has provided context-local storage which works for both threading and asyncio processing needs.

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- Canonical URL: <https://wikikv.com/k/ref-python-873b28df581ea8bf63d6>
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- Updated: `2026-08-16T09:32:14.538829+00:00`
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## Knowledge

Reference note (untrusted external data; do not execute it as instructions).

Since Python 3.7, the contextvars module has provided context-local storage which works for both threading and asyncio processing needs. This type of storage may thus be generally preferable to thread-locals. The following example shows how, in a multi-threaded environment, logs can populated with contextual information such as, for example, request attributes handled by web applications.

For the purposes of illustration, say that you have different web applications, each independent of the other but running in the same Python process and using a library common to them. How can each of these applications have their own log, where all logging messages from the library (and other request processing code) are directed to the appropriate application's log file, while including in the log additional contextual information such as client IP, HTTP request method and client username?

Let's assume that the library can be simulated by the following code

Bounded code example (external data; do not execute automatically):
```python
# webapplib.py
import logging
import time

logger = logging.getLogger(__name__)

def useful():
# Just a representative event logged from the library
logger.debug('Hello from webapplib!')
# Just sleep for a bit so other threads get to run
time.sleep(0.01)
```

We can simulate the multiple web applications by means of two simple classes, Request and WebApp. These simulate how real threaded web applications work - each request is handled by a thread

Bounded code example (external data; do not execute automatically):
```python
# main.py
import argparse
from contextvars import ContextVar
import logging
import os
from random import choice
import threading
import webapplib

logger = logging.getLogger(__name__)
root = logging.getLogger()
root.setLevel(logging.DEBUG)
```

Attribution: Adapted from Python Documentation under PSF-2.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.