Simple context in Python's stdlib logging

demo-contextlog.py

#!/usr/bin/env -S uv run --script
# /// script
# requires-python = ">=3.13"
# dependencies = []
# ///

import logging
import logging.config
from contextlib import contextmanager
from contextvars import ContextVar
from typing import Iterator

context = ContextVar("context", default={})


@contextmanager
def log_context(**kwargs) -> Iterator[dict]:
    """Temporarily bind and restore context in a with-block."""
    token = context.set({**context.get(), **kwargs})
    try:
        yield context.get()
    finally:
        context.reset(token)


class ContextFormatter(logging.Formatter):
    """A formatter that injects contextvars into the record."""

    def format(self, record: logging.LogRecord) -> str:
        record.context = ", ".join(f"{k}={v}" for k, v in context.get().items())
        return super().format(record)


"""
Config
"""

logging_config = {
    "version": 1,
    "disable_existing_loggers": False,
    "formatters": {
        "context": {
            "()": "__main__.ContextFormatter",
            "format": "[ %(levelname)s ] %(name)s  %(message)s  %(context)s",
        },
    },
    "handlers": {
        "console": {
            "class": "logging.StreamHandler",
            "formatter": "context",
            "level": "INFO",
            "stream": "ext://sys.stdout",
        }
    },
    "root": {
        "level": "INFO",
        "handlers": ["console"],
    },
}

logging.config.dictConfig(logging_config)


"""
Demo

Output:

[ INFO ] __main__  Hello from myfunc!  request_id=abc123, user=alice
[ INFO ] __main__  Temporary override  request_id=abc123, user=2433
[ INFO ] __main__  Looping  request_id=abc123, user=alice, loop_iter=0
[ INFO ] __main__  Looping  request_id=abc123, user=alice, loop_iter=1
[ INFO ] __main__  Looping  request_id=abc123, user=alice, loop_iter=2
[ INFO ] __main__  After loop  request_id=abc123, user=alice, probe={'a': [1, 2]}
[ INFO ] __main__  Clean again  
"""

logger = logging.getLogger(__name__)


def myfunc():
    logger.info("Hello from myfunc!")


with log_context(request_id="abc123", user="alice"):
    myfunc()

    with log_context(user=2433):
        logger.info("Temporary override")

    with log_context() as ctx:
        for i in range(3):
            ctx["loop_iter"] = i
            logger.info("Looping")

    with log_context(probe=dict(a=[1, 2])):
        logger.info("After loop")

logger.info("Clean again")