Python logging

Python Logging Patterns

Structured Logging with structlog

import structlog
import logging
import sys

def configure_logging(json_logs: bool = False, log_level: str = "INFO") -> None:
    shared_processors = [
        structlog.contextvars.merge_contextvars,
        structlog.processors.add_log_level,
        structlog.processors.TimeStamper(fmt="iso"),
        structlog.stdlib.add_logger_name,
    ]
    
    if json_logs:
        processors = shared_processors + [structlog.processors.JSONRenderer()]
    else:
        processors = shared_processors + [structlog.dev.ConsoleRenderer()]
    
    structlog.configure(
        processors=processors,
        wrapper_class=structlog.make_filtering_bound_logger(
            logging.getLevelName(log_level)
        ),
        logger_factory=structlog.PrintLoggerFactory(file=sys.stdout),
    )

# Usage
logger = structlog.get_logger(__name__)

async def create_user(data: dict) -> User:
    logger.info("creating_user", email=data["email"])
    try:
        user = await user_repo.create(data)
        logger.info("user_created", user_id=user.id, email=user.email)
        return user
    except DuplicateEmailError:
        logger.warning("duplicate_email", email=data["email"])
        raise
    except Exception:
        logger.exception("user_creation_failed", email=data["email"])
        raise

Correlation IDs in FastAPI

from contextvars import ContextVar
import uuid
import structlog

REQUEST_ID: ContextVar[str] = ContextVar("request_id", default="")

@app.middleware("http")
async def add_correlation_id(request: Request, call_next):
    request_id = request.headers.get("X-Request-ID") or str(uuid.uuid4())
    REQUEST_ID.set(request_id)
    structlog.contextvars.bind_contextvars(request_id=request_id)
    
    response = await call_next(request)
    response.headers["X-Request-ID"] = request_id
    structlog.contextvars.clear_contextvars()
    return response

Standard Library Logging (minimal setup)

import logging

logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s %(name)s %(levelname)s %(message)s",
    handlers=[logging.StreamHandler(sys.stdout)],
)

# In modules: always use module-level logger
logger = logging.getLogger(__name__)

# Log with context
logger.info("Processing order", extra={"order_id": order_id, "user_id": user_id})

# Don't use f-strings in log calls (deferred formatting)
logger.debug("User %s logged in from %s", user.id, ip_address)  # Good
logger.debug(f"User {user.id} logged in from {ip_address}")     # Bad: formats even if DEBUG disabled

Log Levels Guide

Level	Use for
DEBUG	Detailed diagnostic information (disabled in production)
INFO	Operational events (request received, user created)
WARNING	Unexpected but handled situations (rate limit hit, retry)
ERROR	Failures that need attention (DB down, 3rd party API failed)
CRITICAL	System-level failures requiring immediate action