How do I use python logging with uvicorn/FastAPI? [duplicate]

Question

Here is a small application that reproduces my problem:

import fastapi
import logging
import loguru

instance = fastapi.FastAPI()

@instance.on_event("startup")
async def startup_event():
    logger = logging.getLogger("mylogger")
    logger.info("I expect this to log")
    loguru.logger.info("but only this logs")

When I launch this application with uvicorn app.main:instance --log-level=debug I see this in my terminal:

INFO:     Waiting for application startup.
2024-05-02 13:14:45.118 | INFO     | app.main:startup_event:28 - but only this logs
INFO:     Application startup complete.

Why does only the loguru logline work, and how can I make standard python logging work as expected?

Answer 1

It's because the --log-level=debug only applies to uvicorn's logger, not your mylogger logger - whose level remains set at WARNING.

If you add a line to the end of your script such as

logging.basicConfig(level=logging.DEBUG,
                    format="%(asctime)s | %(levelname)-8s | "
                           "%(module)s:%(funcName)s:%(lineno)d - %(message)s")

and run, you'll see something like

$ uvicorn main:instance --log-level=debug
INFO:     Started server process [1311]
INFO:     Waiting for application startup.
2024-05-02 16:44:55,736 | INFO     | main:startup_event:12 - I expect this to log
2024-05-02 16:44:55.736 | INFO     | main:startup_event:13 - but only this logs
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

The extra line configures the root logger and adds a console handler with that format, and events logged to the mylogger logger are passed up the hierarchy to the root logger's console handler.

Answer 2

Try this: uvicorn app.main:instance --no-access-log

The reasoning is that Uvicorn configures the built-in logging module by default. Passing this flag while starting your application will turn off Uvicorn's access log and allow you to configure custom logging. Source: Settings - Uvicorn