fastapi-log-api-route

A drop-in APIRoute subclass for FastAPI that logs each request and response with method, path, headers, body, status, duration, and any extra context you want to attach.

Why

FastAPI's middleware tier doesn't have access to a parsed request body or to the path template (only the rendered path). A custom APIRoute runs after FastAPI has parsed the body via your dependency-injected models, so it sees the same data your handler sees — without re-reading the request stream.

Install

pip install fastapi-log-api-route

Quick start

import logging

logging.basicConfig(
    level=logging.INFO,
    format="%(levelname)s %(name)s %(message)s",
)

from fastapi import APIRouter, FastAPI
from fastapi_log_api_route import LogAPIRoute

app = FastAPI()
router = APIRouter(route_class=LogAPIRoute)


@router.post("/items")
async def create_item(item: dict):
    return {"ok": True}


app.include_router(router)

Calls are emitted on the fastapi_log_api_route logger (INFO / WARNING / ERROR by status — see Hooks). If you omit basicConfig (or equivalent dictConfig), INFO lines often never appear: Python only outputs log records once a handler is attached on the propagation chain, and the root logger often has none until you configure it (Logging configuration).

Example payload (shape depends on config, status, and errors):

{
  "started_at": 1714665600.12,
  "finished_at": 1714665600.18,
  "duration_ms": 57.4,
  "request": {
    "method": "POST",
    "path": "/items",
    "path_params": {},
    "query_params": {},
    "client": "127.0.0.1",
    "headers": {"content-type": "application/json", "user-agent": "curl/8.5.0"},
    "body": {"name": "widget"}
  },
  "response": {"status": 200, "body": {"ok": true}},
  "endpoint_logs": null
}

You can also patch a bare FastAPI app's router directly:

app = FastAPI()
app.router.route_class = LogAPIRoute

If you also create your own APIRouter, pass route_class=LogAPIRoute to it as well — patching the app router doesn't propagate to sub-routers.

Configuration

Subclass LogAPIRoute and override class attributes:

class MyLogAPIRoute(LogAPIRoute):
    LOG_HTTP_METHODS = {"POST", "PUT", "PATCH", "DELETE"}
    REQUEST_HEADERS_WHITELIST = {"x-request-id", "user-agent"}

Hooks

Override these methods on your subclass for richer behaviour:

• custom_fields(request) -> dict — return extra top-level fields merged into the log record (trace ids, tenant ids, service metadata, …).
• should_skip(request) -> bool — completely bypass logging for a request.
• log(log_object) -> None — change how records are emitted (ship to a message bus, write to a file, hand off to structlog, …). The default chooses the stdlib log level from log_object["response"]["status"]: 5xx → ERROR, 4xx → WARNING, otherwise → INFO (missing or non-numeric status is treated as success for level purposes).

Request / response body capture is not hookable in the default class; subclass and replace get_route_handler if you need different body rules.

endpoint_logs (handler-attached payload)

Handlers (or middleware) can attach structured fields meant for downstream log processors by setting request.scope["endpoint_logs"] to a mapping (e.g. dict). They appear under the same key in the emitted log dict — nothing else on the request is copied there automatically. If the value is not a mapping, it is ignored and a warning is logged; the field stays null.

from fastapi import APIRouter, FastAPI, Request
from fastapi_log_api_route import LogAPIRoute

app = FastAPI()
router = APIRouter(route_class=LogAPIRoute)


@router.post("/items")
async def create_item(request: Request, item: dict):
    request.scope["endpoint_logs"] = {"feature_flag": "new-pricing"}
    return {"ok": True}


app.include_router(router)

Example: Datadog trace ids + service metadata

import os
from typing import Any

from ddtrace import tracer
from fastapi import Request
from fastapi_log_api_route import LogAPIRoute


class TracedLogAPIRoute(LogAPIRoute):
    LOG_HTTP_METHODS = {"GET", "POST"}

    def custom_fields(self, request: Request) -> dict[str, Any]:
        span = tracer.current_span()
        return {
            "dd.trace_id": span.trace_id,
            "service": os.getenv("SERVICE_NAME"),
            "pod": os.getenv("POD_NAME"),
        }

Logging configuration

The default LogAPIRoute.log serialises log_object with orjson and emits:

logging.getLogger("fastapi_log_api_route").log(level, json_line, extra={"log_object": log_object})

level follows log_object["response"]["status"]: 5xx → ERROR, 4xx → WARNING, otherwise → INFO (see Hooks).

Why nothing shows up

Python only prints log records once a handler is attached somewhere on the propagation chain toward root. Until you call logging.basicConfig(...) at process startup (or declare your pipeline in dictConfig / YAML), INFO from this package’s logger often has nowhere to go.

Minimal local setup

import logging

logging.basicConfig(
    level=logging.INFO,
    format="%(levelname)s %(name)s %(message)s",
)

This package deliberately does not call basicConfig for you (libraries shouldn’t seize global logging policy).

Tune severity like any logging setup—for example root.setLevel(logging.WARNING) hides successful INFO request lines while still emitting WARNING/ERROR for 4xx/5xx.

Custom sink: override log (print, queue, OTLP, …)

from typing import Any

import orjson

from fastapi_log_api_route import LogAPIRoute


class PrintLogAPIRoute(LogAPIRoute):
    def log(self, log_object: dict[str, Any]) -> None:
        line = orjson.dumps(log_object, default=str).decode("utf-8")
        print(line, flush=True)

Use APIRouter(route_class=PrintLogAPIRoute). You bypass stdlib handlers entirely; wire whatever transport you prefer.

When you stick with logging, importing the packaged symbol can help wrappers attach to the same logger:

from fastapi_log_api_route import logger

If log raises, LogAPIRoute catches it and emits logger.exception(...) on the package logger — a buggy log hook must never break the request.

Response body in logs

When LOG_RESPONSE_BODY is enabled, captured bodies behave as follows:

• media_type == "application/json" — parsed JSON via orjson.loads.
• media_type is None — null.
• media_type == "text/plain" — {"plain_text": "<decoded string>"} using the response charset.
• Any other media type — {"logger_error": "<unsupported message>"} (no raw bytes).

Unhandled exceptions, FastAPI validation errors, and HTTPException paths populate response without a normal routed body (detail, errors, optional traceback shapes).

Development

This project is built with hatch and uses uv for dev workflows; either works.

uv sync --group dev   # installs runtime + dev dependencies
uv run pytest       # or: pytest

License

MIT License – see LICENSE.