Logging errors in FastAPI - Step-by-Step Execution

Concept Flow - Logging errors

Start FastAPI app

↓

Receive request

↓

Process request

↓

Error occurs?

No→Send response

Yes↓

Log error details

↓

Send error response

↓

Wait for next request

This flow shows how FastAPI handles a request, logs errors if they happen, and sends responses.

Execution Sample

FastAPI

from fastapi import FastAPI, Request
import logging
from fastapi.responses import JSONResponse

app = FastAPI()

@app.exception_handler(Exception)
async def log_exceptions(request: Request, exc: Exception):
    logging.error(f"Error on {request.url}: {exc}")
    return JSONResponse(status_code=500, content={"detail": "Internal Server Error"})

This code sets up FastAPI to catch all exceptions, log the error with request info, and return a simple error message.

Execution Table

Step	Action	Input/Condition	Logging Output	Response Sent
1	Start FastAPI app	N/A	No logs yet	N/A
2	Receive request	GET /items/1	No logs yet	N/A
3	Process request	Code runs normally	No logs yet	200 OK with item data
4	Receive request	GET /items/999 (nonexistent)	No logs yet	N/A
5	Process request	Error: Item not found	Error on http://localhost/items/999: Item not found	500 Internal Server Error
6	Send error response	After logging error	Error logged	{"detail": "Internal Server Error"}
7	Wait for next request	N/A	Logs saved	N/A

💡 Execution stops waiting for next request after sending response.

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
request.url	N/A	http://localhost/items/1	http://localhost/items/999	http://localhost/items/999
exc	N/A	N/A	Item not found	Item not found
logging output	Empty	Empty	Error on http://localhost/items/999: Item not found	Error on http://localhost/items/999: Item not found

Key Moments - 2 Insights

Why do we see no logs when the request succeeds?

Because logging.error is only called when an exception occurs, as shown in step 3 vs step 5 in the execution_table.

What does the exception handler catch?

It catches all exceptions during request processing, logs them, and returns a generic error response, as seen in steps 5 and 6.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what is logged?

A No logs yet

B Error on http://localhost/items/999: Item not found

C 200 OK with item data

D Internal Server Error

💡 Hint

Check the Logging Output column at step 5 in the execution_table.

At which step does the app send a 500 Internal Server Error response?

A Step 3

B Step 2

C Step 6

D Step 1

💡 Hint

Look at the Response Sent column for the 500 error in the execution_table.

If no exception occurs, what will the logging output be after processing the request?

A No logs yet

B Error on request URL

C Error logged

D Internal Server Error

💡 Hint

Refer to step 3 in the execution_table where the request succeeds without error.

Concept Snapshot

FastAPI error logging:
- Use @app.exception_handler(Exception) to catch errors
- Log error details with logging.error()
- Return a generic error response
- Only logs errors when exceptions occur
- Normal requests do not produce error logs

Full Transcript

This visual execution shows how FastAPI handles requests and logs errors. When a request comes in, the app processes it. If no error happens, it sends a normal response without logging. If an error occurs, the exception handler logs the error with the request URL and exception message. Then it sends a 500 Internal Server Error response with a simple JSON message. Variables like request.url and exc hold the request info and error details. The logging output updates only when an error happens. This helps developers see what went wrong while keeping normal requests quiet.