Logging errors in FastAPI

Introduction

Logging errors helps you find and fix problems in your FastAPI app. It keeps a record of what went wrong.

When your FastAPI app crashes or behaves unexpectedly.

When you want to track errors users encounter.

When debugging issues during development or after deployment.

When you want to keep a history of errors for later review.

Syntax

FastAPI

import logging

logger = logging.getLogger("myapp")

try:
    # code that may fail
    pass
except Exception as e:
    logger.error("An error occurred", exc_info=True)

Use logger.error() to log error messages.

Set exc_info=True to include the error details and stack trace.

Examples

Logs a division by zero error with details.

FastAPI

import logging

logger = logging.getLogger("myapp")

try:
    1 / 0
except Exception as e:
    logger.error("Division by zero error", exc_info=True)

Logs a file error without stack trace.

FastAPI

import logging

logger = logging.getLogger("myapp")

try:
    open("missing_file.txt")
except Exception as e:
    logger.error(f"File error: {e}")

Sample Program

This FastAPI app has a /divide endpoint that divides two numbers. If division by zero happens, it logs the error with details and returns a friendly message.

FastAPI

from fastapi import FastAPI
import logging

app = FastAPI()
logger = logging.getLogger("uvicorn.error")

@app.get("/divide")
async def divide(a: int, b: int):
    try:
        result = a / b
        return {"result": result}
    except Exception as e:
        logger.error("Error in divide endpoint", exc_info=True)
        return {"error": "Cannot divide by zero"}

Important Notes

FastAPI uses uvicorn as the default server, which has its own logger named uvicorn.error. Use it to integrate with server logs.

Configure logging level and format in your app or server to see error logs clearly.

Summary

Logging errors helps track and fix problems in FastAPI apps.

Use logger.error() with exc_info=True to log error details and stack traces.

Integrate with uvicorn.error logger for consistent logging in FastAPI.

Practice

1. What is the main purpose of using logger.error() with exc_info=True in a FastAPI application?

easy

A. To send an email notification when an error occurs

B. To log detailed error information including the stack trace

C. To automatically restart the FastAPI server on errors

D. To disable error logging temporarily

Solution

1. Step 1: Understand the role of logger.error()

   This method logs error messages in the application logs.

2. Step 2: Explain the effect of exc_info=True

   This argument adds the full exception traceback to the log, helping with debugging.

3. Final Answer:

   To log detailed error information including the stack trace -> Option B

4. Quick Check:

   Logging errors with traceback = A [OK]

Hint: Remember: exc_info=True shows full error details [OK]

Common Mistakes:

• Thinking logger.error() restarts the server
• Confusing error logging with notifications
• Assuming exc_info=True disables logging

2. Which of the following is the correct way to log an error with exception info in FastAPI using Python's logging module?

easy

A. logger.error("Error occurred")

B. logger.error(exc_info=True)

C. logger.error("Error occurred", exception=True)

D. logger.error("Error occurred", exc_info=True)

Solution

1. Step 1: Review the logging.error() method signature

   The method accepts a message string and optional keyword arguments like exc_info.

2. Step 2: Identify the correct usage to include exception info

   Using exc_info=True includes the traceback in the log output.

3. Final Answer:

   logger.error("Error occurred", exc_info=True) -> Option D

4. Quick Check:

   Use exc_info=True to log exceptions [OK]

Hint: Use exc_info=True inside logger.error() for full error trace [OK]

Common Mistakes:

• Omitting exc_info argument
• Passing exc_info as a positional argument
• Using non-existent argument 'exception=True'

3. Given this FastAPI error logging code snippet, what will be the output in the logs?

try:
    1 / 0
except Exception as e:
    logger.error("Division failed", exc_info=True)

medium

A. Logs only the message 'Division failed' without error details

B. Raises a ZeroDivisionError and stops the program

C. Logs the message 'Division failed' with a ZeroDivisionError traceback

D. Logs nothing because exception is caught

Solution

1. Step 1: Analyze the try-except block

   The code attempts division by zero, which raises ZeroDivisionError caught by except.

2. Step 2: Understand logger.error with exc_info=True

   It logs the message plus the full traceback of the caught exception.

3. Final Answer:

   Logs the message 'Division failed' with a ZeroDivisionError traceback -> Option C

4. Quick Check:

   exc_info=True logs error trace [OK]

Hint: exc_info=True logs full error trace, not just message [OK]

Common Mistakes:

• Thinking the error stops the program
• Assuming no traceback is logged
• Believing the log is empty on exception

4. You wrote this FastAPI error logging code but no error details appear in logs:

try:
    open('missing_file.txt')
except Exception:
    logger.error("File open failed")

What is the main fix to log the error details?

medium

A. Add exc_info=True to logger.error call

B. Change except Exception to except FileNotFoundError

C. Use logger.info instead of logger.error

D. Remove the try-except block

Solution

1. Step 1: Identify missing error detail logging

   The current logger.error call logs only the message without exception info.

2. Step 2: Add exc_info=True to include traceback

   Passing exc_info=True logs the full exception details automatically.

3. Final Answer:

   Add exc_info=True to logger.error call -> Option A

4. Quick Check:

   Include exc_info=True to log error details [OK]

Hint: Always add exc_info=True to log exceptions fully [OK]

Common Mistakes:

• Changing exception type without logging fix
• Using logger.info which is lower severity
• Removing try-except causing crashes

5. You want to log errors in a FastAPI app only when an exception occurs, including the traceback, but avoid logging if no error happens. Which code snippet correctly implements this?

hard

A. try:
risky_operation()
except Exception as e:
logger.error("Operation failed", exc_info=True)

B. logger.error("Operation failed", exc_info=True)
risky_operation()

C. try:
risky_operation()
except Exception:
logger.error("Operation failed")

D. try:
risky_operation()
except Exception as e:
logger.error("Operation failed")

Solution

1. Step 1: Ensure error logging happens only on exceptions

   Logging must be inside except block to avoid logging when no error occurs.

2. Step 2: Include exc_info=True to log traceback

   Passing exc_info=True ensures full error details are logged.

3. Final Answer:

   try: risky_operation() except Exception as e: logger.error("Operation failed", exc_info=True) -> Option A

4. Quick Check:

   Log errors with traceback only on exceptions [OK]

Hint: Log errors inside except with exc_info=True [OK]

Common Mistakes:

• Logging errors outside try-except block
• Omitting exc_info=True losing traceback
• Logging without exception object