Logging configuration in FastAPI

Introduction

Logging helps you see what your FastAPI app is doing. It records messages about events, errors, and info so you can understand and fix problems.

You want to track errors in your FastAPI app to fix bugs.

You need to monitor app activity to understand user behavior.

You want to save important events for later review.

You want to debug your app during development.

You want to keep a record of requests and responses.

Syntax

FastAPI

import logging

logging.basicConfig(level=logging.LEVEL, format='FORMAT')

logger = logging.getLogger('your_logger_name')

logger.LEVEL('Your message')

Replace LEVEL with logging levels like DEBUG, INFO, WARNING, ERROR, or CRITICAL.

The format controls how the log message looks, including time, level, and message.

Examples

This sets the log level to INFO and prints messages like INFO:App started.

FastAPI

import logging

logging.basicConfig(level=logging.INFO, format='%(levelname)s:%(message)s')
logger = logging.getLogger('app')
logger.info('App started')

This shows detailed logs with time, level, and message, useful for debugging.

FastAPI

import logging

logging.basicConfig(level=logging.DEBUG, format='%(asctime)s - %(levelname)s - %(message)s')
logger = logging.getLogger('app')
logger.debug('Debugging info')

Sample Program

This FastAPI app logs an info message every time the root endpoint is called.

FastAPI

import logging
from fastapi import FastAPI

logging.basicConfig(level=logging.INFO, format='%(asctime)s - %(levelname)s - %(message)s')
logger = logging.getLogger('fastapi_app')

app = FastAPI()

@app.get('/')
async def root():
    logger.info('Root endpoint was called')
    return {'message': 'Hello World'}

Important Notes

Use different log levels to control what messages appear.

You can configure logging to write to files instead of the console.

Logging helps you find problems faster than print statements.

Summary

Logging records important events in your FastAPI app.

Use logging.basicConfig to set up logging format and level.

Call logger methods like info() or debug() to write messages.

Practice

1. What is the main purpose of configuring logging in a FastAPI application?

easy

A. To track app activity and errors for easier debugging

B. To speed up the app's response time

C. To automatically update the app's dependencies

D. To create user interface themes

Solution

1. Step 1: Understand logging purpose

   Logging records events and errors during app execution.

2. Step 2: Connect to debugging

   Tracking these events helps developers find and fix issues.

3. Final Answer:

   To track app activity and errors for easier debugging -> Option A

4. Quick Check:

   Logging = Track activity/errors [OK]

Hint: Logging is for tracking events and errors [OK]

Common Mistakes:

• Confusing logging with performance optimization
• Thinking logging updates app code automatically
• Mixing logging with UI design

2. Which of the following is the correct way to set up basic logging in a FastAPI app using Python's logging module?

easy

A. log.config(level=logging.DEBUG)

B. logging.setup(level='DEBUG')

C. logging.basicConfig(level=logging.DEBUG)

D. logging.start(level=logging.DEBUG)

Solution

1. Step 1: Recall Python logging setup

   The standard method is logging.basicConfig() to configure logging.

2. Step 2: Check syntax correctness

   logging.basicConfig(level=logging.DEBUG) correctly sets the log level.

3. Final Answer:

   logging.basicConfig(level=logging.DEBUG) -> Option C

4. Quick Check:

   Use basicConfig for logging setup [OK]

Hint: Use logging.basicConfig() to configure logging [OK]

Common Mistakes:

• Using non-existent logging.setup() method
• Misspelling logging as log
• Using start() instead of basicConfig()

3. Given this FastAPI logging setup code:

import logging
logging.basicConfig(level=logging.WARNING)
logging.debug('Debug message')
logging.warning('Warning message')

What will be printed to the console?

medium

A. Warning message

B. Debug message

C. Debug message\nWarning message

D. No output

Solution

1. Step 1: Understand logging levels

   Level WARNING means only WARNING and above are shown, DEBUG is lower level.

2. Step 2: Check which messages print

   DEBUG message is ignored, WARNING message is printed.

3. Final Answer:

   Warning message -> Option A

4. Quick Check:

   Level WARNING shows warnings only [OK]

Hint: Only messages at or above set level print [OK]

Common Mistakes:

• Expecting DEBUG messages to print at WARNING level
• Thinking all messages print regardless of level
• Confusing logging levels order

4. You wrote this code to configure logging:

import logging
logging.basicConfig(level='DEBUG')
logging.info('Info message')

But no output appears. What is the likely problem?

medium

A. You need to import sys module first

B. The level argument should be logging.DEBUG, not a string

C. basicConfig() must be called after logging.info()

D. logging.info() does not print messages

Solution

1. Step 1: Check logging level argument type

   logging.basicConfig expects level as a constant like logging.DEBUG, not a string.

2. Step 2: Understand effect of wrong level

   Passing 'DEBUG' string is ignored, default level WARNING applies, so info messages don't show.

3. Final Answer:

   The level argument should be logging.DEBUG, not a string -> Option B

4. Quick Check:

   Level must be logging.DEBUG constant [OK]

Hint: Use logging.DEBUG constant, not string 'DEBUG' [OK]

Common Mistakes:

• Passing level as string instead of constant
• Calling basicConfig after logging calls
• Assuming info messages never print

5. You want to log messages to a file named app.log with level INFO and include timestamps. Which is the correct logging configuration code?

hard

A. logging.basicConfig(file='app.log', level='INFO', format='%(time)s - %(msg)s')

B. logging.basicConfig(filename='app.log', level=logging.DEBUG, format='%(message)s')

C. logging.setup(filename='app.log', level=logging.INFO, format='%(asctime)s - %(message)s')

D. logging.basicConfig(filename='app.log', level=logging.INFO, format='%(asctime)s - %(message)s')

Solution

1. Step 1: Check correct parameter names

   logging.basicConfig uses filename, level, and format parameters correctly.

2. Step 2: Verify format string and level

   Format '%(asctime)s - %(message)s' adds timestamp; level INFO matches requirement.

3. Final Answer:

   logging.basicConfig(filename='app.log', level=logging.INFO, format='%(asctime)s - %(message)s') -> Option D

4. Quick Check:

   Use filename, level=logging.INFO, correct format [OK]

Hint: Use filename and format with %(asctime)s for timestamps [OK]

Common Mistakes:

• Using wrong parameter names like file instead of filename
• Passing level as string instead of constant
• Incorrect format placeholders