Logging in production in Flask - Step-by-Step Execution

Concept Flow - Logging in production

Start Flask App

↓

Configure Logger

↓

App Running

↓

Event Occurs

↓

Logger Records Event

↓

Log Stored in File or External System

↓

Monitor Logs for Issues

This flow shows how a Flask app starts, sets up logging, records events during runtime, and stores logs for monitoring.

Execution Sample

Flask

import logging
from flask import Flask
app = Flask(__name__)
logging.basicConfig(filename='app.log', level=logging.INFO)
@app.route('/')
def home():
    app.logger.info('Home page accessed')
    return 'Hello, world!'

This code sets up a Flask app with logging to a file and logs a message when the home page is accessed.

Execution Table

Step	Action	Logger State	Log Output	Result
1	Start Flask app	Logger configured, no logs yet	None	App ready to receive requests
2	Receive GET request at '/'	Logger ready	None	home() function called
3	home() logs info message	Logger has one INFO log	INFO:flask.app:Home page accessed	Log written to app.log
4	home() returns response	Logger unchanged	None	Client receives 'Hello, world!'
5	No more requests	Logger unchanged	None	App waits for next event

💡 No more requests, app continues running waiting for events

Variable Tracker

Variable	Start	After Step 3	Final
app.logger	Configured with INFO level	Contains 1 log entry	Contains 1 log entry

Key Moments - 2 Insights

Why don't we see log output immediately on the console?

Because logging is configured to write to a file ('app.log'), so logs are stored there instead of printing to the console. See execution_table step 3.

What happens if the log level is set to WARNING but we log an INFO message?

The INFO message will be ignored and not recorded because the logger only records messages at WARNING level or higher. This is why setting the correct log level is important.

Visual Quiz - 3 Questions

Look at the execution table at step 3. What does the logger do?

A It prints the message to the console

B It writes an INFO message to the log file

C It ignores the message because it's too low level

D It crashes the app

💡 Hint

Check the 'Log Output' column at step 3 in the execution_table

At which step does the Flask app respond to the client?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Result' column for when the client receives a response

If we change logging level to WARNING, what happens to the INFO log in step 3?

A It is ignored and not logged

B It is still logged

C It causes an error

D It is logged twice

💡 Hint

Refer to key_moments about log levels and filtering

Concept Snapshot

Flask logging in production:
- Configure logging with logging.basicConfig(filename, level)
- Use app.logger.info/debug/warning to log events
- Logs go to file or external system, not console by default
- Set appropriate log level to filter messages
- Logs help monitor app behavior and troubleshoot

Full Transcript

This visual execution shows how a Flask app sets up logging to a file and records an info message when the home page is accessed. The app starts, configures the logger to write INFO level logs to 'app.log', then when a client requests the home page, the app logs 'Home page accessed' to the file. The client receives the response 'Hello, world!'. The logger state changes from empty to having one log entry. Key points include understanding that logs go to a file, not console, and that log levels control which messages are recorded. The quiz tests understanding of when logs are written, when responses are sent, and how log levels affect logging.