Great question! Let's break down the differences between DEBUG and INFO in Python's logging library, along with clear scenarios for each.

First, a quick note on logging hierarchy: Python's logging uses levels ordered from lowest to highest as DEBUG < INFO < WARNING < ERROR < CRITICAL. By default, the logging system only outputs messages at INFO level or higher—so DEBUG messages won't show up unless you explicitly configure your logger to include them.

The key semantic difference is:

• DEBUG: Developer-focused, granular details about the inner workings of your code. These are messages you only need when troubleshooting or debugging.
• INFO: Operational/audience-focused, high-level status updates that confirm the program is working as expected. These are useful for monitoring, auditing, or understanding the overall flow of the system without diving into code details.

Use DEBUG when you need to see fine-grained details that help you diagnose issues or verify that a specific part of your code is behaving correctly. Common scenarios include:

• Tracking variable values: Logging the value of a complex data structure, loop counter, or input parameter to ensure it's what you expect.
• Following code flow: Logging when a function is entered/exited, or which branch of a conditional statement was taken (especially in complex logic).
• Debugging external interactions: Logging full request/response payloads when calling APIs, or raw SQL queries sent to a database—this helps spot issues like malformed requests.
• Testing edge cases: Logging details about how the code handles unusual inputs or rare scenarios during development.

Example:

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

def process_order(order_id):
    logging.debug(f"Starting processing for order ID: {order_id}")
    order_details = fetch_order_details(order_id)
    logging.debug(f"Fetched order details: {order_details}")
    # ... rest of the code

Use INFO to log events that are important for understanding the overall health and activity of your application, even in production. These are messages that non-developers (like sysadmins or support teams) might care about. Common scenarios include:

• Application lifecycle events: Logging when the app starts, stops, or finishes initializing (e.g., "Server initialized and listening on port 5000").
• Key user actions: Logging successful user logins, account creations, or major transactions (e.g., "User 'jane_smith' completed purchase of order #1234").
• Completed workflows: Logging when a batch job finishes, a data sync completes, or a long-running task is done (e.g., "Daily inventory sync finished: 2500 records updated").
• System state changes: Logging when critical resources are acquired or released (e.g., "Connected to primary database successfully").

Example:

import logging
logging.basicConfig(level=logging.INFO)

def start_server():
    logging.info("Starting Flask server in production mode")
    app.run(host='0.0.0.0', port=5000)
    logging.info("Server stopped gracefully")

• DEBUG: For developers, during development/debugging—details you don't want cluttering production logs.
• INFO: For everyone monitoring the system—high-level updates that confirm normal operation.

内容的提问来源于stack exchange，提问作者Dull Bananas