Logging levels communicate how severe an event is: DEBUG for development detail, INFO for expected events, WARNING for unusual behavior, ERROR for failed operations and CRITICAL for severe failures.
Understanding the Concept
Logging levels communicate how severe an event is: DEBUG for development detail, INFO for expected events, WARNING for unusual behavior, ERROR for failed operations and CRITICAL for severe failures. Severity is independent of whether the message is printed.
A logging handler and threshold determine what is emitted; some messages may be filtered out.
How this helps diagnose real applications
First decide what question you are trying to answer: whether a particular branch runs, why an operation failed, or what contextual event should be recorded. Logging, interactive debugging, traceback inspection and controlled timing complement each other but produce different kinds of evidence.
Diagnostic output should preserve enough context to reproduce the problem without recording passwords, tokens, full personal records or untrusted payloads. Keep instrumentation focused so it does not hide the original behavior.
Worked Example
Read the code from the first line to the last. Identify which values are inputs, which operation is being demonstrated and what should happen before running it.
import logging
logging.basicConfig(level=logging.WARNING, format="%(levelname)s: %(message)s")
logging.info("Job started")
logging.warning("Retrying in a moment")
logging.error("Job could not finish")Walkthrough: What Each Part Does
- The imports make logging available. These are standard-library building blocks unless the example explicitly names an external tool.
- Observe which message or diagnostic field appears and which condition caused it. Modify the input deliberately to test the failure path.
This demonstration focuses on one practical behavior. Before changing several lines, vary one input or one configuration choice and check whether the result changes for the reason you expected.
The example completed in the local test environment. Its output is not fixed enough to publish as a universal expected result; inspect the result on your own computer.
Important Behavior and Edge Cases
Logging routine success as ERROR or suppressing all warnings just to make output shorter.
A logging handler and threshold determine what is emitted; some messages may be filtered out.
Practice Exercise
Select an appropriate level for a successful login, an invalid file format and a failed database connection.
Write down the result you expect before running your changed code. Check at least one normal case and one boundary or failure case. If your example starts workers or opens resources, verify how they are cleaned up.
Further reading: official documentation and API reference.
Common Beginner Mistakes
- Logging routine success as ERROR or suppressing all warnings just to make output shorter.
- Removing the diagnostic message after seeing it once instead of reproducing the failure and checking the underlying cause.
Key Takeaways
- Logging levels communicate how severe an event is: DEBUG for development detail, INFO for expected events, WARNING for unusual behavior, ERROR for failed operations and CRITICAL for severe failures.
- A logging handler and threshold determine what is emitted; some messages may be filtered out.
- Practice: Select an appropriate level for a successful login, an invalid file format and a failed database connection.