Logging levels and basicConfig
Overview
Python's `logging` module replaces `print()` for production code. It categorizes messages into five severity levels, in increasing order of seriousness: DEBUG, INFO, WARNING, ERROR, CRITICAL. import logging logging.basicConfig(level=logging.INFO) logging.debug('Hidden') # below INFO -- not shown logging.info('Visible') # shown logging.warning('Also shown') `logging.basicConfig()` configures the root logger, setting the minimum level to display and the format of the output — only messages at that level or higher print. By default, before you call `basicConfig()`, the logging level is WARNING, meaning DEBUG and INFO messages are silently ignored.
Unlike `print()`, logging gives you a dial to control output volume without changing code (e.g., hiding DEBUG logs in production). It also automatically includes metadata like timestamps and module names.
Where used: Web servers (Django/FastAPI), Data pipelines, CLI tools
Why learn this
- Write production-ready code that emits searchable, structured logs instead of scattered print statements.
- Control the verbosity of your application's output.
Code walkthrough
import logging
logging.basicConfig(
level=logging.INFO,
format='%(levelname)s - %(message)s'
)
logging.debug('Hidden')
logging.info('Visible')Focus: level=logging.INFO,
Aha moment
import logging
logging.basicConfig(level=logging.INFO)
logging.basicConfig(level=logging.DEBUG)
logging.debug('Can you see me?')Prediction: Will the debug message be printed?
Common guess: Yes, because the level was changed to DEBUG.
It won't print. `basicConfig()` only works the FIRST time it is called. The second call does nothing, so the level remains INFO.
Common mistakes
- Calling basicConfig multiple times: `logging.basicConfig()` does nothing if the root logger is already configured. To reconfigure it, you must pass `force=True`.
- Wondering where INFO logs went: The default logging level is WARNING. If you call `logging.info('hello')` without configuring the logger, nothing will print.
Glossary
- severity levels
- Categories that indicate how serious or important a log message is, e.g. `logging.INFO` for general updates or `logging.CRITICAL` for system failures.
- verbosity
- The amount of detail or the number of messages a program outputs, e.g. setting `level=logging.DEBUG` is high verbosity, `level=logging.ERROR` is low verbosity.
Recall questions
- What is the default logging level if you don't configure the logger?
- Which function is used to configure the root logger's level and format?
- Name the five standard logging levels in increasing order of severity.
Understanding checks
What is printed to the console (ignoring format metadata)?
ERROR:root:B CRITICAL:root:C
The level is set to ERROR. WARNING is below ERROR, so 'A' is ignored. ERROR and CRITICAL are high enough to be printed.
What is printed to the console (assuming fresh execution)?
WARNING:root:World
Because no configuration was explicitly set, the default level is WARNING. The INFO message is discarded.
Practice tasks
Configure Logging Level
Given this code, modify the `basicConfig` call so that ONLY 'Critical issue' is printed.
Challenge
Force Configuration
Write a script that first calls `basicConfig` with INFO level, then uses `basicConfig` again to change the level to DEBUG by passing `force=True`, and finally prints a debug message 'Testing'.
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