Python logging basicconfig format and examples
The logging.basicConfig() function is the easiest way to perform basic setup for the Python logging module. It sets the configuration for the root logger, which is the parent of all other loggers in your application.
This function is ideal for simple scripts, development environments, and applications where you only need a single, global logging configuration.
1. Understanding the basicConfig Parameters
logging.basicConfig() accepts several keyword arguments to define the minimum logging level, the output destination, and the structure of the log messages.
| Parameter | Purpose | Example |
|---|---|---|
level | Sets the minimum severity level to handle. Logs below this level are ignored. | level=logging.INFO |
filename | Specifies a file path to write logs to. If omitted, logs go to the console. | filename='app.log' |
filemode | Specifies the file open mode ('a' for append, 'w' for overwrite). Default is 'a'. | filemode='w' |
format | Defines the exact structure and content of the log message string. | format='%(asctime)s - %(levelname)s - %(message)s' |
stream | Specifies an output stream (e.g., sys.stderr or sys.stdout) if filename is not used. | stream=sys.stdout |
2. The Log Message Format String
The format parameter is a standard Python string that contains special placeholder variables (prefixed with %(...)s). These variables are dynamically replaced with details about the logging event.
Here are the most common and useful format specifiers:
| Specifier | Description | Example Output |
|---|---|---|
%(asctime)s | The time the LogRecord was created. | 2025-12-14 07:05:00,123 |
%(levelname)s | The text logging level for the message. | INFO, ERROR, DEBUG |
%(name)s | The name of the logger that generated the event. | MyApp.Worker1 |
%(message)s | The actual log message content. | Processing complete. |
%(module)s | The name of the module (file) where the call was made. | database_module |
%(lineno)d | The line number within the module where the call was made. | 145 |
Example Format String
A robust format string for production might combine several specifiers:
FORMAT = '%(asctime)s [%(levelname)s] (%(filename)s:%(lineno)d) %(message)s'
Example Output: 2025-12-14 07:05:00,123 [ERROR] (api.py:145) Failed to connect to server.
3. Basic Configuration Examples
Example A: Development Console Output
This configuration is perfect for local development. It sets the level to DEBUG and outputs a detailed, yet simple, format to the console.
import logging
import sys
# Configure for DEBUG level output to standard output (stdout)
logging.basicConfig(
level=logging.DEBUG,
stream=sys.stdout,
format='%(levelname)s: %(message)s (%(filename)s:%(lineno)d)'
)
logger = logging.getLogger("DevApp")
logger.debug("Debugging data structure creation.") # Will show
logger.info("Service initialized.") # Will show
logger.warning("Resource limits are high.") # Will show
Output:
DEBUG: Debugging data structure creation. (script_name.py:10)
INFO: Service initialized. (script_name.py:11)
WARNING: Resource limits are high. (script_name.py:12)
Example B: Production File Output
This configuration sets a higher severity level (WARNING) and directs all output to a file, using a timestamped format.
import logging
LOG_FILE = 'prod_audit.log'
# Configure for WARNING level output to a persistent file
logging.basicConfig(
level=logging.WARNING,
filename=LOG_FILE,
filemode='a', # Append to the file
format='%(asctime)s - %(name)s - %(levelname)s - %(message)s',
datefmt='%Y-%m-%d %H:%M:%S' # Custom date format
)
logger = logging.getLogger("AuditService")
logger.info("Startup complete.") # Will NOT be written (level < WARNING)
logger.error("Critical failure during transaction.") # Will be written
Output in prod_audit.log:
2025-12-14 07:08:15 - AuditService - ERROR - Critical failure during transaction.
4. Important Gotcha: basicConfig Runs Only Once
The most crucial rule of logging.basicConfig() is that it must be called before any loggers are created or any logging calls are made.
If the root logger already has handlers attached (which happens if basicConfig() or another configuration function has been called previously), subsequent calls to basicConfig() will be silently ignored.
Incorrect Usage (The Second Call is Ignored):
import logging
# 1. First call sets INFO level
logging.basicConfig(level=logging.INFO)
logging.info("First config is active.")
# 2. Second call tries to set DEBUG level, but is IGNORED
logging.basicConfig(level=logging.DEBUG)
logging.debug("This DEBUG message will NOT show!")
Solution: Only call basicConfig() once at the very start of your application, or use the more advanced logging.config.dictConfig() method for dynamic configuration.
5. Advanced: Custom Time Formatting
The default format for %(asctime)s is ISO 8601-like, but you can specify a precise format using the datefmt parameter, which accepts standard strftime directives.
import logging
import sys
# Configure the format and the date format
logging.basicConfig(
level=logging.INFO,
stream=sys.stdout,
format='%(asctime)s - %(message)s',
datefmt='%Y/%m/%d %I:%M:%S %p' # e.g., 2025/12/14 07:08:15 AM
)
logging.info("Custom time format test.")