Logging

Video ref: https://www.youtube.com/watch?v=pxuXaaT1u3k

Overview

Python's built-in logging module is the standard for production logging. Avoid using print() in production — it has no severity levels, no formatting, and can't be disabled without changing code.

Log Levels

Default level is WARNING — DEBUG/INFO are suppressed unless configured.

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Basic Setup

import logging

logging.basicConfig(
    level=logging.DEBUG,
    format="%(asctime)s - %(name)s - %(levelname)s - %(message)s",
    datefmt="%Y-%m-%d %H:%M:%S",
)

logger = logging.getLogger(__name__)  # use module name, not root logger

logger.debug("Debug message")
logger.info("Server started on port 8080")
logger.warning("Disk usage above 80%%")
logger.error("Failed to connect to database")
logger.critical("Out of memory — shutting down")

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Logger Hierarchy

Loggers are named in a hierarchy using dots: app, app.db, app.api. Child loggers propagate to parent unless propagate = False.

# Module-level pattern — best practice
logger = logging.getLogger(__name__)
# In myapp/db.py this becomes "myapp.db"

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Handlers

Handlers send log records to different destinations:

import logging

logger = logging.getLogger("myapp")
logger.setLevel(logging.DEBUG)

# Console handler
console = logging.StreamHandler()
console.setLevel(logging.INFO)

# File handler
file_handler = logging.FileHandler("app.log")
file_handler.setLevel(logging.DEBUG)

# Formatter
fmt = logging.Formatter("%(asctime)s [%(levelname)s] %(name)s: %(message)s")
console.setFormatter(fmt)
file_handler.setFormatter(fmt)

logger.addHandler(console)
logger.addHandler(file_handler)

Common handlers:

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Structured Logging (Production)

For production/cloud environments, log JSON instead of plain text so tools like Datadog, CloudWatch, or ELK can parse fields.

import logging
import json

class JSONFormatter(logging.Formatter):
    def format(self, record):
        log_data = {
            "time": self.formatTime(record),
            "level": record.levelname,
            "name": record.name,
            "message": record.getMessage(),
        }
        if record.exc_info:
            log_data["exc_info"] = self.formatException(record.exc_info)
        return json.dumps(log_data)

Or use python-json-logger library:

pip install python-json-logger

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Exception Logging

try:
    result = 1 / 0
except ZeroDivisionError:
    logger.exception("Division failed")  # logs ERROR + full traceback automatically
    # equivalent to: logger.error("msg", exc_info=True)

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Key Rules

• Always use logging.getLogger(__name__) — never use the root logger directly in libraries
• Set level on the logger, not just the handler
• Use logger.exception() inside except blocks — captures traceback automatically
• Use %s style formatting in log messages (logger.debug("val: %s", x)) — lazy evaluation, no string built if level disabled