Logging Levels in Python0%

Logging Levels in Python

Intermediate8 min read•Updated: 2026-10-01

Logging Levels in Python

Python's logging module categorizes diagnostic and operational messages using a standardized severity hierarchy. By assigning appropriate severity levels to log messages, you can control the volume of logging output without altering your source code—viewing granular debug details during local development while filtering down to warnings and errors in production.

1. The Five Standard Logging Levels

Python defines five standard levels, each associated with an integer severity weight:

Level Name	Numeric Value	When It Should Be Used
`DEBUG`	`10`	Detailed diagnostic information, variable state inspection, and function parameter tracking for developers.
`INFO`	`20`	Routine confirmation that operations and milestones are proceeding as expected (e.g. server booted, user signed in).
`WARNING`	`30`	Indication that something unexpected happened or a potential issue is emerging (e.g., deprecated API usage, low disk space), but execution continues normally.
`ERROR`	`40`	Due to a more serious problem, the software was unable to complete a specific task or transaction.
`CRITICAL`	`50`	A fatal event has occurred indicating the application or server itself may crash or be unable to continue executing.

Threshold Rule: Setting a logger's threshold to level L logs all messages with numeric value $\ge$ L, while silently suppressing everything below L.

2. Practical Examples for Every Level

1import logging
2 
3logging.basicConfig(
4    level=logging.DEBUG,
5    format="%(levelname)-8s | %(message)s"
6)
7 
8# 1. DEBUG: Granular details
9logging.debug("Connecting to database socket at 192.168.1.50:5432 with timeout=3s")
10 
11# 2. INFO: High-level operational events
12logging.info("Database connection established successfully. Pool size: 10")
13 
14# 3. WARNING: Potential hazards or degradations
15logging.warning("Query response took 1,420ms (exceeds 500ms SLA target)")
16 
17# 4. ERROR: Business transaction failure
18logging.error("Failed to charge credit card for Invoice #891: Card expired")
19 
20# 5. CRITICAL: Unrecoverable system failure
21logging.critical("Database storage drive mounted as READ-ONLY! Shutting down write worker.")

3. Capturing Exception Tracebacks: `logger.exception()`

When catching runtime exceptions, standard logging.error() records only your text message. To automatically include the complete Python traceback without manually formatting it, use logger.exception() (or pass exc_info=True to logger.error()):

1import logging
2 
3logging.basicConfig(level=logging.INFO)
4 
5def compute_ratio(numerator, denominator):
6    try:
7        return numerator / denominator
8    except ZeroDivisionError:
9        # Automatically appends the complete stack traceback to the log!
10        logging.exception("Division operation failed unexpectedly:")
11        return None
12 
13compute_ratio(100, 0)

Generated Output:

1ERROR:root:Division operation failed unexpectedly:
2Traceback (most recent call last):
3  File "app.py", line 7, in compute_ratio
4    return numerator / denominator
5ZeroDivisionError: division by zero

4. Dynamic Level Switching via Environment / CLI

In real-world applications, you configure the log level dynamically using environment variables or command-line flags:

1import logging
2import os
3 
4# Read log level from environment variable, defaulting to INFO
5env_level = os.getenv("APP_LOG_LEVEL", "INFO").upper()
6 
7numeric_level = getattr(logging, env_level, logging.INFO)
8 
9logging.basicConfig(
10    level=numeric_level,
11    format="%(asctime)s [%(levelname)s] %(message)s"
12)
13 
14logging.debug("This only appears if APP_LOG_LEVEL=DEBUG")
15logging.info("Application initialized.")

5. Summary Hierarchy Diagram

1Level           Numeric Value       Production Setting
2-------------------------------------------------------
3CRITICAL        50                  Always recorded
4ERROR           40                  Always recorded
5WARNING         30                  Standard Production Threshold (Default)
6INFO            20                  Staging / Operational Audits
7DEBUG           10                  Local Development & Troubleshooting

⚠️ Common Intermediate Mistakes & Gotchas

1. Calling `logging.basicConfig()` Multiple Times

logging.basicConfig() only does work the first time it is invoked. Subsequent calls are silently ignored unless force=True is set (Python 3.8+).

2. Misusing Log Severity Levels

Use DEBUG for fine-grained diagnostic details.
Use INFO for general operational confirmation.
Use WARNING for unexpected but recoverable events.
Use ERROR for failures affecting a specific operation.
Use CRITICAL for application-wide fatal crashes.

💻 Try It Yourself: Multi-Handler Logger (Console + File)

Scenario

Create a logger that prints INFO and higher messages to the console while simultaneously recording detailed DEBUG and higher messages to a persistent app.log file.

Complete Solution

1import logging
2 
3logger = logging.getLogger("AppLogger")
4logger.setLevel(logging.DEBUG)
5 
6# File handler (records everything)
7file_handler = logging.FileHandler("app.log", encoding="utf-8")
8file_handler.setLevel(logging.DEBUG)
9file_fmt = logging.Formatter("%(asctime)s [%(levelname)s] (%(filename)s:%(lineno)d) - %(message)s")
10file_handler.setFormatter(file_fmt)
11 
12# Console handler (records only INFO and above)
13console_handler = logging.StreamHandler()
14console_handler.setLevel(logging.INFO)
15console_fmt = logging.Formatter("[%(levelname)s] %(message)s")
16console_handler.setFormatter(console_fmt)
17 
18logger.addHandler(file_handler)
19logger.addHandler(console_handler)
20 
21logger.debug("Database connection pool initialized with 5 workers.")
22logger.info("Server started on http://localhost:8000")
23logger.warning("Disk space usage is at 82%.")

Expected Output

1[INFO] Server started on http://localhost:8000
2[WARNING] Disk space usage is at 82%.

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

What is the default logging level if no level is explicitly configured in logging.basicConfig()?

Question 2

Which logging level has the lowest numeric severity value?

Question 3

What method automatically records an ERROR level message along with the full exception traceback?

Question 4

If the active logging level is set to logging.ERROR, which of the following statements will be emitted?

Question 5

Why should passwords and sensitive API keys never be logged, even at the DEBUG level?

🚀 What's Next?

In the next lesson, we will continue your intermediate Python journey with Basic Debugging with pdb (9: Debugging and Logging).

👉 Continue to Next Lesson: Basic Debugging with pdb →

Next Lesson

Basic Debugging with pdb

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Using print vs logging	Basic Debugging with pdb

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

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Basic Debugging with pdb

Logging Levels in Python

Intermediate8 min read•Updated: 2026-10-01

Study Materials Cheatsheet

Logging Levels in Python

1. The Five Standard Logging Levels

Python defines five standard levels, each associated with an integer severity weight:

Level Name	Numeric Value	When It Should Be Used
`DEBUG`	`10`	Detailed diagnostic information, variable state inspection, and function parameter tracking for developers.
`INFO`	`20`	Routine confirmation that operations and milestones are proceeding as expected (e.g. server booted, user signed in).
`WARNING`	`30`	Indication that something unexpected happened or a potential issue is emerging (e.g., deprecated API usage, low disk space), but execution continues normally.
`ERROR`	`40`	Due to a more serious problem, the software was unable to complete a specific task or transaction.
`CRITICAL`	`50`	A fatal event has occurred indicating the application or server itself may crash or be unable to continue executing.

Threshold Rule: Setting a logger's threshold to level L logs all messages with numeric value $\ge$ L, while silently suppressing everything below L.

2. Practical Examples for Every Level

1import logging
2 
3logging.basicConfig(
4    level=logging.DEBUG,
5    format="%(levelname)-8s | %(message)s"
6)
7 
8# 1. DEBUG: Granular details
9logging.debug("Connecting to database socket at 192.168.1.50:5432 with timeout=3s")
10 
11# 2. INFO: High-level operational events
12logging.info("Database connection established successfully. Pool size: 10")
13 
14# 3. WARNING: Potential hazards or degradations
15logging.warning("Query response took 1,420ms (exceeds 500ms SLA target)")
16 
17# 4. ERROR: Business transaction failure
18logging.error("Failed to charge credit card for Invoice #891: Card expired")
19 
20# 5. CRITICAL: Unrecoverable system failure
21logging.critical("Database storage drive mounted as READ-ONLY! Shutting down write worker.")

3. Capturing Exception Tracebacks: `logger.exception()`

1import logging
2 
3logging.basicConfig(level=logging.INFO)
4 
5def compute_ratio(numerator, denominator):
6    try:
7        return numerator / denominator
8    except ZeroDivisionError:
9        # Automatically appends the complete stack traceback to the log!
10        logging.exception("Division operation failed unexpectedly:")
11        return None
12 
13compute_ratio(100, 0)

Generated Output:

1ERROR:root:Division operation failed unexpectedly:
2Traceback (most recent call last):
3  File "app.py", line 7, in compute_ratio
4    return numerator / denominator
5ZeroDivisionError: division by zero

4. Dynamic Level Switching via Environment / CLI

In real-world applications, you configure the log level dynamically using environment variables or command-line flags:

1import logging
2import os
3 
4# Read log level from environment variable, defaulting to INFO
5env_level = os.getenv("APP_LOG_LEVEL", "INFO").upper()
6 
7numeric_level = getattr(logging, env_level, logging.INFO)
8 
9logging.basicConfig(
10    level=numeric_level,
11    format="%(asctime)s [%(levelname)s] %(message)s"
12)
13 
14logging.debug("This only appears if APP_LOG_LEVEL=DEBUG")
15logging.info("Application initialized.")

5. Summary Hierarchy Diagram

1Level           Numeric Value       Production Setting
2-------------------------------------------------------
3CRITICAL        50                  Always recorded
4ERROR           40                  Always recorded
5WARNING         30                  Standard Production Threshold (Default)
6INFO            20                  Staging / Operational Audits
7DEBUG           10                  Local Development & Troubleshooting

⚠️ Common Intermediate Mistakes & Gotchas

1. Calling `logging.basicConfig()` Multiple Times

logging.basicConfig() only does work the first time it is invoked. Subsequent calls are silently ignored unless force=True is set (Python 3.8+).

2. Misusing Log Severity Levels

Use DEBUG for fine-grained diagnostic details.
Use INFO for general operational confirmation.
Use WARNING for unexpected but recoverable events.
Use ERROR for failures affecting a specific operation.
Use CRITICAL for application-wide fatal crashes.

💻 Try It Yourself: Multi-Handler Logger (Console + File)

Scenario

Create a logger that prints INFO and higher messages to the console while simultaneously recording detailed DEBUG and higher messages to a persistent app.log file.

Complete Solution

1import logging
2 
3logger = logging.getLogger("AppLogger")
4logger.setLevel(logging.DEBUG)
5 
6# File handler (records everything)
7file_handler = logging.FileHandler("app.log", encoding="utf-8")
8file_handler.setLevel(logging.DEBUG)
9file_fmt = logging.Formatter("%(asctime)s [%(levelname)s] (%(filename)s:%(lineno)d) - %(message)s")
10file_handler.setFormatter(file_fmt)
11 
12# Console handler (records only INFO and above)
13console_handler = logging.StreamHandler()
14console_handler.setLevel(logging.INFO)
15console_fmt = logging.Formatter("[%(levelname)s] %(message)s")
16console_handler.setFormatter(console_fmt)
17 
18logger.addHandler(file_handler)
19logger.addHandler(console_handler)
20 
21logger.debug("Database connection pool initialized with 5 workers.")
22logger.info("Server started on http://localhost:8000")
23logger.warning("Disk space usage is at 82%.")

Expected Output

1[INFO] Server started on http://localhost:8000
2[WARNING] Disk space usage is at 82%.

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

What is the default logging level if no level is explicitly configured in logging.basicConfig()?

Question 2

Which logging level has the lowest numeric severity value?

Question 3

What method automatically records an ERROR level message along with the full exception traceback?

Question 4

If the active logging level is set to logging.ERROR, which of the following statements will be emitted?

Question 5

Why should passwords and sensitive API keys never be logged, even at the DEBUG level?

🚀 What's Next?

In the next lesson, we will continue your intermediate Python journey with Basic Debugging with pdb (9: Debugging and Logging).

👉 Continue to Next Lesson: Basic Debugging with pdb →

Next Lesson

Basic Debugging with pdb

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Using print vs logging	Basic Debugging with pdb

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

Logging Levels in Python

1. The Five Standard Logging Levels

2. Practical Examples for Every Level

3. Capturing Exception Tracebacks: logger.exception()

Generated Output:

4. Dynamic Level Switching via Environment / CLI

5. Summary Hierarchy Diagram

⚠️ Common Intermediate Mistakes & Gotchas

1. Calling logging.basicConfig() Multiple Times

2. Misusing Log Severity Levels

💻 Try It Yourself: Multi-Handler Logger (Console + File)

Scenario

Complete Solution

Expected Output

Practice Quiz

🚀 What's Next?

Basic Debugging with pdb

Related Lessons

Practice Quiz

Loading Page Content...

Logging Levels in Python

1. The Five Standard Logging Levels

2. Practical Examples for Every Level

3. Capturing Exception Tracebacks: logger.exception()

Generated Output:

4. Dynamic Level Switching via Environment / CLI

5. Summary Hierarchy Diagram

⚠️ Common Intermediate Mistakes & Gotchas

1. Calling logging.basicConfig() Multiple Times

2. Misusing Log Severity Levels

💻 Try It Yourself: Multi-Handler Logger (Console + File)

Scenario

Complete Solution

Expected Output

Practice Quiz

🚀 What's Next?

Basic Debugging with pdb

Related Lessons

Practice Quiz

3. Capturing Exception Tracebacks: `logger.exception()`

1. Calling `logging.basicConfig()` Multiple Times

3. Capturing Exception Tracebacks: `logger.exception()`

1. Calling `logging.basicConfig()` Multiple Times