A Comprehensive Guide to Logging in Python

Logging is an invaluable tool for developing reliable software applications. It provides a way to record and track events in the software, leaving an audit trail that captures the sequence of operations occurring within the system.

This audit trail of events can be used to diagnose problems and understand program behavior at runtime. Logs can also be used to track user behavior, giving product teams and designers insight into the effectiveness of a design decision by analyzing user interactions with it.

The Python logging module offers a comprehensive and flexible logging system that can be used to log messages of various severity levels and to control how log messages are processed and displayed. This article will provide a comprehensive overview of this module and how to configure its behavior to meet your specific needs.

Whether you are a Python beginner or an experienced developer, you will gain a solid understanding of how to use the logging module to configure an effective logging strategy in your applications. Here's a summary of you will learn in this tutorial:

Prerequisites

Before we begin, you should have the latest Python version installed on your machine (v3.10 at the time of writing). If you are missing Python, you can find the installation instructions here.

Getting started with the logging module

Python's built-in logging module is incredibly powerful and boasts the necessary logging features that one might require from a separate logging framework like pino for Node.js or Log4j for Java. Therefore, it remains the most popular way to log in the Python ecosystem despite the existence of a few third-party logging frameworks like Loguru.

Providing a logging module in the standard library ensures that all Python programs can benefit from having consistent logging features, making it easier to adopt and understand when working on different projects. To get started with the logging module, you need to import it to your program first, as shown below:

import logging

logging.debug("A debug message")
logging.info("An info message")
logging.warning("A warning message")
logging.error("An error message")
logging.critical("A critical message")

In the example above, we imported the logging module and used several of its methods. When you execute the above program, you should observe the following output:

WARNING:root:A warning message
ERROR:root:An error message
CRITICAL:root:A critical message

Notice that the debug() and info() messages are missing from the output while the others are present. This is due to the default log level configured on the logging module, which we will get to shortly. For now, understand that this is the intended behavior of the example.

In each record above, you will observe that the log level (WARNING, ERROR, etc) is printed in all caps followed by root, which is the name of the default logger. Finally you have the log message that was passed as an argument to the function. This output format is summarized below:

<LEVEL>:<name>:<message>

We will discuss how you can customize this log format to include other information such as timestamp, file name, and other details in a subsequent section. With the basics of the logging module out of the way, let's discuss the various log levels available and how you can leverage them in your programs.

Understanding log levels in Python

Log levels define the severity of the event that is being logged. They convey implicit meaning about the program state when the message was recorded, which is crucial when sieving through large logs for specific events. For example a message logged at the INFO level indicates a normal and expected event, while one that is logged at the ERROR level signifies that some unexpected error has occurred.

Each log level in Python is associated with a number (from 10 to 50) and has a corresponding module-level method in the logging module as demonstrated in the previous example. The available log levels in the logging module are listed below in increasing order of severity:

1. DEBUG (10): used to log messages that are useful for debugging.
2. INFO (20): used to log events within the parameters of expected program behavior.
3. WARNING (30): used to log unexpected events which may impede future program function but not severe enough to be an error.
4. ERROR (40): used to log unexpected failures in the program. Often, an exception needs to be raised to avoid further failures, but the program may still be able to run.
5. CRITICAL (50): used to log severe errors that can cause the application to stop running altogether.

It's important to always use the most appropriate log level so that you can quickly find the information you need. For instance, logging a message at the WARNING level will help you find potential problems that need to be investigated, while logging at the ERROR or CRITICAL level helps you discover problems that need to be rectified immediately. If an inappropriate log level is used, you will likely miss important events and your application will be worse off as a result.

By default, the logging module will only produce records for events that have been logged at a severity level of WARNING and above. This is why the debug() and info() messages were omitted in the previous example since they are less severe than WARNING. You can change this behavior using the logging.basicConfig() method as demonstrated below:

import logging

logging.basicConfig(level=logging.INFO)

logging.debug("A debug message")
logging.info("An info message")
logging.warning("A warning message")
logging.error("An error message")
logging.critical("A critical message")

Ensure to place the call to logging.basicConfig() before any methods such as info(), warning(), and others are used. It should also be called once as it is a one-off configuration facility. If called multiple times, only the first one will have an effect.

When you execute the program now, you will observe the output below:

INFO:root:An info message
WARNING:root:A warning message
ERROR:root:An error message
CRITICAL:root:A critical message

Since we've set the minimum level to INFO, we are now able to view the log message produced by logging.info() in addition levels with a greater severity while the DEBUG message remains suppressed.

Setting a default log level in the manner shown above is useful for controlling the amount of logs that is generated by your application. For example, during development you could set it to DEBUG to see all the log messages produced by the application, while production environments could use INFO or WARNING.

Adding custom log levels to Python

If you're coming to Python from other languages, you may have used other log levels like TRACE or FATAL which are not present in Python by default. However, the logging module is quite extensible so it is easy to add custom levels as you see fit. Here's an example that adds a TRACE level and associates it with the constant number 5 (5 less than DEBUG).

import logging


# Adopted from https://stackoverflow.com/a/35804945/1691778
# Adds a new logging method to the logging module
def addLoggingLevel(levelName, levelNum, methodName=None):
    if not methodName:
        methodName = levelName.lower()

    if hasattr(logging, levelName):
        raise AttributeError("{} already defined in logging module".format(levelName))
    if hasattr(logging, methodName):
        raise AttributeError("{} already defined in logging module".format(methodName))
    if hasattr(logging.getLoggerClass(), methodName):
        raise AttributeError("{} already defined in logger class".format(methodName))

    def logForLevel(self, message, *args, **kwargs):
        if self.isEnabledFor(levelNum):
            self._log(levelNum, message, args, **kwargs)

    def logToRoot(message, *args, **kwargs):
        logging.log(levelNum, message, *args, **kwargs)

    logging.addLevelName(levelNum, levelName)
    setattr(logging, levelName, levelNum)
    setattr(logging.getLoggerClass(), methodName, logForLevel)
    setattr(logging, methodName, logToRoot)

# Create the TRACE level
addLoggingLevel("TRACE", logging.DEBUG - 5)
logging.basicConfig(level=logging.TRACE)

# Use the TRACE level
logging.trace("A trace message")
logging.debug("A debug message")
logging.info("An info message")
logging.warning("A warning message")
logging.error("An error message")
logging.critical("A critical message")

With this in place, you can use logging.trace() wherever you want and it will work in exactly the same way as the built-in levels.

TRACE:root:A trace message
DEBUG:root:A debug message
INFO:root:An info message
WARNING:root:A warning message
ERROR:root:An error message
CRITICAL:root:A critical message

Customizing the log format

Beyond the log message itself, it is essential to include as much diagnostic information about the context surrounding each event being logged. Standardizing the structure of your logs entries makes it much easier to parse them and extract only the necessary details.

Generally, a good log entry should include at least the following properties:

1. Timestamp: indicating the time that the event being logged occurred.
2. Log level: indicating the severity of the event.
3. Message: describing the details of the event.

These properties allow for basic filtering by log management tools so you must ensure they are present in all your Python log entries. At the moment, the timestamp is missing from our examples so far which prevents us from knowing when each log entry was created. Let's fix this by changing the log format through the logging.basicConfig() method as shown below:

import logging

logging.basicConfig(format="%(levelname)s | %(asctime)s | %(message)s")
logging.warning("Something bad is going to happen")

The format argument above configures how log messages are displayed. It uses the following LogRecord attributes for formatting the logs:

• %(levelname)s: the log level name (INFO, DEBUG, WARNING, etc).
• %(asctime)s: human-readable time indicating when the log was created.
• %(message)s: the log message.

With the above configuration in place, all log entries produced by the logging module will now be formatted in following manner:

WARNING | 2023-02-05 11:41:31,862 | Something bad is going to happen

Notice that the root name no longer appears in the log entry because the log format has been redefined to exclude it. The fields are also now separated by spaces and the | character to make them easier to read. After the log level, you have the log creation time which can be customized further using the datefmt argument to basicConfig():

import logging

logging.basicConfig(
    format="%(levelname)s | %(asctime)s | %(message)s",
    datefmt="%Y-%m-%dT%H:%M:%SZ",
)
logging.warning("Something bad is going to happen")

WARNING | 2023-02-05T11:45:31Z | Something bad is going to happen

The datefmt argument accepts the same directives as the time.strftime() method. We recommend using the ISO-8601 format shown above because it is widely recognized and supported which makes it easy to process and sort timestamps in a consistent manner.

Please refer to the LogRecord documentation to examine all the available attributes that you can use to format your log entries. Here's another example that uses a few more of these attributes to customize the log format:

import logging

logging.basicConfig(
    format="%(name)s: %(asctime)s | %(levelname)s | %(filename)s:%(lineno)s | %(process)d >>> %(message)s",
    datefmt="%Y-%m-%dT%H:%M:%SZ",
)

logging.warning("system disk is 85% full")
logging.error("unexpected error")

root: 2023-02-05T14:11:56Z | WARNING | example.py:8 | 428223 >>> system disk is 85% full
root: 2023-02-05T14:11:56Z | ERROR | example.py:9 | 428223 >>> unexpected error

Setting up custom loggers

So far in this tutorial, we've used the default logger (named root) to write logs in the examples. The default logger is accessible and configurable through module-level methods on the logging module (such as logging.info(), logging.basicConfig()). However, it is generally considered a bad idea to log in this manner due to a number of reasons:

1. Namespace collisions: If multiple modules in your application use the root logger, you risk having messages from different modules logged under the same name, which can make it more difficult to understand where log messages are coming from.

2. Lack of control: When using the root logger, it can be difficult to control the log level for different aspects of your application. This can lead to logging too much information or not enough information, depending on your needs.

3. Loss of context: Log messages generated by the root logger may not contain enough information to provide context about where the log message came from, which can make it difficult to determine the source of issues in your application.

In general, it's best to avoid using the root logger and instead create a separate logger for each module in your application. This allows you to easily control the logging level and configuration for each module, and provide better context for log messages. Creating a new custom logger can be achieved by calling the getLogger() method as shown below:

import logging

logger = logging.getLogger("example")

logger.info("An info")
logger.warning("A warning")

A warning

The getLogger() method returns a Logger object whose name is set to the specified name argument (example in this case). A Logger provides all the functions for you to log messages in your application. Unlike the root logger, its name is not included in the log output by default. In fact, we don't even get the log level, only the log message is present. However, it defaults to logging to the standard error just like the root logger.

To change the output and behaviour of a custom logger, you have to use the Formatter and Handler classes provided by the logging module. The Formatter does what you'd expect; it helps with formatting the output of the logs, while the Handler specifies the log destination which could be the console, a file, an HTTP endpoint, and more. We also have Filter objects which provide sophisticated filtering capabilities for your Loggers and Handlers.

Using Handlers

A Handler object can be instantiated and added to a Logger to send the logs to a given destination, such as the console, a file, network socket, or HTTP API. There are several types of Handler objects, each with a preconfigured destination. For example, StreamHandler sends logs to streams, while FileHandler sends logs to disk files.

You can also configure multiple Handlers on a single Logger instance so that the logs produced through that Logger can be sent to multiple destinations. We will show an example of this later on in this tutorial. For now, let's configure the StreamHandler class on our logger so that it logs to the standard output instead of the standard error.

import sys
import logging

logger = logging.getLogger("example")

stdout = logging.StreamHandler(stream=sys.stdout)
stdout.setLevel(logging.INFO)

logger.addHandler(stdout)

logger.info("An info")
logger.warning("A warning")

The StreamHandler() class accepts a stream argument which is defined to be the standard output in this example. The resulting object is stored in the stdout variable and a minimum level of INFO is set on this object so that only records of INFO severity or higher are logged to the standard output.

However, when you execute the above program, you'll notice that only the WARNING level was produced as before:

A warning

This is because while the stdout handler allows INFO logs or higher to pass through, the minimum level on the logger is still at WARNING so the INFO log will continue to be suppressed. To fix this you must set the minimum level on the Logger object itself as shown below:

. . .
logger.setLevel(logging.INFO)

logger.info("An info")
logger.warning("A warning")

At this point, both entries will now be present in the output:

An info
A warning

The usefulness of setting a minimum level on the Handler object will become apparent later when we demonstrate using multiple handlers. For now though, let's look at how to customize the log format.

Using Formatters

The logging.Formatter class determines the format of the log entries produced by a Logger. By default, a Logger will not have a specific format, so all it outputs is the log message (that is %(message)s as seen in the previous section). We can create and attach a Formatter object to the Logger to give it a specific format.

import sys
import logging

logger = logging.getLogger("example")

stdout = logging.StreamHandler(stream=sys.stdout)

fmt = logging.Formatter(
    "%(name)s: %(asctime)s | %(levelname)s | %(filename)s:%(lineno)s | %(process)d >>> %(message)s"
)

stdout.setFormatter(fmt)
logger.addHandler(stdout)

logger.setLevel(logging.INFO)

logger.info("An info")
logger.warning("A warning")

The Formatter class above is configured using the same LogRecord attributes from our earlier logging.basicConfig() examples. It also accepts some other options but this is enough for us to get started.

When you execute the program now, you'll observe the following output:

example: 2023-02-05 21:21:46,634 | INFO | example.py:17 | 653630 >>> An info
example: 2023-02-05 21:21:46,634 | WARNING | example.py:18 | 653630 >>> A warning

Notice that the logger name (example) corresponds to the string argument passed to the getLogger() method. You can also set the logger name to be the name of the current module by using the special __name__ variable:

logger = logging.getLogger(__name__)

This will produce the following output:

__main__: 2023-02-05 21:42:50,882 | INFO | example.py:17 | 675470 >>> An info
__main__: 2023-02-05 21:42:50,882 | WARNING | example.py:18 | 675470 >>> A warning

Now, the logger name is registered as __main__ indicating that the records were logged from the module where execution starts. If the example.py file is imported in some other file and that file is executed, the __name__ variable will correspond to the module name (example).

import example

python exec.py

example: 2023-02-05 21:21:46,634 | INFO | example.py:17 | 653630 >>> An info
example: 2023-02-05 21:21:46,634 | WARNING | example.py:18 | 653630 >>> A warning

Coloring Python log output

If you're logging to the console as we've been doing so far, it may be helpful to color your log output so that the different fields and levels are easily distinguishable at a glance. Here's an example that uses the colorlog to achieve log output coloring in Python:

import sys
import logging
import colorlog

logger = logging.getLogger("example")

stdout = colorlog.StreamHandler(stream=sys.stdout)

fmt = colorlog.ColoredFormatter(
    "%(name)s: %(white)s%(asctime)s%(reset)s | %(log_color)s%(levelname)s%(reset)s | %(blue)s%(filename)s:%(lineno)s%(reset)s | %(process)d >>> %(log_color)s%(message)s%(reset)s"
)

stdout.setFormatter(fmt)
logger.addHandler(stdout)

logger.setLevel(logging.DEBUG)

logger.debug("A debug message")
logger.info("An info message")
logger.warning("A warning message")
logger.error("An error message")
logger.critical("A critical message")

Ensure to install the package first before executing the above program:

pip install colorlog

You should now observe the following output:

See the colorlog documentation for more details on the different options available.

Using Filters

Filter objects in Python's logging module allow you to selectively include or exclude log messages based on specific criteria. They implement a single method filter(record), which returns either True to include the log message or False to exclude the log message. Here's an example:

import sys
import logging


class LevelFilter(logging.Filter):
    def __init__(self, level):
        self.level = level

    def filter(self, record):
        if record.levelno == self.level:
            return True


logger = logging.getLogger(__name__)

stdout = logging.StreamHandler(stream=sys.stdout)

fmt = logging.Formatter(
    "%(name)s: %(asctime)s | %(levelname)s | %(filename)s%(lineno)s | %(process)d >>> %(message)s"
)

level_filter = LevelFilter(logging.WARNING)
logger.addFilter(level_filter)

stdout.setFormatter(fmt)
logger.addHandler(stdout)

logger.setLevel(logging.INFO)

logger.info("An info")
logger.warning("A warning")
logger.error("An error")

The LevelFilter class above is used to filter out any log record that does not match the exact specified log level when the filter object was created (logging.WARNING). This is different from the standard log levels which also allow levels of a greater severity to be recorded. When you run this example, you'll observe that only WARNING records are produced while INFO and ERROR are suppressed because they've been filtered out.

__main__: 2023-02-06 05:58:12,079 | WARNING | example.py:32 | 727478 >>> A warning

In this manner, you can use filters to implement a wide range of logging policies, such as including only messages from certain parts of your application, only messages of a certain severity, or only messages that contain certain text. By using filters, you will have fine-grained control over what records are included in your logs.

Adding contextual data to your logs

In many situations, you may need to add some variable data to your log records in order to fully capture the event that occurred. For example, you might need to log the ID of a transaction, or the user ID that performed an action. So that instead of recording a message that like this:

user logged in

You can have something like this:

user 'james' logged in

Adding contextual variables in the manner shown above can be done by using a formatting directive in the log message and passing variable data as arguments.

import sys
import logging


logger = logging.getLogger(__name__)

stdout = logging.StreamHandler(stream=sys.stdout)

fmt = logging.Formatter(
    "%(name)s: %(asctime)s | %(levelname)s | %(filename)s%(lineno)s | %(process)d >>> %(message)s"
)

stdout.setFormatter(fmt)
logger.addHandler(stdout)

logger.setLevel(logging.INFO)

name = "james"
browser = "firefox"
logger.info("user %s logged in with %s", name, browser)

__main__: 2023-02-09 08:37:47,860 | INFO | main.py20 | 161954 >>> user james logged in with firefox

You can also use the newer formatted string literals (f-strings) syntax which are easier to read:

. . .
logger.info(f"user {name} logged in with {browser}")

However, note that the logging module is optimized to use the % formatting style and the use of f-strings might have an extra cost since formatting will occur even if the message isn't logged. So even though f-strings are easier to read, you should probably stick to the % style to guarantee best performance.

Another way to add context to your logs is by passing the extra parameter to any level method to supply a dictionary of custom attributes to the record. This is useful in cases where all your application logs need to have certain information present in the entry, but those details are dependent on the execution context.

Here's an example that demonstrates the extra argument:

import sys
import logging


logger = logging.getLogger(__name__)

stdout = logging.StreamHandler(stream=sys.stdout)

fmt = logging.Formatter(
    "%(name)s: %(asctime)s | %(levelname)s | %(filename)s%(lineno)s | %(process)d >>> %(message)s"
)

stdout.setFormatter(fmt)
logger.addHandler(stdout)

logger.setLevel(logging.INFO)

logger.info("Info message", extra={"user": "johndoe", "session_id": "abc123"})
logger.warning("Warning message", extra={"user": "joegage", "session_id": "3fe-uz"})
logger.error("Error message", extra={"user": "domingrange", "session_id": "2fe-a1"})

Assuming your logs need to include information on the current user and session ID, you can include those details in a dictionary and pass them to the extra argument. However, when you run this code you'll notice that the values in the extra argument don't appear in the output:

__main__: 2023-02-09 08:43:30,827 | INFO | main.py18 | 175097 >>> Info message
__main__: 2023-02-09 08:43:30,827 | WARNING | main.py19 | 175097 >>> Warning message
__main__: 2023-02-09 08:43:30,827 | ERROR | main.py20 | 175097 >>> Error message

This is because our current log format does not include any of the fields in the extra dictionary. We can fix this by modifying the format argument as follows:

. . .
fmt = logging.Formatter(
    "%(name)s: %(asctime)s | %(levelname)s | %(filename)s%(lineno)s | %(process)d | %(user)s | %(session_id)s >>> %(message)s"
)
. . .

Notice that the user and session_id fields have now been added to the log format. This ensures that the corresponding values are substituted accordingly:

__main__: 2023-02-09 08:46:07,588 | INFO | main.py18 | 178735 | johndoe | abc123 >>> Info message
__main__: 2023-02-09 08:46:07,588 | WARNING | main.py19 | 178735 | joegage | 3fe-uz >>> Warning message
__main__: 2023-02-09 08:46:07,588 | ERROR | main.py20 | 178735 | domingrange | 2fe-a1 >>> Error message

There are two main things to note when using the extra property:

First, the field names used in the extra dictionary should not clash with any of the LogRecord attributes otherwise you will get a KeyError when you run the program:

# the message field here clashes with the `message` LogRecord attribute
logger.info(
  "Info message",
  extra={"user": "johndoe", "session_id": "abc123", "message": "override info
  message"},
)

. . .
File "/usr/lib64/python3.11/logging/__init__.py", line 1606, in makeRecord
 raise KeyError("Attempt to overwrite %r in LogRecord" % key)
KeyError: "Attempt to overwrite 'message' in LogRecord"

Another noteworthy behaviour is that modifying the log format with custom attributes (such as user and session_id) means that these values must always be supplied at log point in the extra parameter. If you omit them, you'll get a different exception which looks like this:

# notice that the `user` field is missing here
logger.info("Info message", extra={"session_id": "abc123"})

--- Logging error ---
Traceback (most recent call last):
  File "/usr/lib64/python3.11/logging/__init__.py", line 449, in format
    return self._format(record)
           ^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/logging/__init__.py", line 445, in _format
    return self._fmt % values
           ~~~~~~~~~~^~~~~~~~
KeyError: 'user'

These limitations, especially the second one, can make using the extra parameter impractical for many use cases but I will show how to get around them in the next section of this tutorial.

Structured JSON logging in Python

So far in this tutorial, we've been logging in a custom plain text format that is human-readable. However, most logs aren't read directly by humans anymore but processed with log aggregation tools first to quickly extract the insights required. With our current plain text format, most of these tools will be unable to automatically parse our logs (at least without the help of some complex regular expression). Therefore, we need to log in a standard structured format (such as JSON) to ensure that the logs are easily machine-parseable.

To achieve structured JSON logging through Python's logging module, you can utilize the techniques provided in the logging cookbook or just use the python-json-logger library. We will go with the second option in this tutorial. Go ahead and install the package through the command below:

pip install python-json-logger

Afterward, import it into your program as follows:

import sys
import logging
from pythonjsonlogger import jsonlogger

logger = logging.getLogger(__name__)

stdout = logging.StreamHandler(stream=sys.stdout)

fmt = jsonlogger.JsonFormatter(
    "%(name)s %(asctime)s %(levelname)s %(filename)s %(lineno)s %(process)d %(message)s"
)

stdout.setFormatter(fmt)
logger.addHandler(stdout)

logger.setLevel(logging.INFO)

logger.info("An info")
logger.warning("A warning")
logger.error("An error")

You will observe the following output:

{"name": "__main__", "asctime": "2023-02-06 07:14:13,616", "levelname": "INFO", "filename": "example.py", "lineno": 19, "process": 810435, "message": "An info"}
{"name": "__main__", "asctime": "2023-02-06 07:14:13,617", "levelname": "WARNING", "filename": "example.py", "lineno": 20, "process": 810435, "message": "A warning"}
{"name": "__main__", "asctime": "2023-02-06 07:14:13,617", "levelname": "ERROR", "filename": "example.py", "lineno": 21, "process": 810435, "message": "An error"}

The LogRecord attributes passed as arguments to JsonFormatter() are all present in the output and they present under their original names. If you want to rename the fields, you can use the rename_fields argument as shown below. You can also customize the date format using the datefmt property as before:

fmt = jsonlogger.JsonFormatter(
    "%(name)s %(asctime)s %(levelname)s %(filename)s %(lineno)s %(process)d %(message)s",
    rename_fields={"levelname": "severity", "asctime": "timestamp"},
)

{"name": "__main__", "timestamp": "2023-02-06T09:04:09Z", "severity": "INFO", "filename": "example.py", "lineno": 21, "process": 858552, "message": "An info"}
{"name": "__main__", "timestamp": "2023-02-06T09:04:09Z", "severity": "WARNING", "filename": "example.py", "lineno": 22, "process": 858552, "message": "A warning"}
{"name": "__main__", "timestamp": "2023-02-06T09:04:09Z", "severity": "ERROR", "filename": "example.py", "lineno": 23, "process": 858552, "message": "An error"}

One of the advantages of using the python-json-logger library is that is allows you to add context to your logs through the extra property without needing to modify the log format. The keys in the extra dictionary will be added to the JSON output as top-level keys so you only need to be careful not to use the default LogRecord attribute names (which will cause a KeyError exception).

. . .

logger.info("An info")
logger.warning("A warning")
logger.error("An error", extra={"user_id": "james103", "session_id": "eheo3e"})

{"name": "__main__", "timestamp": "2023-02-06T09:08:04Z", "severity": "INFO", "filename": "example.py", "lineno": 21, "process": 872558, "message": "An info"}
{"name": "__main__", "timestamp": "2023-02-06T09:08:04Z", "severity": "WARNING", "filename": "example.py", "lineno": 22, "process": 872558, "message": "A warning"}
{"name": "__main__", "timestamp": "2023-02-06T09:08:04Z", "severity": "ERROR", "filename": "example.py", "lineno": 23, "process": 872558, "message": "An error", "user_id": "james103", "session_id": "eheo3e"}

Notice how the user_id and session_id fields are present in the ERROR log without needing to modify the log format. This means you can use any arbitrary properties necessary at log point and it will just work as expected. The rest of this tutorial will assume use of the python-json-logger package for structured logging.

Logging errors in Python

Errors are often the most common target for logging so its important to understand what tools the logging module provides for logging errors in Python so that all the necessary information about the error is captured properly to aid the debugging process.

The ERROR level is the primary way for recording errors in your programs. If an problem is particularly severe to the function of your program, you may log it at the CRITICAL level instead. The logging module also provides an exception() method on a Logger which is essentially an alias for logging.error(<msg>, exc_info=1). Let's see it in action:

import sys
import logging
from pythonjsonlogger import jsonlogger

logger = logging.getLogger(__name__)

stdoutHandler = logging.StreamHandler(stream=sys.stdout)

jsonFmt = jsonlogger.JsonFormatter(
    "%(name)s %(asctime)s %(levelname)s %(filename)s %(lineno)s %(process)d %(message)s",
    rename_fields={"levelname": "severity", "asctime": "timestamp"},
    )
    datefmt="%Y-%m-%dT%H:%M:%SZ",

stdoutHandler.setFormatter(jsonFmt)
logger.addHandler(stdoutHandler)

logger.setLevel(logging.INFO)

try:
    1 / 0
except ZeroDivisionError as e:
    logger.error(e, exc_info=True)
    logger.exception(e)
    logger.critical(e, exc_info=True)

{"name": "__main__", "timestamp": "2023-02-06T09:28:54Z", "severity": "ERROR", "filename": "example.py", "lineno": 26, "process": 913135, "message": "division by zero", "exc_info": "Traceback (most recent call last):\n  File \"/home/betterstack/community/python-logging/example.py\", line 24, in <module>\n    1 / 0\n    ~~^~~\nZeroDivisionError: division by zero"}
{"name": "__main__", "timestamp": "2023-02-06T09:28:54Z", "severity": "CRITICAL", "filename": "example.py", "lineno": 27, "process": 913135, "message": "division by zero", "exc_info": "Traceback (most recent call last):\n  File \"/home/betterstack/community/python-logging/example.py\", line 24, in <module>\n    1 / 0\n    ~~^~~\nZeroDivisionError: division by zero"}
{"name": "__main__", "timestamp": "2023-02-06T09:28:54Z", "severity": "ERROR", "filename": "example.py", "lineno": 28, "process": 913135, "message": "division by zero", "exc_info": "Traceback (most recent call last):\n  File \"/home/betterstack/community/python-logging/example.py\", line 24, in <module>\n    1 / 0\n    ~~^~~\nZeroDivisionError: division by zero"}

The error() and exception() methods produced exactly the same output, while critical() differs only in the severity property. In all three cases, the exception info is added to the record under the exc_info property. Note that the exc_info argument should only be used in an exception context otherwise exc_info will be set to NoneType: None in the output.

{"name": "__main__", "timestamp": "2023-02-06T09:35:27Z", "severity": "ERROR", "filename": "example.py", "lineno": 21, "process": 923890, "message": "error", "exc_info": "NoneType: None"}

If you want to add a stack trace to any of your logs outside an exception context, use the stack_info argument instead:

logger.debug("debug", stack_info=True)
logger.error("error", stack_info=True)

The stack trace can be found under the stack_info property:

{"name": "__main__", "timestamp": "2023-02-06T09:39:47Z", "severity": "DEBUG", "filename": "example.py", "lineno": 21, "process": 934209, "message": "debug", "stack_info": "Stack (most recent call last):\n  File \"/home/betterstack/community/python-logging/example.py\", line 21, in <module>\n    logger.debug(\"debug\", stack_info=True)"}
{"name": "__main__", "timestamp": "2023-02-06T09:39:47Z", "severity": "ERROR", "filename": "example.py", "lineno": 22, "process": 934209, "message": "error", "stack_info": "Stack (most recent call last):\n  File \"/home/betterstack/community/python-logging/example.py\", line 22, in <module>\n    logger.error(\"error\", stack_info=True)"}

Logging uncaught exceptions

Uncaught exceptions are caused by a failure to handle a thrown exception in the program with a try/catch block. When an such an exception occurs, the program terminates abruptly and an error message (traceback) is printed to the console.

It is helpful to log uncaught exceptions at the CRITICAL level so that you can identify the root cause of the problem and take appropriate action to fix it. If you're sending your logs to a log management tool like Logtail, you can configure alerting on such errors so that they are speedily resolved to prevent recurrence.

Here's how you can ensure such exceptions are logged properly in Python:

import sys
import logging
from pythonjsonlogger import jsonlogger

logger = logging.getLogger(__name__)

stdoutHandler = logging.StreamHandler(stream=sys.stdout)

jsonFmt = jsonlogger.JsonFormatter(
    "%(name)s %(asctime)s %(levelname)s %(filename)s %(lineno)s %(process)d %(message)s",
    rename_fields={"levelname": "severity", "asctime": "timestamp"},
    datefmt="%Y-%m-%dT%H:%M:%SZ",
)

stdoutHandler.setFormatter(jsonFmt)

logger.addHandler(stdoutHandler)
logger.setLevel(logging.DEBUG)


def handle_exception(exc_type, exc_value, exc_traceback):
    if issubclass(exc_type, KeyboardInterrupt):
        sys.__excepthook__(exc_type, exc_value, exc_traceback)
        return

    logger.critical("Uncaught exception", exc_info=(exc_type, exc_value, exc_traceback))


sys.excepthook = handle_exception

# Cause an unhandled exception
raise RuntimeError("Test unhandled")

{"name": "__main__", "timestamp": "2023-02-07T14:36:42Z", "severity": "CRITICAL", "filename": "example.py", "lineno": 26, "process": 1437321, "message": "Uncaught exception", "exc_info": "Traceback (most recent call last):\n  File \"/home/betterstack/community/python-logging/example.py\", line 31, in <module>\n    raise RuntimeError(\"Test unhandled\")\nRuntimeError: Test unhandled"}

With this setup in place, you'll always know when an uncaught exception occurs and diagnosing the cause it should be a breeze since all the details have been recorded in the log.

Logging to a file

Next, let's discuss how to store Python logs in a file. This can be done in many ways but the most basic way involves setting up a Handler that transports the logs to a file. Fortunately the logging.FileHandler class exists for this purpose:

import sys
import logging
from pythonjsonlogger import jsonlogger


logger = logging.getLogger(__name__)

stdoutHandler = logging.StreamHandler(stream=sys.stdout)
fileHandler = logging.FileHandler("logs.txt")

jsonFmt = jsonlogger.JsonFormatter(
    "%(name)s %(asctime)s %(levelname)s %(filename)s %(lineno)s %(process)d %(message)s",
    rename_fields={"levelname": "severity", "asctime": "timestamp"},
    datefmt="%Y-%m-%dT%H:%M:%SZ",
)

stdoutHandler.setFormatter(jsonFmt)
fileHandler.setFormatter(jsonFmt)

logger.addHandler(stdoutHandler)
logger.addHandler(fileHandler)

logger.setLevel(logging.DEBUG)

logger.debug("A debug message")
logger.error("An error message")

Once a new FileHandler object is created, you can set your preferred format for the handler and add it to the Logger as shown above. When you execute the program, the logs will be printed to the standard output as before but also stored in a logs.txt file in the current working directory.

cat logs.txt

{"name": "__main__", "timestamp": "2023-02-06T10:19:34Z", "severity": "DEBUG", "filename": "example.py", "lineno": 30, "process": 974925, "message": "A debug message"}
{"name": "__main__", "timestamp": "2023-02-06T10:19:34Z", "severity": "ERROR", "filename": "example.py", "lineno": 31, "process": 974925, "message": "An error message"}

If you're logging to multiple Handlerss (as above) you don't have to use the same format for all of them. For example, structured JSON logs can be a little hard to read in development so you can retain the plain text formatting for the stdoutHandler while JSON formatted logs go to a file to be further processed through log management tools.

import sys
import logging

logger = logging.getLogger(__name__)

stdoutHandler = logging.StreamHandler(stream=sys.stdout)
fileHandler = logging.FileHandler("logs.txt")

stdoutFmt = logging.Formatter(
    "%(name)s: %(asctime)s | %(levelname)s | %(filename)s:%(lineno)s | %(process)d >>> %(message)s"
)

jsonFmt = jsonlogger.JsonFormatter(
    "%(name)s %(asctime)s %(levelname)s %(filename)s %(lineno)s %(process)d %(message)s",
    rename_fields={"levelname": "severity", "asctime": "timestamp"},
    datefmt="%Y-%m-%dT%H:%M:%SZ",
)

stdoutHandler.setFormatter(stdoutFmt)
fileHandler.setFormatter(jsonFmt)

logger.addHandler(stdoutHandler)
logger.addHandler(fileHandler)

logger.setLevel(logging.DEBUG)

logger.debug("A debug message")
logger.error("An error message")

python example.py

__main__: 2023-02-06 10:34:25,172 | DEBUG | example.py:30 | 996769 >>> A debug message
__main__: 2023-02-06 10:34:25,173 | ERROR | example.py:31 | 996769 >>> An error message

cat logs.txt

{"name": "__main__", "timestamp": "2023-02-06T10:19:34Z", "severity": "DEBUG", "filename": "example.py", "lineno": 30, "process": 974925, "message": "A debug message"}
{"name": "__main__", "timestamp": "2023-02-06T10:19:34Z", "severity": "ERROR", "filename": "example.py", "lineno": 31, "process": 974925, "message": "An error message"}

You can take this further by setting a different minimum log levels on the Handlers or using a Filter object to prevent certain logs from being sent to any of the destinations. I'll leave you to experiment further with that on your own.

Automatically rotating log files

When you're logging to files, you need to be careful not to allow the file grow too large and consume a huge amount of disk space. By rotating log files, older logs can be compressed or deleted, freeing up space and reducing the risk of disk usage issues. Additionally, rotating logs helps to maintain an easily manageable set of log files, and can also be used to reduce the risk of sensitive information exposure by removing logs after a set period of time.

We generally recommend using Logrotate to manage log file rotation since it is the standard way to do it on Linux servers. However, the logging module also provides two Handlers to help with solving this problem: RotatingFileHandler and TimedRotatingFileHandler.

from logging.handlers import RotatingFileHandler

fileHandler = RotatingFileHandler("logs.txt", backupCount=5, maxBytes=5000000)

The RotatingFileHandler class takes the filename as before but also a few other properties. The most crucial of these are backupCount and maxBytes. The former determines how many backup files will be kept while the latter determines the maximum size of a log file before it is rotated. In this manner, each file is kept to a reasonable size and older logs don't clog up storage space unnecessarily.

With this setting in place, the logs.txt file will be created and written to as before until it reaches 5 megabytes. It will subsequently be renamed to logs.txt.1 and a new logs.txt file will be created once again. When the new file gets to 5 MB, it will be renamed to logs.txt.1 and the previous logs.txt.1 file will be renamed to logs.txt.2. This process continues until we get to logs.txt.5. At that point, the oldest file (logs.txt.5) gets deleted to make way for the newer logs.

The TimedRotatingFileHandler class works in a similar manner, except that it uses timed intervals to determine when the log file should be rotated. For example, you can use the following configuration to rotate files once a day at midnight, while keeping a maximum of 5 backup files.

from logging.handlers import TimedRotatingFileHandler

fileHandler = TimedRotatingFileHandler(
    "logs.txt", backupCount=5, when="midnight"
)

The Python logging hierarchy

Before closing out this tutorial, let's touch base on Python's logging hierarchy and how it works. The Python logging hierarchy is a way of organizing loggers into a tree structure based on their names with the root logger at the top.

Each custom logger has a unique name, and loggers with similar names form a hierarchy. When a logger is created, it inherits log levels and handlers from the nearest ancestor that does if it doesn't have those settings on itself. This allows for fine-grained control over how log messages are handled.

For example, a logger named app.example is the child of the app logger which in turn is the child of the "root" logger:

import logging

app_logger = logging.getLogger("app")
module_logger = logging.getLogger("app.module")

print(app_logger.parent)
print(module.parent)

<RootLogger root (WARNING)>
<Logger app (WARNING)>

Notice how the log level of the app logger is WARNING despite not setting a log level on it directly. This is because the default log level for custom loggers is a special NOTSET value which causes Python traverse its chain of ancestors until one with a level other than NOTSET is found, or the root is reached. Since root is set to WARNING by default, the effective level of app (and app.example) is also warning. However, if root also has a level of NOTSET, then all messages will be processed.

The propagate argument in the Python logging module is used to control whether log messages should be passed up the logging hierarchy to parent loggers. By default, this argument is set to True, meaning that log messages are propagated up the hierarchy.

If the propagate argument is set to False for a particular logger, log messages emitted by that logger will not be passed up the hierarchy to its parent loggers. This allows for greater control over log message handling, as it allows you to prevent messages from being handled by parent loggers.

For example, consider a logging hierarchy with a root logger, an app logger, and a app.module1 logger. If the propagate argument is set to False for the app.module1 logger, log messages emitted by this logger will only be handled by handlers attached directly to it and will not be passed up to the app logger or the root logger.

In general, it's a good practice to set the propagate argument to False only when necessary, as it can lead to unexpected behavior and make it harder to manage logging in a complex application. However, in some cases, it may be desirable to turn off propagation in order to provide greater control over logging behavior.

Best practices for logging in Python

Let's wrap up this article by looking at a few best practices to help you put together an effective logging strategy in your Python applications:

1. Use meaningful logger names: Give loggers meaningful names that reflect their purpose, using dots as separators to create a hierarchy. For example, a logger for a module could be named module.submodule. You can use the __name__ variable to achieve this naming convention.

2. Avoid using the root logger: The root logger is a catch-all logger that can be difficult to manage. Instead, create specific loggers for different parts of your application.

3. Set the appropriate log levels: For example, you can use WARNING in production and DEBUG in development or testing environments.

4. Centralize your logging configuration: Centralizing your logging configuration in a single location will make it much easier to manage.

5. Aggregate your logs: Consider using a library like logtail-python for centralizing your logs, as it provides advanced features like centralized logging, aggregation, and alerting.

6. Include as much context as necessary: Ensure that relevant context surrounding the event being logged is included in the log record. At a minimum, records should always include the severity level, the logger name, and the time the message was emitted.

7. Avoid logging sensitive information: Avoid logging sensitive information, such as passwords, security keys or user data, as it could compromise the security of your application.

8. Test your logging configuration: Test your logging configuration in different scenarios to ensure that it behaves as expected before deploying to production.

9. Rotate log files regularly: Regularly rotate log files to keep them from growing too large and becoming difficult to manage. We recommend using Logrotate but you can also use the RotatingFileHandler or TimedRotatingFileHandler as demonstrated in this article.

Final thoughts and next steps

We've covered a lot of ground so far in this tutorial, and I'm sure you'd like to know how all these features come together in a real-world application. Therefore, we've provided another comprehensive tutorial discussing a practical example of how to use the Pythonlogging module in a Django web application and how to use a log management platform to analyze and manage your application logs. When your application is ready, you can check how to optimize your deployment process across different environments.

By following through with the tutorial you'll see many of the logging features we've discussed here in action, and get a more solid understanding of how to use it in your projects.

Thanks for reading, and happy logging!

Article by

Ayooluwa Isaiah

Ayo is the Head of Content at Better Stack. His passion is simplifying and communicating complex technical ideas effectively. His work was featured on several esteemed publications including LWN.net, Digital Ocean, and CSS-Tricks. When he’s not writing or coding, he loves to travel, bike, and play tennis.

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.