Logging in Python



Logging is a option to retailer details about your script and observe occasions that happen. When writing any advanced script in Python, logging is crucial for debugging software program as you develop it. With out logging, discovering the supply of an issue in your code could also be extraordinarily time consuming.

After finishing this tutorial, you’ll know:

  • Why we wish to use the logging module
  • The right way to use the logging module
  • The right way to customise the logging mechanism

Let’s get began.

Logging in Python
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Tutorial Overview

This tutorial is split into 4 elements; they’re:

  • The advantages of logging
  • Primary logging
  • Superior configuration to logging
  • An instance of the usage of logging

Advantages of Logging

You could ask: “Why not simply use printing?”

If you run an algorithm and need to verify it’s doing what you anticipated, it’s pure so as to add some print() statements at strategic places to indicate this system’s state. Printing will help debug easier scripts, however as your code will get an increasing number of advanced, printing lacks the pliability and robustness that logging has.

With logging, you possibly can pinpoint the place a logging name got here from, differentiate severity between messages, and write data to a file, which printing can not do. For instance, we will activate and off the message from a specific module of a bigger program. We will additionally enhance or lower the verbosity of the logging messages with out altering lots of code.

Primary Logging

Python has a built-in library, logging, for this goal. It’s easy to create a “logger” to log messages or data that you just wish to see.

The logging system in Python operates below a hierarchical namespace and totally different ranges of severity. The Python script can create a logger below a namespace, and each time a message is logged, the script should specify its severity. The logged message can go to totally different locations relying on the handler we arrange for the namespace. The commonest handler is to easily print on the display, like the ever present print() perform. After we begin this system, we might register a brand new handler and arrange the extent of severity to which the handler will react.

There are 5 totally different logging ranges that point out the severity of the logs, proven in rising severity:

  1. DEBUG
  2. INFO
  4. ERROR

A quite simple instance of logging is proven beneath, utilizing the default logger or the basis logger:

These will emit log messages of various severity. Whereas there are 5 strains of logging, you might even see solely three strains of output if you happen to run this script, as follows:

It’s because the basis logger, by default, solely prints the log messages of a severity degree of WARNING or above. Nonetheless, utilizing the basis logger this fashion just isn’t a lot totally different from utilizing the print() perform.

The settings for the basis logger aren’t set in stone. We will configure the basis logger to output to a specific file, change its default severity degree, and format the output. Right here’s an instance:

Operating this script will produce no output to the display however can have the next within the newly created file file.log:

The decision to logging.basicConfig() is to change the basis logger. In our instance, we set the handler to output to a file as an alternative of the display, regulate the logging degree such that every one log messages of degree DEBUG or above are dealt with, and likewise change the format of the log message output to incorporate the time.

Observe that now all 5 messages have been output, so the default degree that the basis logger logs is now “DEBUG.” The log document attributes (akin to %(asctime)s) that can be utilized to format the output might be discovered within the logging documentation.

Though there’s a default logger, we often need to make and use different loggers that may be configured individually. It’s because we might desire a totally different severity degree or format for various loggers. A brand new logger might be created with:

Internally, the loggers are organized in a hierarchy. A logger created with:

might be a baby logger created below the logger with the title “mum or dad,” which, in flip, is below the basis logger. Utilizing a dot within the string signifies that the kid logger is a baby of the mum or dad logger. Within the above case, a logger with the title “mum or dad.baby” is created in addition to one with the title "mum or dad" implicitly.

Upon creation, a baby logger has all of the properties of its mum or dad logger till reconfigured. We will exhibit this with the next instance:

This code snippet will output just one line:

which is created by the StreamHandler object with the custom-made format string. It occurs solely after we reconfigured the logger for mum or dad as a result of in any other case, the basis logger’s configuration prevails, and no messages at degree INFO might be printed.

Superior Configuration to Logging

As we noticed within the final instance, we will configure the loggers we made.

Threshold of Stage

Like the fundamental configuration of the basis logger, we will additionally configure the output vacation spot, severity degree, and formatting of a logger. The next is how we will set the threshold of the extent of a logger to INFO:

Now instructions with severity degree INFO and better might be logged by the parent_logger. But when that is all you probably did, you’ll not see something from parent_logger.information("messages") as a result of there are not any handlers assigned for this logger. In truth, there are not any handlers for root logger as properly except you arrange one with logging.basicConfig().

Log Handlers

We will configure the output vacation spot of our logger with handlers. Handlers are accountable for sending the log messages to the proper vacation spot. There are a number of sorts of handlers; the commonest ones are StreamHandler and FileHandler. With StreamHandler, the logger will output to the terminal, whereas with FileHandler, the logger will output to a specific file.

Right here’s an instance of utilizing StreamHandler to output logs to the terminal:

Within the code above, there are two handlers created: A FileHandler created by logging.basicConfig() for the basis logger and a StreamHandler created for the mum or dad logger.

Observe that despite the fact that there’s a StreamHandler that sends the logs to the terminal, logs from the mum or dad logger are nonetheless being despatched to file.log since it’s a baby of the basis logger, and the basis logger’s handler can be lively for the kid’s log messages. We will see that the logs to the terminal embody INFO degree messages and above:

However the output to the terminal just isn’t formatted, as we have now not used a Formatter but. The log to file.log, nonetheless, has a Formatter arrange, and will probably be like the next:

Alternatively, we will use FileHandler within the above instance of parent_logger:

The instance above demonstrated which you can additionally set the extent of a handler. The extent of parent_fhandler filters out logs that aren’t WARNING degree or larger. Nonetheless, if you happen to set the handler’s degree to DEBUG, that will be the identical as not setting the extent as a result of DEBUG logs would already be filtered out by the logger’s degree, which is INFO.

On this case, the output to mum or dad.log is:

whereas that of file.log is similar as earlier than. In abstract, when a log message is recorded by a logger,

  1. The logger’s degree will decide if the message is extreme sufficient to be dealt with. If the logger’s degree just isn’t set, the extent of its mum or dad (and finally the basis logger) might be used for this consideration.
  2. If the log message might be dealt with, all handlers ever added alongside the logger hierarchy as much as the basis logger will obtain a replica of the message. Every handler’s degree will decide if this explicit handler ought to honor this message.


To configure the format of the logger, we use a Formatter. It permits us to set the format of the log, equally to how we did so within the root logger’s basicConfig(). That is how we will add a formatter to our handler:

First, we create a formatter, then set our handler to make use of that formatter. If we wished to, we might make a number of totally different loggers, handlers, and formatters in order that we might combine and match primarily based on our preferences.

On this instance, the mum or dad.log can have:

and the file.log related to the handler at root logger can have:

Beneath is the visualization of the move of loggers, handlers, and formatters from the documentation of the logging module:

Movement chart of loggers and handlers within the logging module

An Instance of the Use of Logging

Let’s think about the Nadam algorithm for instance:

The only use case is to make use of logging to switch the print() perform. We will make the next change: First, create a logger with the title nadam earlier than we run any code and assign a StreamHandler:

We should assign a handler as a result of we by no means configured the basis logger, and this is able to be the one handler ever created. Then, within the perform nadam(), we re-create a logger nadam, however because it has already been arrange, the extent and handlers endured. On the finish of every outer for-loop in nadam(), we changed the print() perform with logger.information() so the message might be dealt with by the logging system:

If we have an interest within the deeper mechanics of the Nadam algorithm, we might add extra logs. The next is the whole code:

We ready two degree of loggers, nadam and nadam.iter, and set them in numerous ranges. Within the internal loop of nadam(), we use the kid logger to print some inside variables. If you run this script, it should print the next:

Setting totally different loggers not solely permits us to set a special degree or handlers, nevertheless it additionally lets us differentiate the place the log message comes from by trying on the logger’s title from the message printed.

In truth, one useful trick is to create a logging decorator and apply the decorator to some features. We will maintain observe of each time that perform is known as. For instance, we created a decorator beneath and utilized it to the features goal() and spinoff():

Then we’ll see the next within the log:

the place we will see the parameters and return values of every name to these two features within the message logged by the nadam.perform logger.

As we get an increasing number of log messages, the terminal display will turn out to be very busy. One option to make it simpler to observe for points is to focus on the logs in coloration with the colorama module. It’s good to have the module put in first:

Right here’s an instance of how you should utilize the colorama module with the logging module to alter your log colours and textual content brightness:

From the terminal, you’d see the next:

the place the Fore, Again, and Fashion from the colorama module management the foreground, background, and brightness type of the textual content printed. That is leveraging the ANSI escape characters and works solely on ANSI-supported terminals. Therefore this isn’t appropriate for logging to a textual content file.

In truth, we might derive the Formatter class with:

and use this as an alternative of logging.Formatter. The next is how we will additional modify the Nadam instance so as to add coloration:

If we run it on a supporting terminal, we’ll see the next output:

Observe that the colourful output will help us spot any irregular conduct simpler. Logging helps with debugging and likewise permits us to simply management how a lot element we need to see by altering just a few strains of code.

Additional Studying

This part gives extra sources on the subject in case you are trying to go deeper.




On this tutorial, you discovered the best way to implement logging strategies in your scripts.

Particularly, you discovered:

  • Primary and superior logging strategies
  • The right way to apply logging to a script and the advantages of doing so