Python logging - better than just print

Logging module is one of most useful modules out of the box. In every single software development, there is a chance the program runs into an error. And that’s a reason we should prevent the case with this.

All programmers are familiar with print function. We use it to debug some values. However, it would be greatly better to understand how to effectively utilize Logging module because we may need to debug from somewhere more persistent than console log.

For more info please follow the link to read the module’s detail, but we’re gonna see some basic use cases of this module through this blog.

logging — Logging facility for Python

Source code: Lib/logging/__init__.py Important: This page contains the API reference information. For tutorial information and discussion of more advanced topics, see Basic Tutorial, Advanced Tutor...

Python documentation

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Simple use

Just a baby step. We can start from this.

There are 5 levels of log there + 1 “NOTSET” which I never use it as there is no use cases for me. Learn more about levels by this link.

The result will be messages from Logging.

And what if we want more info of the message such as time or formatting level? We can add argument format into the object like this.

And it will produce the message based on the given format.

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Handlers

Wanna try more advance? Let’s make a logger that perform logging on both console and file writing.

For this requirement, we are going to review two main handlers for injecting log records to these displays.

Console

Other than using basicConfig() as above, we will use StreamHandler(). The example is below:

And let’s see what happen when we run it.

File

We can parse the log outputs to a file via FileHandler.

See the output file here.

Combine together

Of course, we can mix both handlers together to inject logs through console and file at the same time. Try this:

Now run it and there are console log along with file log. Yes!

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More fancy examples

Dynamic level per message

We can supply the message level by calling .log() instead of .info() or other level-specific methods.

Date time format

We would add argument datefmt to logging.Formatter(). Visit this link to learn more about format codes.

Multiple loggers

How to manage multiple loggers?

We can create many loggers as we want but we need to make sure we named each logger distinguishly. Let’s see the case.

• Firstly, program calls outer.
• outer create a logger name “outer” to write logs in file “outer.log”. The messages are “[outer] start” and “[outer] done”.
• Between both messages from outer, outer also call inner through a loop.
• There are 2 items and it will call inner 2 times.
• First item is a text “abc” that inner will log in the filepath “msg1/inner.log” with “[inner]” beginning the line.
• Second is a text “def” that inner will log in the filepath “msg2/inner.log” with “[inner]” beginning the line as well.
• inner has a logger name “inner”.

Looking good but the result was not as expected. The log file “msg1/inner.log” should have only “[inner] abc” but it inproperly has “[inner] def”.

What we missed? We had given the same log name for inner so that the first inner call receive the consequent logs.

We have to fix by giving different name. One solution is to use UUID to generate a random UUID (line 27) for each inner call.

And this time we got the correct logs.

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Repo

All examples is in my repo here.

GitHub - bluebirz/sample-python-logging

Contribute to bluebirz/sample-python-logging development by creating an account on GitHub.

GitHub