2.4 Advanced Linting Features

Last updated on 2025-04-08 | Edit this page

Overview

Questions

  • FIXME

Objectives

  • FIXME

More Verbose Reporting

We can also obtain a more verbose report by adding --reports y to the command, which gives us a lot more detail:

BASH 

pylint --reports y file

Here’s a part of that output:

OUTPUT 

...
Messages
--------

+---------------------------+------------+
|message id                 |occurrences |
+===========================+============+
|redefined-outer-name       |4           |
+---------------------------+------------+
|invalid-name               |4           |
+---------------------------+------------+
|missing-function-docstring |2           |
+---------------------------+------------+
|unused-import              |1           |
+---------------------------+------------+
|unspecified-encoding       |1           |
+---------------------------+------------+
|superfluous-parens         |1           |
+---------------------------+------------+
|missing-module-docstring   |1           |
+---------------------------+------------+
|consider-using-with        |1           |
+---------------------------+------------+
...

QUESTION: for those doing activity, who’s managed to run this command? YES/NO

It gives you some overall statistics, plus comparisons with the last time you ran it, on aspects such as:

  • How many modules, classes, methods and functions were looked at
  • Raw metrics (which we’ll look at in a minute)
  • Extent of code duplication (none, which is good)
  • Number of messages by category (again, we can see that it’s mainly convention issues)
  • A sorted count of the messages we received

Looking at raw metrics, we can see that it breaks down our program into how many lines are code lines, python docstrings, standalone comments, and empty lines. This is very useful, since it gives us an idea of how well commented our code is. In this case - not very well commented at all! For normal comments, the usually accepted wisdom is to add them to explain why you are doing something, or perhaps to explain how necessarily complex code works, but not to explain the obvious, since clearly written code should do that itself.

Increasing our Pylint Score - Adding a Docstring

QUESTION: Who’s familiar with Python docstrings? Yes/No

Docstrings are a special kind of comment for a function, that explain what the function does, the parameters it expects, and what is returned. You can also write docstrings for classes, methods, and modules, but you should usually aim to add docstring comments to your code wherever you can, particularly for critical or complex functions.

Let’s add one to our code now, within the fahr_to_celsius function.

PYTHON 

    """Convert fahrenheit to Celsius.

    :param fahr: temperature in Fahrenheit
    :returns: temperature in Celsius
    """

Re-run pylint - can see we have one less docstring error, and a slightly higher score.

If you’d like to know more about docstrings and commenting, there’s an in-depth RealPython tutorial on these and the different ways you can format them.

Ignoring Issues

We can instruct pylint to ignore any particular types of issues, which is useful if they are not seen as important or pedantic, or we need to see other types more clearly. For example, to ignore any unused imports:

BASH 

pylint --disable=W0611 climate_analysis.py

Or, to disable all issues of type “warning”:

BASH 

pylint --disable=W climate_analysis.py

This can be particularly useful if we wish to ignore particularly pedantic rules, such as long line lengths over 100 characters.

Challenge

Edit the climate_analysis.py file and add in a comment line that exceeds 100 characters. Then re-run pylint and determine the issue identifier for this message, and re-run pylint again disabling this specific issue.

BASH 

pylint climate_analysis.py

OUTPUT 

************* Module climate_analysis
climate_analysis.py:3:0: C0301: Line too long (111/100) (line-too-long)
climate_analysis.py:17:0: C0325: Unnecessary parens after '=' keyword (superfluous-parens)
climate_analysis.py:1:0: C0114: Missing module docstring (missing-module-docstring)
...

We can see that the identifier is C0301, so:

BASH 

pylint --disable=C0301 climate_analysis.py

However, if we wanted to ignore this issue for the foreseeable future, typing this in every time would be tiresome. Fortunately we can specify a configuration file to pylint which specifies how we want to interpret issues.

We do this by first using pylint to generate a default .pylintrc file. It directs this as output to the shell, so we need to redirect it to a file to capture it. Ensure you are in the repository root directory, then:

BASH 

pylint --generate-rcfile > .pylintrc

If you edit this generated file you’ll notice there are many things we can specify to pylint. For now, look for disable= and add C0301 to the list of ignored issues already present that are separated by commas, e.g.:

# no Warning level messages displayed, use "--disable=all --enable=classes
# --disable=W".
disable=C0301,
        raw-checker-failed,
        bad-inline-option,
        locally-disabled,
        file-ignored,
        suppressed-message,
        useless-suppression,
        deprecated-pragma,
        use-implicit-booleaness-not-comparison-to-string,
        use-implicit-booleaness-not-comparison-to-zero,
        use-symbolic-message-instead

Every time you re-run it now, the C0301 issue will not be present.

Summary

Code linters like pylint help us to identify problems in our code, such as code styling issues and potential errors, and importantly, if we work in a team of developers such tools help us keep our code style consistent. Attempting to understand a code base which employs a variety of coding styles (perhaps even in the same source file) can be remarkably difficult.

But there are some aspects we should be careful of when using linters and interpreting their results:

  • They don’t tell us that the code actually works and they don’t tell us if the results our code produces are actually correct, so we still need to test our code.
  • They don’t give us any Idea of whether it’s a good implementation, and that the technical choices are good ones. For example, this code contains functions to conduct temperature conversions, but it turns out there’s a number of well-maintained Python packages that do this (e.g. pytemperature)so we should be using a tried and tested package instead of reinventing the wheel.
  • They also don’t tell us if the implementation is actually fit for purpose. Even if the code is a good implementation, and it works as expected, is it actually solving the intended problem?
  • They also don’t tell us anything about the data the program uses which may have its own problems.
  • A high score or zero warnings may give us false confidence. Just because we have reached a 10.00 score, doesn’t mean the code is actually good code, just that it’s likely well formatted and hopefully easier to read and understand.

So we have to be a bit careful. These are all valid, high-level questions to ask while you’re writing code, both as a team, and also individually. In the fog of development, it can be surprisingly easy to lose track of what’s actually being implemented and how it’s being implemented. A good idea is to revisit these questions regularly, to be sure you can answer them!

However, whilst taking these shortcomings into account, linters are a very low effort way to help us improve our code and keep it consistent.

Key Points

  • FIXME