What is Pylint?

Pylint is a tool that checks for errors in Python code, tries to enforce a coding standard and looks for bad code smells. This is similar but nevertheless different from what pychecker provides, especially since pychecker explicitly does not bother with coding style. The default coding style used by Pylint is close to Guido’s style guide). For more information about code smells, refer to Martin Fowler’s refactoring book

Pylint will display a number of messages as it analyzes the code, as well as some statistics about the number of warnings and errors found in different files. The messages are classified under various categories such as errors and warnings (more below). If you run Pylint twice, it will display the statistics from the previous run together with the ones from the current run, so that you can see if the code has improved or not.

Last but not least, the code is given an overall mark, based on the number an severity of the warnings and errors. This has proven to be very motivating for some programmers.

Pylint was born in 2003 at Logilab, that funded Sylvain Thénault to lead its development up to now.

What Pylint is not?

What Pylint says is not to be taken as gospel and Pylint isn’t smarter than you are: it may warn you about things that you have conscientiously done.

Pylint tries hard to report as few false positives as possible for errors, but it may be too verbose with warnings. That’s for example because it tries to detect things that may be dangerous in a context, but are not in others, or because it checks for some things that you don’t care about. Generally, you shouldn’t expect Pylint to be totally quiet about your code, so don’t necessarily be alarmed if it gives you a hell lot of messages for your project!

Quoting Alexandre Fayolle:

My usage pattern for Pylint is to generally run pylint -E quite often to get stupid errors flagged before launching an application (or before committing). I generally run Pylint with all the bells and whistles activated some time before a release, when I want to cleanup the code. And when I do that I simply ignore tons of the false warnings (and I can do that without being driven mad by this dumb program which is not smart enough to understand the dynamicity of Python because I only run it once or twice a week in this mode)

Quoting Marteen Ter Huurne:

In our project we just accepted that we have to make some modifications in our code to please Pylint:

  • stick to more naming conventions (unused variables ending in underscores, mix-in class names ending in “Mixin”)
  • making all abstract methods explicit (rather than just not defining them in the superclass)
  • add # pylint: disable=X0123 comments:
    • for messages which are useful in general, but not in a specific case
    • for Pylint bugs
    • for Pylint limitations (for instance Twisted’s modules create a lot of definitions dynamically so Pylint does not know about them)

The effort is worth it, since Pylint helps us a lot in keeping the code clean and finding errors early. Although most errors found by Pylint would also be found by the regression tests, by fixing them before committing, we save time. And our regression tests do not cover all code either, just the most complex parts.

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