Flake8

Christian Külker

0.1.2

2023-04-28

Flake8 is a popular Python linting tool that combines the functionality of several linting tools (PyFlakes, pycodestyle). It helps identify errors, PEP 8 style guide violations, and code complexity issues, making Python code more readable, maintainable, and efficient. This document will guide you through the installation and use of Flake8 with Python on a Debian-based Linux distribution.

With flake8 it is possible to enforce a style guide on hand written Python code. By default, flake8 includes checks provided by PyFlakes, PEP-0008 inspired style checks provided by PyCodeStyle, and McCabe complexity checking.

Installation

aptitude install flake8

This assumes that you are using the system version of Python. “It is very important to install Flake8 on the correct version of Python for your needs. If you want Flake8 to properly parse new language features in Python 3.5 (for example), you need it to be installed on 3.5 for Flake8 to understand those features. In many ways, Flake8 is tied to the version of Python on which it runs.” (Note from https://flake8.pycqa.org/en/latest/)

Versions

Debian Stretch

flake8 --version
3.2.1 (mccabe: 0.5.3, pyflakes: 1.3.0, pycodestyle: 2.2.0) CPython 3.5.3 on Linux

Debian Buster

flake8 --version
3.6.0 (mccabe: 0.6.1, pycodestyle: 2.4.0, pyflakes: 2.0.0) CPython 3.7.3 on Linux

Debian Bullseye

flake8 --version
3.8.4 (mccabe: 0.6.1, pycodestyle: 2.6.0, pyflakes: 2.2.0) CPython 3.9.2 on Linux

Sources

Usage

There are basically three ways to use and influence flake8:

  1. Command line (file or directory)
  2. Code Comments
  3. Configuration file

The command line tool flake8 can be used on a single file or on multiple files, while comments control flake8 for special cases and the configuration file can control flake8 for one or all files aka the project.

Single File Usage

python3 flake8 /PATH/TO/file.py

Multiple File Usage

python3 flake8 /PATH/TO/FILES/*.py

Output

The flake8 command prints a list of problems found. While most of them are style-related, it also prints if it found a variable that was assigned but not used. (Perl does this automatically via the -w option).

Useful Command Line Options

While flake8 can be used without options, it can be customized via a configuration file or various options. This section shows only a small subset of useful command line options.

  • -q --quite: Just print the name of file in which issues are found
  • --config=CONFIG: specify the configuration file (for custom options)
  • --max-line-length=n: set different line length (default 79)

Ignoring Errors (Violations)

Ignoring Files Or Directory

To ignore whole files or directories

flake8 --exclude path/to/file

Ignoring Error (Violation) Types

To ignore certain violation types, it is possible to specify the ID on the command line. (It is also possible to specify it in a configuration file)

New exclude list:

flake8 --ignore=E1,E23,W503 path/to/files/

Extending the exclude list:

flake8 --extend-ignore=E1,E23 path/to/files/

Ignoring Lines

If the line is short enough, it is possible to ignore a line with a special comment # noqa.

example = lambda: 'example'  # noqa: E731

Flake8 Configuration

Create a .flake8 file in the project directory.

touch .flake8
vim .flake8
[flake8]
ignore = E226,E302
max-line-length = 80
exclude = migrations,venv,tests

Here we’re ignoring E226 (missing whitespace around arithmetic operators) and E302 (expected two blank lines, found 0). We’ve also set the maximum line length to 80 characters and excluded directories named “migrations”, “venv” and “tests” from scanning. The call to flake8 is as usual.

Discussion

Having flake8 in Python is nice. Compared to other solutions, such as perltidy, it seems that this tool has a long way to go before it is as easy to use, useful, and time-saving. A short example: When using an editor (like vim), it is possible to define a hotkey to activate perltidy to format the displayed code. So as a programmer, you do not have to write all the indentation and other fancy spacing or alignment. Just press the key and the code looks nice. With flake8, the programmer just gets a report and has to fix everything manually. perltidy can batch convert files, check files embedded in other markup (such as HTML), and much more. Since Python, unlike Perl, has the “there is only one way” syntax philosophy, it is of course no wonder that flake8 does not offer much in the way of syntax customization, but a real advantage of flake8 is that it prints IDs referencing the rules, which can be used to look up and find more information about specific rules.

Alternatives

History

Version Date Notes
0.1.2 2023-04-28 Improve writing, add configuration file .flake8
0.1.1 2022-07-06 History, shell->bash, Debian 11 Bullseye
0.1.0 2020-05-08 Initial release
  • Flake8