Introducing Linters

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A linter will look at your code and comment on it. It could give you tips about mistakes you’ve made, correct trailing spaces, and even predict bugs you may have introduced.

Passive Linting With flake8

A popular linter that comments on the style of your code in relation to the PEP 8 specification is flake8.

You can install flake8 using pip:

pip install flake8

You can then run flake8 over a single file, a folder, or a pattern:

!flake8 lint_example.py

lint_example.py:2:3: E111 indentation is not a multiple of 4
lint_example.py:4:1: E302 expected 2 blank lines, found 1
lint_example.py:7:1: W293 blank line contains whitespace
lint_example.py:10:4: E111 indentation is not a multiple of 4
lint_example.py:11:4: E111 indentation is not a multiple of 4
lint_example.py:11:44: W292 no newline at end of file

flake8 is configurable on the command line or inside a configuration file in your project. If you wanted to ignore certain rules, like E302 shown above, you can set them in the configuration. flake8 will inspect a .flake8 file in the project folder or a setup.cfg file. If you decided to use Tox, you can put the flake8 configuration section inside tox.ini.

This example ignores the .git and __pycache__ directories as well as the E302 rule. Also, it sets the max line length to 90 instead of 80 characters. You will likely find that the default constraint of 79 characters for line-width is very limiting for tests, as they contain long method names, string literals with test values, and other pieces of data that can be longer. It is common to set the line length for tests to up to 120 characters:

[flake8]
ignore = E302
exclude = .git,__pycache__
max-line-length = 90

Alternatively, you can provide these options on the command line:

!flake8 --ignore E302 --exclude .git,__pycache__ --max-line-length=90 lint_example.py

lint_example.py:2:3: E111 indentation is not a multiple of 4
lint_example.py:7:1: W293 blank line contains whitespace
lint_example.py:10:4: E111 indentation is not a multiple of 4
lint_example.py:11:4: E111 indentation is not a multiple of 4
lint_example.py:11:44: W292 no newline at end of file

A full list of configuration options is available on the Documentation Website.

Aggressive Linting with a Code Formatter

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flake8 is a passive linter: it recommends changes, but you have to go and change the code. A more aggressive approach is a code formatter. Code formatters will change your code automatically to meet a collection of style and layout practices.

black is a very unforgiving formatter. It doesn’t have any configuration options, and it has a very specific style. This makes it great as a drop-in tool to put in your test pipeline.

You can install black via pip:

pip install black

Then to run black at the command line, provide the file or directory you want to format:

black test.py