7.3 Linting#

Estimated time for this notebook: 15 minutes

There are automated tools which enforce coding conventions and check for common mistakes. These are called linters.

Do not blindly believe all these automated tools! Style guides are guides not rules.

Linters Starter Pack#

A good starting point for any Python project is to use flake8, black, and isort. All three should improve the style and consistency of your code whilst requiring minimal setup, and generally they are not opinionated about the way your code is designed, only the way it is formatted and syntax or convention errors.


Combines two main tools:

  • PyFlakes - checks Python code for syntax errors

  • pycodestyle - checks whether Python code is compliant with PEP8 conventions

flake8 only checks code and flags any syntax/style errors, it does not attempt to fix them.

For example, consider this piece of code:

%%writefile flake8_example.py

from constants import e

def circumference(r):
    return 2 * pi * r
Overwriting flake8_example.py

Running flake8 on it gives the following warnings:

! flake8 flake8_example.py
flake8_example.py:2:1: F401 'constants.e' imported but unused
flake8_example.py:4:1: E302 expected 2 blank lines, found 1
flake8_example.py:5:16: F821 undefined name 'pi'

The first warning tells us we have imported a variable called e but not used it, and the last that we’re trying to use a variable called pi but haven’t defined it anywhere. The 2nd warning indicates that in the PEP8 conventions there should be two blank lines before a function definition, but we only have 1.

Running on multiple files

All the examples here run a linter on a single file, but they can be run on all the files in a project at once as well (e.g. by just running flake8 without a filename).


A highly opinionated code formatter, which enforces control of minutiae details of your code. The name comes from a Henry Ford quote: “Any customer can have a car painted any color that he wants, so long as it is black.”

For example, consider this piece of code:

%%writefile black_example.py

import numpy as np

def my_complex_function(important_argument_1,important_argument_2,optional_argument_3 = 3,optional_argument_4 = 4):
    return np.random.random()*important_argument_1*important_argument_2*optional_argument_3*optional_argument_4

def hello(name,greet='Hello',end="!"):
    print(greet,    name,    end)
Overwriting black_example.py
! black black_example.py
reformatted black_example.py

All done! ✨ 🍰 ✨
1 file reformatted.

After running black on the file its contents become:

!cat black_example.py
import numpy as np

def my_complex_function(
    return (
        * important_argument_1
        * important_argument_2
        * optional_argument_3
        * optional_argument_4

def hello(name, greet="Hello", end="!"):
    print(greet, name, end)

Changes made by black:

  • Ensured there are two blank lines before and after function definitions

  • Wrapped long lines intelligently

  • Removed excess whitespace (e.g. between the arguments in the print statement on the last line)

  • Used double quotes " for all strings (rather than a mix of ' and ")

Note that black will automatically fix most of the whitespace-related warnings picked up by flake8 (but it would not fix the import or undefined name errors in the flake8 example above).

Line length

black is not compliant with PEP8 in one way - by default it uses a maximum line length of 88 characters (PEP8 suggests 79 characters). This is discussed in the black documentation.


“Sorts” imports alphabetically in groups in the following order:

  1. standard library imports (e.g. import os).

  2. third-party imports (e.g. import pandas).

  3. local application/library specific imports (e.g. from .my_python_file import MyClass).

with a blank line between each group.

For example, consider the following code:

%%writefile isort_example.py

import pandas as pd
import os
from matplotlib import pyplot as plt
import black_example
import numpy as np
import json
Overwriting isort_example.py
! isort isort_example.py
Fixing /home/runner/work/rse-course/rse-course/module07_construction_and_design/isort_example.py

If we run isort it becomes:

!cat isort_example.py
import json
import os

import numpy as np
import pandas as pd
from matplotlib import pyplot as plt

import black_example

Note that from imports are placed at the bottom of each group.

Other Linters#


If you use type annotations in your code, mypy can check it for errors that may result from variables being assigned the wrong type. For example, consider the following code:

%%writefile mypy_example.py

def hello(name: str, greet: str = "Hello", rep: int = 1) -> str:
    message: str = ""
    for _ in range(rep):
        message += f"{greet} {name}\n"
    return message

print(hello("Bob", 5))
Overwriting mypy_example.py

If we run mypy on it:

! mypy mypy_example.py
mypy_example.py:9: error: Argument 2 to "hello" has incompatible type "int"; expected "str"
Found 1 error in 1 file (checked 1 source file)

The error tells us we have passed an int as the 2nd argument to hello, but in the function definition the second argument (greet) is defined to be a str. We probably meant to write hello("Bob", rep=5).


pylint analyses your code for errors, coding standards, and makes suggestions around where code could be refactored. It checks for a much wider range of code quality issues than flake8 but is also much more likely to pick up false positives, i.e. pylint is more likely to give you warnings about things you don’t want to change.

Let’s run it on the same code we used for our flake8 example earlier:

%%writefile pylint_example.py

from constants import e

def circumference(r):
    return 2 * pi * r
Overwriting pylint_example.py
! pylint pylint_example.py
************* Module pylint_example
pylint_example.py:1:0: C0114: Missing module docstring (missing-module-docstring)
pylint_example.py:2:0: E0401: Unable to import 'constants' (import-error)
pylint_example.py:4:0: C0116: Missing function or method docstring (missing-function-docstring)
pylint_example.py:4:18: C0103: Argument name "r" doesn't conform to snake_case naming style (invalid-name)
pylint_example.py:5:15: E0602: Undefined variable 'pi' (undefined-variable)
pylint_example.py:2:0: W0611: Unused e imported from constants (unused-import)

Your code has been rated at 0.00/10 (previous run: 0.00/10, +0.00)

Compared to flake8, in this case pylint also warns us that:

  • The circumference function doesn’t have a docstring

  • The constants library we try to import is not available on our system

  • The variable name r doesn’t follow conventions (single letter variables are discouraged by convention, we could use radius instead)


nbqa allows you to run many Python quality tools (including all the ones we’ve introduced here) on jupyter notebooks. For example:

! nbqa flake8 07_02_coding_conventions.ipynb
07_02_coding_conventions.ipynb:cell_3:1:1: F811 redefinition of unused 'ClassName' from line 2


pylama wraps many code quality tools (including isort, mypy, pylint and much of flake8) in a single command.

! pylama --linters isort,mccabe,mypy,pycodestyle,pydocstyle,pyflakes,pylint flake8_example.py
ERROR: /home/runner/work/rse-course/rse-course/module07_construction_and_design/flake8_example.py Imports are incorrectly sorted and/or formatted.
flake8_example.py:0:1  Incorrectly sorted imports. [isort]
flake8_example.py:1:1 C0114 Missing module docstring [pylint]
flake8_example.py:1:1 D100 Missing docstring in public module [pydocstyle]
flake8_example.py:2:1 E0401 Unable to import 'constants' [pylint]
flake8_example.py:2:1 W0611 Unused e imported from constants [pylint]
flake8_example.py:2:1  Cannot find implementation or library stub for module named "constants" [mypy]
flake8_example.py:2:1  See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports [mypy]
flake8_example.py:4:1 C0116 Missing function or method docstring [pylint]
flake8_example.py:4:19 C0103 Argument name "r" doesn't conform to snake_case naming style [pylint]
flake8_example.py:4:1 D103 Missing docstring in public function [pydocstyle]
flake8_example.py:4:1 E302 expected 2 blank lines, found 1 [pycodestyle]
flake8_example.py:5:16 E0602 Undefined variable 'pi' [pylint]


Compatibility between linters#

If you’re using multiple linters in your project you may need to configure them to be compatible with each other. For example, flake8 warns about lines longer than 79 characters (the PEP8 convention) but black will allow lines up to 88 characters by default.

This repository has an example setup for using black, isort, and flake8 together. The .flake8 and pyproject.toml configuration files set flake8 and isort to run in modes compatible with black.

Ignoring lines of code or linting rules#

There will be times where a linter warns you about something in your code but there’s a valid reason it’s structured that way and you don’t want to change it. Most linters can be configured to ignore specific warnings, either by the type of warning, by file, or by individual code line. For example, adding a # noqa comment to a line will make flake8 ignore it.

Each linter does this differently so check their documentation (e.g. flake8, isort, mypy, pylint).

Running Linters#

It’s a good idea to run linters regularly, or even better to have them setup to run automatically so you don’t have to remember. There are various tools to help with that:

IDE Integration#

Many editors/IDEs have integrations with common linters or have their own built-in. This can include highlighting problems inline, or automatically running linters when files are saved, for example. Here is the VS Code documentation for linting in Python.

There are also tools like editorconfig to help sharing the conventions used within a project, where each contributor uses different IDEs and tools.


pre-commit is a manager for creating git “hooks” - scripts that run before making a commit. If a hook errors the commit won’t be made, and you’ll be prompted to fix the problems first. There are pre-commit plugins for all the linters discussed here, and it’s a good way to ensure all code committed to your repo has had a level of quality control applied to it.

Continuous Integration#

As well as automating unit tests on a CI system like GitHub Actions it’s a good idea to configure them to run linters on your code too.

Here is an example from a repo using isort, flake8 and black in a GitHub Action. Note that in a CI setup tools that usually change your code, like black and isort, will be configured to only check whether there are changes that need to be made.