Detect PEP-8 violations on pull requests

I like the Zen of Python. There should be one - and preferably only one - obvious way to do it reflects perfectly my philosophy of life. That is why I also like PEP-8: it solves the problem of different coding conventions across many teams. Sure, you have to sacrifice a bit of your freedom as a developer, and sometimes it is difficult to keep the line under 79 characters, but still I think it is worth it in the long run.

Every Python developer has an editor or an IDE configured to display all PEP-8 violations (initially, of course, since in time you learn those rules by heart and you no longer need any hints). But sometimes you need to push changes before you can install a linter plugin or you simply don’t notice that imported but unused [F401] message. What then? How to ensure that no PEP-8 violations will ever find their way into your codebase?

Don’t worry, you don’t have to manually validate every pull request. You can configure an automatic linter. Once in place, it will analyse the changes in a branch once a pull request is created. If no errors are found, it will add a comment with a nice message. If there are any errors, it will add a comment with a full description for each incorrect line of code.

Application setup

First you need to create an app on Heroku. In case you haven’t used Heroku before, here is a guide that will help you get started.

To create a new app, go to Dashboard -> New -> Create new app. You can specify the name of your app if you want.

Once the app is created, provide it with the RabbitMQ Bigwig add-on. Go to Resources, find the add-on and click Provision. Unfortunately, the process requires you to provide billing information (you need to configure a credit card for your account). But don’t worry, the add-on itself is free.

Now we move to the command line. First, you should install Heroku CLI. Just run this command:

wget -O- https://toolbelt.heroku.com/install-ubuntu.sh | sh

Next you will need to clone the repository with PEP-8 linter bot:

git clone https://github.com/pfertyk/lint-review.git
cd lint-review

The time has come to tell Heroku who you are. Run heroku login an provide it with your Heroku credentials (email and password). Next you have to configure a git remote to be able to push the code to Heroku. The name of the remote can be found in your app’s Settings:

git remote add heroku https://git.heroku.com/pep8-linter.git

For some reason, when I tried to deploy this app, Heroku insisted on treating it as if it was written in Ruby. So, to be sure that it will be recognized as Python code, you should set a proper buildpack:

heroku buildpacks:set heroku/python

When this is done, you can deploy the app to Heroku:

git push heroku master

Bear in mind that this process can take a while.

Configuration

Once the app is deployed, you will notice that the celery worker’s status if OFF:

To fix this, go to Resources, click the edit icon next to worker, switch the state and confirm.

Next go to Settings and click Reveal Config Vars. You should see the following variables:

• RABBITMQ_BIGWIG_URL
• RABBITMQ_BIGWIG_TX_URL
• RABBITMQ_BIGWIG_RX_URL

You need to configure the settings file and workspace for your app. You also have to specify the name of the server, which is the same as your app’s domain (you can find it in Settings -> Domains). In my case, the additional configuration looked like this:

New GitHub account

Your automatic linter will need a GitHub account. You can use your own, but it’s more fun to create a new one.

Once the account is created, you will have to generate a token. Go to Settings -> Personal access tokens -> Generate new token. Choose a good description and select the notifications scope and the whole repo scope (or just public_repo if you are going to use this bot only for public repositories).

Copy the token and go back to your Heroku app’s Settings. Add two new config variables: GITHUB_USER with the name of newly created GitHub profile (in my case PEPing-tom) and GITHUB_OAUTH_TOKEN with the token you just generated.

Testing

Let’s see your new bot in action. Create a test repository on GitHub. The bot will look for linter configuration in a file called .lintrc, so let’s create one with the following content:

[tools]
linters = flake8

There are two more things you need to configure in every repository that you want this linter to check. First, you have to add your bot’s GitHub profile as a collaborator (Settings -> Collaborators), and the bot has to accept the invitation. Second, you need to add a webhook to your repository to inform the bot about pull requests. Go to Settings -> Webhooks -> Add webhook. The value in Payload URL should be {HEROKU_APP_DOMAIN}/review/start (in my case it was https://pep8-linter.herokuapp.com/review/start). Leave application/json as content type and choose Let me select individual events. The only event you need is Pull request. Make sure that Active is checked and add a webhook.

Now let’s see how it works in practice. Create a new branch in your test repository and add some atrocious Python code:

def x():
    a=x

Push the new branch to GitHub and create a new pull request. A moment later, you should see some comments:

Let’s fix these errors:

def x():
    a = 1
    print(a)

Now your bot informs you that there are no problems:

That’s it! Now you can be sure that no PEP-8 violation will sneak into your clean and standard-compliant codebase. Unless, of course, you decide to ignore these comments…

I hope that you found this tutorial useful. Please contact me if there is anything missing or if you encounter any problems with the whole process.