Code Style

As a project, Horizon adheres to code quality standards.


We follow PEP8 for all our Python code, and use (available via the shortcut tox -e pep8) to validate that our code meets proper Python style guidelines.


Additionally, we follow Django’s style guide for templates, views, and other miscellany.


The following standards are divided into required and recommended sections. Our main goal in establishing these best practices is to have code that is reliable, readable, and maintainable.



  • The code has to work on the stable and latest versions of Firefox, Chrome, Safari, and Opera web browsers, and on Microsoft Internet Explorer 11 and later.

  • If you turned compression off during development via COMPRESS_ENABLED = False in, re-enable compression and test your code before submitting.

  • Use === as opposed to == for equality checks. The == will do a type cast before comparing, which can lead to unwanted results.


    If typecasting is desired, explicit casting is preferred to keep the meaning of your code clear.

  • Keep document reflows to a minimum. DOM manipulation is expensive, and can become a performance issue. If you are accessing the DOM, make sure that you are doing it in the most optimized way. One example is to build up a document fragment and then append the fragment to the DOM in one pass instead of doing multiple smaller DOM updates.

  • Use “strict”, enclosing each JavaScript file inside a self-executing function. The self-executing function keeps the strict scoped to the file, so its variables and methods are not exposed to other JavaScript files in the product.


    Using strict will throw exceptions for common coding errors, like accessing global vars, that normally are not flagged.


      'use strict';
      // code...
  • Use forEach | each when looping whenever possible. AngularJS and jQuery both provide for each loops that provide both iteration and scope.


    angular.forEach(objectToIterateOver, function(value, key) {
      // loop logic


    $.each(objectToIterateOver, function(key, value) {
      // loop logic
  • Do not put variables or functions in the global namespace. There are several reasons why globals are bad, one being that all JavaScript included in an application runs in the same scope. The issue with that is if another script has the same method or variable names they overwrite each other.

  • Always put var in front of your variables. Not putting var in front of a variable puts that variable into the global space, see above.

  • Do not use eval( ). The eval (expression) evaluates the expression passed to it. This can open up your code to security vulnerabilities and other issues.

  • Do not use ‘with object {code}’. The with statement is used to access properties of an object. The issue with with is that its execution is not consistent, so by reading the statement in the code it is not always clear how it is being used.

Readable & Maintainable

  • Give meaningful names to methods and variables.

  • Avoid excessive nesting.

  • Avoid HTML and CSS in JS code. HTML and CSS belong in templates and stylesheets respectively. For example:

    • In our HTML files, we should focus on layout.

      1. Reduce the small/random <script> and <style> elements in HTML.

      2. Avoid in-lining styles into element in HTML. Use attributes and classes instead.

    • In our JS files, we should focus on logic rather than attempting to manipulate/style elements.

      1. Avoid statements such as element.css({property1,property2...}) they belong in a CSS class.

      2. Avoid statements such as $("<div><span>abc</span></div>") they belong in a HTML template file. Use show | hide | clone elements if dynamic content is required.

      3. Avoid using classes for detection purposes only, instead, defer to attributes. For example to find a div:

        <div class="something"></div>
          $(".something").html("Don't find me this way!");

        is better found like:

        <div data-something></div>
          $("div[data-something]").html("You found me correctly!");
  • Avoid commented-out code.

  • Avoid dead code.


  • Avoid creating instances of the same object repeatedly within the same scope. Instead, assign the object to a variable and re-use the existing object. For example:

    $(document).on('click', function() { /* do something. */ });
    $(document).on('mouseover', function() { /* do something. */ });

    A better approach:

    var $document = $(document);
    $document.on('click', function() { /* do something. */ });
    $document.on('mouseover', function() { /* do something. */ });

    In the first approach a jQuery object for document is created each time. The second approach creates only one jQuery object and reuses it. Each object needs to be created, uses memory, and needs to be garbage collected.



This section is intended as a quick intro to contributing with AngularJS. For more detailed information, check the AngularJS Topic Guide.

“John Papa Style Guide”

The John Papa Style Guide is the primary point of reference for Angular code style. This style guide has been endorsed by the AngularJS team:

"The most current and detailed Angular Style Guide is the
community-driven effort led by John Papa and Todd Motto."


The style guide is found at the below location:

When reviewing / writing, please refer to the sections of this guide. If an issue is encountered, note it with a comment and provide a link back to the specific issue. For example, code should use named functions. A review noting this should provide the following link in the comments:

In addition to John Papa, the following guidelines are divided into required and recommended sections.


  • Scope is not the model (model is your JavaScript Objects). The scope references the model. Use isolate scopes wherever possible.

  • Since Django already uses {{ }}, use {$ $} or {% verbatim %} instead.


ESLint is a great tool to be used during your code editing to improve JavaScript quality by checking your code against a configurable list of checks. Therefore, JavaScript developers should configure their editors to use ESLint to warn them of any such errors so they can be addressed. Since ESLint has a ton of configuration options to choose from, links are provided below to the options Horizon wants enforced along with the instructions for setting up ESLint for Eclipse, Sublime Text, Notepad++ and WebStorm/PyCharm.

Instructions for setting up ESLint: ESLint setup instructions


ESLint is part of the automated unit tests performed by Jenkins. The automated test use the default configurations, which are less strict than the configurations we recommended to run in your local development environment.


Style guidelines for CSS are currently quite minimal. Do your best to make the code readable and well-organized. Two spaces are preferred for indentation so as to match both the JavaScript and HTML files.

JavaScript and CSS libraries using xstatic

We do not bundle third-party code in Horizon’s source tree. Instead, we package the required files as xstatic Python packages and add them as dependencies to Horizon.

To create a new xstatic package:

  1. Check if the library is already packaged as xstatic on PyPi, by searching for the library name. If it already is, go to step 5. If it is, but not in the right version, contact the original packager to have them update it.

  2. Package the library as an xstatic package by following the instructions in xstatic documentation. Install the xstatic-release script and follow the instructions that come with it.

  3. Create a new repository under OpenStack. Use “xstatic-core” and “xstatic-ptl” groups for the ACLs. Make sure to include the -pypi-wheel-upload job in the project config.

  4. Set up PyPi to allow OpenStack (the “openstackci” user) to publish your package.

  5. Add the new package to global-requirements.

To make a new release of the package, you need to:

  1. Ensure the version information in the xstatic/pkg/<package name>/ file is up to date, especially the BUILD.

  2. Push your updated package up for review in gerrit.

  3. Once the review is approved and the change merged, request a release by updating or creating the appropriate file for the xstatic package in the releases repository under deliverables/_independent. That will cause it to be automatically packaged and released to PyPi.


Note that once a package is released, you can not “un-release” it. You should never attempt to modify, delete or rename a released package without a lot of careful planning and feedback from all projects that use it.

For the purpose of fixing packaging mistakes, xstatic has the build number mechanism. Simply fix the error, increment the build number and release the newer package.

Integrating a new xstatic package into Horizon

Having done a release of an xstatic package:

  1. Look for the upper-constraints.txt edit related to the xstatic release that was just performed. One will be created automatically by the release process in the openstack/requirements project with the topic new-release. You should -1 that patch until you are confident Horizon does not break (or you have generated a patch to fix Horizon for that release.) If no upper-constraints.txt patch is automatically generated, ensure the releases yaml file created in the releases repository has the “include-pypi-link: yes” setting.

  2. Pull that patch down so you have the edited upper-constraints.txt file locally.

  3. Set the environment variable TOX_CONSTRAINTS_FILE to the edited upper-constraints.txt file name and run tests or local development server through tox. This will pull in the precise version of the xstatic package that you need.

  4. Move on to releasing once you’re happy the Horizon changes are stable.

Releasing a new compatible version of Horizon to address issues in the new xstatic release:

  1. Continue to -1 the upper-constraints.txt patch above until this process is complete. A +1 from a Horizon developer will indicate to the requirements team that the upper-constraints.txt patch is OK to merge.

  2. When submitting your changes to Horizon to address issues around the new xstatic release, use a Depends-On: referencing the upper-constraints.txt review. This will cause the OpenStack testing infrastructure to pull in your updated xstatic package as well.

  3. Merge the upper-constraints.txt patch and the Horizon patch noting that Horizon’s gate may be broken in the interim between these steps, so try to minimise any delay there. With the Depends-On it’s actually safe to +W the Horizon patch, which will be held up until the related upper-constraints.txt patch merges.

  4. Once the upper-constraints.txt patch merges, you should propose a patch to global-requirements which bumps the minimum version of the package up to the upper-constraints version so that deployers / packagers who don’t honor upper-constraints still get compatible versions of the packages.


Again, readability is paramount; however be conscientious of how the browser will handle whitespace when rendering the output. Two spaces is the preferred indentation style to match all front-end code.

Exception Handling

Avoid propogating direct exception messages thrown by OpenStack APIs to the UI. It is a precaution against giving obscure or possibly sensitive data to a user. These error messages from the API are also not translatable. Until there is a standard error handling framework implemented by the services which presents clean and translated messages, horizon catches all the exceptions thrown by the API and normalizes them in horizon.exceptions.handle().


Horizon’s documentation is written in reStructuredText (reST) and uses Sphinx for additional parsing and functionality, and should follow standard practices for writing reST. This includes:

  • Flow paragraphs such that lines wrap at 80 characters or less.

  • Use proper grammar, spelling, capitalization and punctuation at all times.

  • Make use of Sphinx’s autodoc feature to document modules, classes and functions. This keeps the docs close to the source.

  • Where possible, use Sphinx’s cross-reference syntax (e.g. :class:` when referring to other Horizon components. The better-linked our docs are, the easier they are to use.

Be sure to generate the documentation before submitting a patch for review. Unexpected warnings often appear when building the documentation, and slight reST syntax errors frequently cause links or cross-references not to work correctly.

Documentation is generated with Sphinx using the tox command. To create HTML docs and man pages:

$ tox -e docs

The results are in the doc/build/html and doc/build/man directories respectively.


Simply by convention, we have a few rules about naming:

  • The term “project” is used in place of Keystone’s “tenant” terminology in all user-facing text. The term “tenant” is still used in API code to make things more obvious for developers.

  • The term “dashboard” refers to a top-level dashboard class, and “panel” to the sub-items within a dashboard. Referring to a panel as a dashboard is both confusing and incorrect.