Best practices for writing extensions

Follow CKAN’s coding standards

See Contributing guide.

Use the plugins toolkit instead of importing CKAN

Try to limit your extension to interacting with CKAN only through CKAN’s plugin interfaces and plugins toolkit. It’s a good idea to keep your extension code separate from CKAN as much as possible, so that internal changes in CKAN from one release to the next don’t break your extension.

Don’t edit CKAN’s database tables

An extension can create its own tables in the CKAN database, but it should not write to core CKAN tables directly, add columns to core tables, or use foreign keys against core tables.

Implement each plugin class in a separate Python module

This keeps CKAN’s plugin loading order simple, see ckan.plugins.

Avoid name clashes

Many of the names you pick for your identifiers and files must be unique in relation to the names used by core CKAN and other extensions. To avoid conflicts you should prefix any public name that your extension introduces with the name of your extension. For example:

• The names of configuration settings introduced by your extension should have the form my_extension.my_config_setting.

• The names of templates and template snippets introduced by your extension should begin with the name of your extension:

  snippets/my_extension_useful_snippet.html
  

  If you have add a lot of templates you can also put them into a separate folder named after your extension instead.

• The names of template helper functions introduced by your extension should begin with the name of your extension. For example:

      def get_helpers(self):
          '''Register the most_popular_groups() function above as a template
          helper function.
  
          '''
          # Template helper function names should begin with the name of the
          # extension they belong to, to avoid clashing with functions from
          # other extensions.
          return {'example_theme_most_popular_groups': most_popular_groups}
  

• The names of JavaScript modules introduced by your extension should begin with the name of your extension. For example fanstatic/example_theme_popover.js:

  // Enable JavaScript's strict mode. Strict mode catches some common
  // programming errors and throws exceptions, prevents some unsafe actions from
  // being taken, and disables some confusing and bad JavaScript features.
  "use strict";
  
  ckan.module('example_theme_popover', function ($) {
    return {
      initialize: function () {
        console.log("I've been initialized for element: ", this.el);
      }
    };
  });
  

• The names of API action functions introduced by your extension should begin with the name of your extension. For example my_extension_foobarize_everything.

• The names of background job queues introduced by your extension should begin with the name of your extension. For example my_extension:super-special-job-queue.

In some situations, a resource may even be shared between multiple CKAN instances, which requires an even higher degree of uniqueness for the corresponding names. In that case, you should also prefix your identifiers with the CKAN site ID, which is available via

try:
    # CKAN 2.7 and later
    from ckan.common import config
except ImportError:
    # CKAN 2.6 and earlier
    from pylons import config

site_id = config[u'ckan.site_id']

Currently this only affects the Redis database:

• All keys in the Redis database created by your extension should be prefixed with both the CKAN site ID and your extension’s name.

Internationalize user-visible strings

All user-visible strings should be internationalized, see String internationalization.

Add third party libraries to requirements.txt

If your extension requires third party libraries, rather than adding them to setup.py, they should be added to requirements.txt, which can be installed with:

pip install -r requirements.txt

To prevent accidental breakage of your extension through backwards-incompatible behaviour of newer versions of your dependencies, their versions should be pinned, such as:

requests==2.7.0

On the flip side, be mindful that this could also create version conflicts with requirements of considerably newer or older extensions.

Do not automatically modify the database structure

If your extension uses custom database tables then it needs to modify the database structure, for example to add the tables after its installation or to migrate them after an update. These modifications should not be performed automatically when the extension is loaded, since this can lead to dead-locks and other problems.

Instead, create a paster command which can be run separately.