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CKAN Coding Standards

This section documents our CKAN-specific coding standards, which are guidelines for writing code that is consistent with the intended design and architecture of CKAN.

For more general coding standards, see also:

Encapsulate SQLAlchemy in ckan.model

Ideally SQLAlchemy should only be used within ckan.model and not from other packages such as ckan.logic. For example instead of using an SQLAlchemy query from the logic package to retrieve a particular user from the database, we add a get() method to ckan.model.user.User:

def get(cls, user_id):
    query = ...
    return query.first()

Now we can call this method from the logic package.

Database Migrations

When changes are made to the model classes in ckan.model that alter CKAN’s database schema, a migration script has to be added to migrate old CKAN databases to the new database schema when they upgrade their copies of CKAN. See Creating a new migration script.

Always go through the Action Functions

Whenever some code, for example in ckan.lib or ckan.controllers, wants to get, create, update or delete an object from CKAN’s model it should do so by calling a function from the ckan.logic.action package, and not by accessing ckan.model directly.

Action Functions are Exposed in the API

The functions in ckan.logic.action are exposed to the world as the The CKAN API. The API URL for an action function is automatically generated from the function name, for example ckan.logic.action.create.package_create() is exposed at /api/action/package_create. See Steve Yegge’s Google platforms rant for some interesting discussion about APIs.

All publicly visible functions in the ckan.logic.action.{create,delete,get,update} namespaces will be exposed through the The CKAN API. This includes functions imported by those modules, as well as any helper functions defined within those modules. To prevent inadvertent exposure of non-action functions through the action api, care should be taken to:

  1. Import modules correctly (see Imports). For example:

    import as search
  2. Hide any locally defined helper functions:

    def _a_useful_helper_function(x, y, z):
       '''This function is not exposed because it is marked as private```
       return x+y+z
  3. Bring imported convenience functions into the module namespace as private members:

    _get_or_bust = logic.get_or_bust

Use get_action()

Don’t call logic.action functions directly, instead use get_action(). This allows plugins to override action functions using the IActions plugin interface. For example:


Instead of


Auth Functions and check_access()

Each action function defined in ckan.logic.action should use its own corresponding auth function defined in ckan.logic.auth. Instead of calling its auth function directly, an action function should go through ckan.logic.check_access (which is aliased _check_access in the action modules) because this allows plugins to override auth functions using the IAuthFunctions plugin interface. For example:

def package_show(context, data_dict):
    _check_access('package_show', context, data_dict)

check_access will raise an exception if the user is not authorized, which the action function should not catch. When this happens the user will be shown an authorization error in their browser (or will receive one in their response from the API).


The data_dict parameter of logic action functions may be user provided, so required files may be invalid or absent. Naive Code like:

id = data_dict['id']

may raise a KeyError and cause CKAN to crash with a 500 Server Error and no message to explain what went wrong. Instead do:

id = _get_or_bust(data_dict, "id")

which will raise ValidationError if "id" is not in data_dict. The ValidationError will be caught and the user will get a 400 Bad Request response and an error message explaining the problem.

Validation and ckan.logic.schema

Logic action functions can use schema defined in ckan.logic.schema to validate the contents of the data_dict parameters that users pass to them.

An action function should first check for a custom schema provided in the context, and failing that should retrieve its default schema directly, and then call _validate() to validate and convert the data. For example, here is the validation code from the user_create() action function:

schema = context.get('schema') or ckan.logic.schema.default_user_schema()
session = context['session']
validated_data_dict, errors = _validate(data_dict, schema, context)
if errors:
    raise ValidationError(errors)

Controller & Template Helper Functions

ckan.lib.helpers contains helper functions that can be used from ckan.controllers or from templates. When developing for ckan core, only use the helper functions found in ckan.lib.helpers.__allowed_functions__.


  • Functional tests which test the behaviour of the web user interface, and the APIs should be placed within ckan/tests/functional. These tests can be a lot slower to run that unit tests which don’t access the database or solr. So try to bear that in mind, and attempt to cover just what is neccessary, leaving what can be tested via unit-testing in unit-tests.
  • and are only available in Python>=2.7. So import them from ckan.tests, which will provide alternatives if they’re not available.
  • the mock library can be used to create and interrogate mock objects.

See Testing CKAN for further information on testing in CKAN.

Writing Extensions

Please see Writing CKAN extensions for information about writing ckan extensions, including details on the API available to extensions.


  • Anything that may be used by extensions (see Writing CKAN extensions) needs to maintain backward compatibility at call-site. ie - template helper functions and functions defined in the plugins toolkit.

  • The length of time of deprecation is evaluated on a function-by-function basis. At minimum, a function should be marked as deprecated during a point release.

  • To mark a helper function, use the deprecated decorator found in ckan.lib.maintain eg:

    def facet_items(*args, **kwargs):
        DEPRECATED: Use the new facet data structure, and `unselected_facet_items()`
        # rest of function definition.