Writing extensions tutorial

Installing CKAN from source

Writing extensions tutorial¶
This tutorial will walk you through the process of creating a simple CKAN
extension, and introduce the core concepts that CKAN extension developers need
to know along the way. As an example, we’ll use the
example_iauthfunctions extension that’s packaged with CKAN.
This is a simple CKAN extension that customizes some of CKAN’s authorization
rules.

Installing CKAN¶
Before you can start developing a CKAN extension, you’ll need a working source
install of CKAN on your system. If you don’t have a CKAN source install
already, follow the instructions in
Installing CKAN from source before continuing.

Creating a new extension¶

Extensions
A CKAN extension is a Python package that modifies or extends CKAN.
Each extension contains one or more plugins that must be added to your
CKAN config file to activate the extension’s features.

You can use the paster create command to create an “empty” extension from
a template. First, activate your CKAN virtual environment:
. /usr/lib/ckan/default/bin/activate

When you run the paster create command, your new extension’s directory will
be created in the current working directory by default (you can override this
with the -o option), so change to the directory that you want your
extension to be created in. Usually you’ll want to track your extension code
using a version control system such as git, so you wouldn’t want to create
your extension in the ckan source directory because that directory already
contains the CKAN git repo. Let’s use the parent directory instead:
cd /usr/lib/ckan/default/src

Now run the paster create command to create your extension:
paster --plugin=ckan create -t ckanext ckanext-iauthfunctions

Note
The last argument to the paster create command
(ckanext-iauthfunctions in this example) is the name for your next
extension. CKAN extension names have to begin with ckanext-.

The command will ask you to answer a few questions. The answers you give will
end up in your extension’s setup.py file (where you can edit them later if
you want).
Once this command has completed, your new CKAN extension’s project
directory will have been created and will contain a few directories and files
to get you started:
ckanext-iauthfunctions/
    ckanext/
        __init__.py
        iauthfunctions/
            __init__.py
    ckanext_iauthfunctions.egg-info/
    setup.py

ckanext_iauthfunctions.egg_info is a directory containing automatically
generated metadata about your project. It’s used by Python’s packaging and
distribution tools. In general, you don’t need to edit or look at anything in
this directory, and you should not add it to version control.
setup.py is the setup script for your project. As you’ll see later, you use
this script to install your project into a virtual environment. It contains
several settings that you’ll update as you develop your project.
ckanext/iauthfunctions is the Python package directory where we’ll add the
source code files for our extension.

Creating a plugin class¶

Plugins
Each CKAN extension contains one or more plugins that provide the
extension’s features.

Now create the file ckanext-iauthfunctions/ckanext/iauthfunctions/plugin.py
with the following contents:
# encoding: utf-8

import ckan.plugins as plugins

class ExampleIAuthFunctionsPlugin(plugins.SingletonPlugin):
    pass

Our plugin is a normal Python class, named
ExampleIAuthFunctionsPlugin
in this example, that inherits from CKAN’s
SingletonPlugin class.

Note
Every CKAN plugin class should inherit from
SingletonPlugin.

Adding the plugin to setup.py¶
Now let’s add our class to the entry_points in setup.py.  This
identifies the plugin class to CKAN once the extension is installed in CKAN’s
virtualenv, and associates a plugin name with the class.  Edit
ckanext-iauthfunctions/setup.py and add a line to
the entry_points section like this:
entry_points='''
    [ckan.plugins]
    example_iauthfunctions=ckanext.iauthfunctions.plugin:ExampleIAuthFunctionsPlugin
''',

Installing the extension¶
When you install CKAN, you create a
Python virtual environment in a directory on
your system (/usr/lib/ckan/default by default) and install the CKAN Python package and
the other packages that CKAN depends on into this virtual environment.  Before
we can use our plugin, we must install our extension into our CKAN virtual
environment.
Make sure your virtualenv is activated, change to the extension’s
directory, and run python setup.py develop:
. /usr/lib/ckan/default/bin/activate
cd /usr/lib/ckan/default/src/ckanext-iauthfunctions
python setup.py develop

Enabling the plugin¶
An extension’s plugins must be added to the ckan.plugins setting in your
CKAN config file so that CKAN will call the plugins’ methods.  The name that
you gave to your plugin class in the left-hand-side of the assignment in
the setup.py file (example_iauthfunctions in this example) is
the name you’ll use for your plugin in CKAN’s config file:
ckan.plugins = stats text_view recline_view example_iauthfunctions

You should now be able to start CKAN in the development web server and have it
start up without any problems:
$ paster serve /etc/ckan/default/development.ini
Starting server in PID 13961.
serving on 0.0.0.0:5000 view at http://127.0.0.1:5000

If your plugin is in the ckan.plugins setting and CKAN starts without
crashing, then your plugin is installed and CKAN can find it. Of course, your
plugin doesn’t do anything yet.

Troubleshooting¶

PluginNotFoundException¶
If CKAN crashes with a PluginNotFoundException
like this:
ckan.plugins.core.PluginNotFoundException: example_iauthfunctions

then:

Check that the name you’ve used for your plugin in your CKAN config file is
the same as the name you’ve used in your extension’s setup.py file
Check that you’ve run python setup.py develop in your extension’s
directory, with your CKAN virtual environment activated. Every time you add
a new plugin to your extension’s setup.py file, you need to run
python setup.py develop again before you can use the new plugin.

ImportError¶
If you get an ImportError from CKAN relating to your plugin, it’s probably
because the path to your plugin class in your setup.py file is wrong.

Implementing the IAuthFunctions plugin interface¶

Plugin interfaces
CKAN provides a number of
plugin interfaces that plugins must
implement to hook into CKAN and modify or extend it. Each plugin interface
defines a number of methods that a plugin that implements the interface must
provide. CKAN will call your plugin’s implementations of these methods, to
allow your plugin to do its stuff.

To modify CKAN’s authorization behavior, we’ll implement the
IAuthFunctions plugin interface.  This
interface defines just one method, that takes no parameters and returns a
dictionary:

get_auth_functions()
Return the authorization functions provided by this plugin.

Action functions and authorization functions
At this point, it’s necessary to take a short diversion to explain how
authorization works in CKAN.
Every action that can be carried out using the CKAN web interface or API is
implemented by an action function in one of the four files
ckan/logic/action/{create,delete,get,update}.py.
For example, when creating a dataset either using the web interface or using
the package_create() API call,
ckan.logic.action.create.package_create() is called. There’s also
ckan.logic.action.get.package_show(),
ckan.logic.action.update.package_update(), and
ckan.logic.action.delete.package_delete().
For a full list of the action functions available in CKAN, see the
Action API reference.
Each action function has a corresponding authorization function in one of
the four files ckan/logic/auth/{create,delete,get,update}.py,
CKAN calls this authorization function to decide whether
the user is authorized to carry out the requested action. For example, when
creating a new package using the web interface or API,
ckan.logic.auth.create.package_create() is called.
The IAuthFunctions plugin interface
allows CKAN plugins to hook into this authorization system to add their own
authorization functions or override the default authorization functions. In
this way, plugins have complete control to customize CKAN’s auth.

Whenever a user tries to create a new group via the web interface or the API,
CKAN calls the group_create() authorization
function to decide whether to allow the action. Let’s override this function
and simply prevent anyone from creating new groups. Edit your plugin.py
file so that it looks like this:
# encoding: utf-8

import ckan.plugins as plugins

def group_create(context, data_dict=None):
    return {'success': False, 'msg': 'No one is allowed to create groups'}

class ExampleIAuthFunctionsPlugin(plugins.SingletonPlugin):
    plugins.implements(plugins.IAuthFunctions)

    def get_auth_functions(self):
        return {'group_create': group_create}

Our ExampleIAuthFunctionsPlugin
class now calls implements() to tell CKAN that it
implements the IAuthFunctions interface, and
provides an implementation of the interface’s
get_auth_functions() method that
overrides the default group_create() function
with a custom one. This custom function simply returns {'success': False}
to refuse to let anyone create a new group.
If you now restart CKAN and reload the /group page, as long as you’re not a
sysadmin user you should see the Add Group button disappear. The CKAN web
interface automatically hides buttons that the user is not authorized to use.
Visiting /group/new  directly will redirect you to the login page. If you
try to call group_create() via the API, you’ll
receive an Authorization Error from CKAN:
$ http 127.0.0.1:5000/api/3/action/group_create Authorization:*** name=my_group
HTTP/1.0 403 Forbidden
Access-Control-Allow-Headers: X-CKAN-API-KEY, Authorization, Content-Type
Access-Control-Allow-Methods: POST, PUT, GET, DELETE, OPTIONS
Access-Control-Allow-Origin: *
Cache-Control: no-cache
Content-Length: 2866
Content-Type: application/json;charset=utf-8
Date: Wed, 12 Jun 2013 13:38:01 GMT
Pragma: no-cache
Server: PasteWSGIServer/0.5 Python/2.7.4

{
    "error": {
        "__type": "Authorization Error",
        "message": "Access denied"
    },
    "help": "Create a new group...",
    "success": false
}

If you’re logged in as a sysadmin user however, you’ll still be able to create
new groups. Sysadmin users can always carry out any action, they bypass the
authorization functions.

Using the plugins toolkit¶
Let’s make our custom authorization function a little smarter, and allow only
users who are members of a particular group named curators to create new
groups.
First run CKAN, login and then create a new group called curators.  Then
edit plugin.py so that it looks like this:

Note
This version of plugin.py will crash if the user is not logged in or if
the site doesn’t have a group called curators. You’ll want to create
a curators group in your CKAN before editing your plugin to look like
this. See Exception handling below.

# encoding: utf-8

import ckan.plugins as plugins
import ckan.plugins.toolkit as toolkit

def group_create(context, data_dict=None):

    # Get the user name of the logged-in user.
    user_name = context['user']

    # Get a list of the members of the 'curators' group.
    members = toolkit.get_action('member_list')(
        data_dict={'id': 'curators', 'object_type': 'user'})

    # 'members' is a list of (user_id, object_type, capacity) tuples, we're
    # only interested in the user_ids.
    member_ids = [member_tuple[0] for member_tuple in members]

    # We have the logged-in user's user name, get their user id.
    convert_user_name_or_id_to_id = toolkit.get_converter(
        'convert_user_name_or_id_to_id')
    user_id = convert_user_name_or_id_to_id(user_name, context)

    # Finally, we can test whether the user is a member of the curators group.
    if user_id in member_ids:
        return {'success': True}
    else:
        return {'success': False,
                'msg': 'Only curators are allowed to create groups'}

class ExampleIAuthFunctionsPlugin(plugins.SingletonPlugin):
    plugins.implements(plugins.IAuthFunctions)

    def get_auth_functions(self):
        return {'group_create': group_create}

context¶
The context parameter of our
group_create() function is
a dictionary that CKAN passes to all authorization and action functions
containing some computed variables. Our function gets the name of the logged-in
user from context:
    user_name = context['user']

data_dict¶
The data_dict parameter of our
group_create() function is
another dictionary that CKAN passes to all authorization and action functions.
data_dict contains any data posted by the user to CKAN, eg. any fields
they’ve completed in a web form they’re submitting or any JSON fields
they’ve posted to the API. If we inspect the contents of the data_dict
passed to our group_create() authorization function, we’ll see that it
contains the details of the group the user wants to create:
{'description': u'A really cool group',
 'image_url': u'',
 'name': u'my_group',
 'title': u'My Group',
 'type': 'group',
 'users': [{'capacity': 'admin', 'name': u'seanh'}]}

The plugins toolkit¶
CKAN’s plugins toolkit is a Python module containing
core CKAN functions, classes and exceptions for use by CKAN extensions.
The toolkit’s get_action() function returns a CKAN
action function. The action functions available to extensions are the same
functions that CKAN uses internally to carry out actions when users make
requests to the web interface or API. Our code uses
get_action() to get the
member_list() action function, which it uses to
get a list of the members of the curators group:
    members = toolkit.get_action('member_list')(
        data_dict={'id': 'curators', 'object_type': 'user'})

Calling member_list() in this way is equivalent to
posting the same data dict to the /api/3/action/member_list API endpoint.
For other action functions available from
get_action(), see Action API reference.
The toolkit’s get_validator() function returns
validator and converter functions from ckan.logic.converters for plugins to use.  This
is the same set of converter functions that CKAN’s action functions use to
convert user-provided data. Our code uses
get_validator() to get the
convert_user_name_or_id_to_id() converter
function, which it uses to convert the name of the logged-in user to their user
id:
    convert_user_name_or_id_to_id = toolkit.get_converter(
        'convert_user_name_or_id_to_id')
    user_id = convert_user_name_or_id_to_id(user_name, context)

Finally, we can test whether the logged-in user is a member of the curators
group, and allow or refuse the action:
    if user_id in member_ids:
        return {'success': True}
    else:
        return {'success': False,
                'msg': 'Only curators are allowed to create groups'}

Exception handling¶
There are two bugs in our plugin.py file that need to be fixed using
exception handling. First, the class will crash if the site does not have a
group named curators.

Tip
If you’ve already created a curators group and want to test what happens
when the site has no curators group, you can use CKAN’s command line
interface to :ref`clean and reinitialize your database
<database management>`.

Try visiting the /group page in CKAN with our example_iauthfunctions
plugin activated in your CKAN config file and with no curators group in
your site. If you have debug = false in your CKAN config file, you’ll see
something like this in your browser:
Error 500

Server Error

An internal server error occurred

If you have debug = true in your CKAN config file, then you’ll see a
traceback page with details about the crash.
You’ll also get a 500 Server Error if you try to create a group using the
group_create API action.
To handle the situation where the site has no curators group without
crashing, we’ll have to handle the exception that CKAN’s
member_list() function raises when it’s asked to
list the members of a group that doesn’t exist. Replace the member_list
line in your plugin.py file with these lines:
    try:
        members = toolkit.get_action('member_list')(
            data_dict={'id': 'curators', 'object_type': 'user'})
    except toolkit.ObjectNotFound:
        # The curators group doesn't exist.
        return {'success': False,
                'msg': "The curators groups doesn't exist, so only sysadmins "
                       "are authorized to create groups."}

With these try and except clauses added, we should be able to load the
/group page and add groups, even if there isn’t already a group called
curators.
Second, plugin.py will crash if a user who is not logged-in tries to create
a group. If you logout of CKAN, and then visit /group/new you’ll see
another 500 Server Error. You’ll also get this error if you post to the
group_create() API action without
providing an API key.
When the user isn’t logged in, context['user'] contains the user’s IP
address instead of a user name:
{'model': <module 'ckan.model' from ...>,
 'user': u'127.0.0.1'}

When we pass this IP address as the user name to
convert_user_name_or_id_to_id(), the converter
function will raise an exception because no user with that user name exists.
We need to handle that exception as well, replace the
convert_user_name_or_id_to_id line in your plugin.py file with these
lines:
    convert_user_name_or_id_to_id = toolkit.get_converter(
        'convert_user_name_or_id_to_id')
    try:
        user_id = convert_user_name_or_id_to_id(user_name, context)
    except toolkit.Invalid:
        # The user doesn't exist (e.g. they're not logged-in).
        return {'success': False,
                'msg': 'You must be logged-in as a member of the curators '
                       'group to create new groups.'}

We’re done!¶
Here’s our final, working plugin.py module in full:
# encoding: utf-8

import ckan.plugins as plugins
import ckan.plugins.toolkit as toolkit

def group_create(context, data_dict=None):
    # Get the user name of the logged-in user.
    user_name = context['user']

    # Get a list of the members of the 'curators' group.
    try:
        members = toolkit.get_action('member_list')(
            data_dict={'id': 'curators', 'object_type': 'user'})
    except toolkit.ObjectNotFound:
        # The curators group doesn't exist.
        return {'success': False,
                'msg': "The curators groups doesn't exist, so only sysadmins "
                       "are authorized to create groups."}

    # 'members' is a list of (user_id, object_type, capacity) tuples, we're
    # only interested in the user_ids.
    member_ids = [member_tuple[0] for member_tuple in members]

    # We have the logged-in user's user name, get their user id.
    convert_user_name_or_id_to_id = toolkit.get_converter(
        'convert_user_name_or_id_to_id')
    try:
        user_id = convert_user_name_or_id_to_id(user_name, context)
    except toolkit.Invalid:
        # The user doesn't exist (e.g. they're not logged-in).
        return {'success': False,
                'msg': 'You must be logged-in as a member of the curators '
                       'group to create new groups.'}

    # Finally, we can test whether the user is a member of the curators group.
    if user_id in member_ids:
        return {'success': True}
    else:
        return {'success': False,
                'msg': 'Only curators are allowed to create groups'}

class ExampleIAuthFunctionsPlugin(plugins.SingletonPlugin):
    plugins.implements(plugins.IAuthFunctions)

    def get_auth_functions(self):
        return {'group_create': group_create}

In working through this tutorial, you’ve covered all the key concepts needed
for writing CKAN extensions, including:

Creating an extension
Creating a plugin within your extension
Adding your plugin to your extension’s setup.py file,
and installing your extension
Making your plugin implement one of CKAN’s
plugin interfaces
Using the plugins toolkit
Handling exceptions

Troubleshooting¶

AttributeError¶
If you get an AttributeError like this one:
AttributeError: 'ExampleIAuthFunctionsPlugin' object has no attribute 'get_auth_functions'

it means that your plugin class does not implement one of the plugin
interface’s methods. A plugin must implement every method of every plugin
interface that it implements.

Todo
Can you user inherit=True to avoid having to implement them all?

Other AttributeErrors can happen if your method returns the wrong type of
value, check the documentation for each plugin interface method to see what
your method should return.

TypeError¶
If you get a TypeError like this one:
TypeError: get_auth_functions() takes exactly 3 arguments (1 given)

it means that one of your plugin methods has the wrong number of parameters.
A plugin has to implement each method in a plugin interface with the same
parameters as in the interface.

Writing extensions tutorial¶

Installing CKAN¶

Creating a new extension¶

Creating a plugin class¶

Adding the plugin to `setup.py`¶

Installing the extension¶

Enabling the plugin¶

Troubleshooting¶

`PluginNotFoundException`¶

`ImportError`¶

Implementing the `IAuthFunctions` plugin interface¶

Using the plugins toolkit¶

`context`¶

`data_dict`¶

The plugins toolkit¶

Exception handling¶

We’re done!¶

Troubleshooting¶

`AttributeError`¶

`TypeError`¶