.. highlight:: python
   :linenothreshold: 5

.. _`pluginsetup`:

Creating a plugin
=================

It is possible to extend pretix with custom Python code using the official plugin
API. Every plugin has to be implemented as an independent Django 'app' living
in its own python package installed like any other python module. There are also some
official plugins inside the ``pretix/plugins/`` directory of your pretix installation.

The communication between pretix and the plugins happens mostly using Django's
`signal dispatcher`_ feature. The core modules of pretix, ``pretix.base``,
``pretix.control`` and ``pretix.presale`` expose a number of signals which are documented
on the next pages.

To create a new plugin, create a new python package which must be a valid `Django app`_
and must contain plugin metadata, as described below.
There is some boilerplate that you will need for every plugin to get started. To save your
time, we created a `cookiecutter`_ template that you can use like this::

   $ pip install cookiecutter
   $ cookiecutter https://github.com/pretix/pretix-plugin-cookiecutter

This will ask you some questions and then create a project folder for your plugin.

The following pages go into detail about the several types of plugins currently
supported. While these instructions don't assume that you know a lot about pretix,
they do assume that you have prior knowledge about Django (e.g. its view layer,
how its ORM works, etc.).

Plugin metadata
---------------

The plugin metadata lives inside a ``PretixPluginMeta`` class inside your app's
configuration class. The metadata class must define the following attributes:

.. rst-class:: rest-resource-table

================== ==================== ===========================================================
Attribute          Type                 Description
================== ==================== ===========================================================
name               string               The human-readable name of your plugin
author             string               Your name
version            string               A human-readable version code of your plugin
description        string               A more verbose description of what your plugin does.
visible            boolean (optional)   ``True`` by default, can hide a plugin so it cannot be normally activated.
restricted         boolean (optional)   ``False`` by default, restricts a plugin such that it can only be enabled
                                        for an event by system administrators / superusers.
================== ==================== ===========================================================

A working example would be::

    from django.apps import AppConfig
    from django.utils.translation import ugettext_lazy as _


    class PaypalApp(AppConfig):
        name = 'pretix_paypal'
        verbose_name = _("PayPal")

        class PretixPluginMeta:
            name = _("PayPal")
            author = _("the pretix team")
            version = '1.0.0'
            visible = True
            restricted = False
            description = _("This plugin allows you to receive payments via PayPal")


    default_app_config = 'pretix_paypal.PaypalApp'

The ``AppConfig`` class may implement a property ``compatiblity_errors``, that checks
whether the pretix installation meets all requirements of the plugin. If so,
it should contain ``None`` or an empty list, otherwise a list of strings containing
human-readable error messages. We recommend using the ``django.utils.functional.cached_property``
decorator, as it might get called a lot. You can also implement ``compatibility_warnings``,
those will be displayed but not block the plugin execution.

Plugin registration
-------------------

Somehow, pretix needs to know that your plugin exists at all. For this purpose, we
make use of the `entry point`_ feature of setuptools. To register a plugin that lives
in a separate python package, your ``setup.py`` should contain something like this::

    setup(
        args...,
        entry_points="""
    [pretix.plugin]
    pretix_paypal=pretix_paypal:PretixPluginMeta
    """
    )


This will automatically make pretix discover this plugin as soon as it is installed e.g.
through ``pip``. During development, you can just run ``python setup.py develop`` inside
your plugin source directory to make it discoverable.

Signals
-------

The various components of pretix define a number of signals which your plugin can
listen for. We will go into the details of the different signals in the following
pages. We suggest that you put your signal receivers into a ``signals`` submodule
of your plugin. You should extend your ``AppConfig`` (see above) by the following
method to make your receivers available::

    class PaypalApp(AppConfig):
        …

        def ready(self):
            from . import signals  # NOQA

You can optionally specify code that is executed when your plugin is activated for an event
in the ``installed`` method::

    class PaypalApp(AppConfig):
        …

        def installed(self, event):
            pass  # Your code here


Note that ``installed`` will *not* be called if the plugin in indirectly activated for an event
because the event is created with settings copied from another event.

Views
-----

Your plugin may define custom views. If you put an ``urls`` submodule into your
plugin module, pretix will automatically import it and include it into the root
URL configuration with the namespace ``plugins:<label>:``, where ``<label>`` is
your Django app label.

.. WARNING:: If you define custom URLs and views, you are currently on your own
   with checking that the calling user is logged in, has appropriate permissions,
   etc. We plan on providing native support for this in a later version.

.. _Django app: https://docs.djangoproject.com/en/1.7/ref/applications/
.. _signal dispatcher: https://docs.djangoproject.com/en/1.7/topics/signals/
.. _namespace packages: http://legacy.python.org/dev/peps/pep-0420/
.. _entry point: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#locating-plugins
.. _cookiecutter: https://cookiecutter.readthedocs.io/en/latest/
