.. highlight:: python
   :linenothreshold: 5

Writing an e-mail placeholder plugin
====================================

An email placeholder is a dynamic value that pretix users can use in their email templates.

Please read :ref:`Creating a plugin <pluginsetup>` first, if you haven't already.

Placeholder registration
------------------------

The placeholder API does not make a lot of usage from signals, however, it
does use a signal to get a list of all available email placeholders. Your plugin
should listen for this signal and return an instance of a subclass of ``pretix.base.email.BaseMailTextPlaceholder``::

    from django.dispatch import receiver

    from pretix.base.signals import register_mail_placeholders


    @receiver(register_mail_placeholders, dispatch_uid="placeholder_custom")
    def register_mail_renderers(sender, **kwargs):
        from .email import MyPlaceholderClass
        return MyPlaceholder()


Context mechanism
-----------------

Emails are sent in different "contexts" within pretix. For example, many emails are sent in the
the context of an order, but some are not, such as the notification of a waiting list voucher.

Not all placeholders make sense in every email, and placeholders usually depend some parameters
themselves, such as the ``Order`` object. Therefore, placeholders are expected to explicitly declare
what values they depend on and they will only be available in an email if all those dependencies are
met. Currently, placeholders can depend on the following context parameters:

* ``event``
* ``order``
* ``position``
* ``waiting_list_entry``
* ``invoice_address``
* ``payment``

There are a few more that are only to be used internally but not by plugins.

The placeholder class
---------------------

.. class:: pretix.base.email.BaseMailTextPlaceholder

   .. autoattribute:: identifier

      This is an abstract attribute, you **must** override this!

   .. autoattribute:: required_context

      This is an abstract attribute, you **must** override this!

   .. automethod:: render

      This is an abstract method, you **must** implement this!

   .. automethod:: render_sample

      This is an abstract method, you **must** implement this!

Helper class for simple placeholders
------------------------------------

pretix ships with a helper class that makes it easy to provide placeholders based on simple
functions::

     placeholder = SimpleFunctionalMailTextPlaceholder(
         'code', ['order'], lambda order: order.code, sample='F8VVL'
     )

