.. highlight:: python
   :linenothreshold: 5


.. Tests are skipped because they fail to import from processing_provider under CI
.. everything runs fine when testing locally with make -f docker.mk doctest

.. testsetup:: processing

    iface = start_qgis()

.. index:: Plugins; Processing algorithm
.. _processing_plugin:

****************************
Writing a Processing plugin
****************************

.. contents::
   :local:

Depending on the kind of plugin that you are going to develop, it might be a better
option to add its functionality as a Processing algorithm (or a set of them).
That would provide a better integration within QGIS, additional functionality (since
it can be run in the components of Processing, such as the modeler or the batch
processing interface), and a quicker development time (since Processing will take of
a large part of the work).

To distribute those algorithms, you should create a new plugin that adds them to the
Processing Toolbox. The plugin should contain an algorithm provider, which has to be
registered when the plugin is instantiated.

Creating from scratch
=====================

To create a plugin from scratch which contains an algorithm provider, you can
follow these steps using the Plugin Builder:

#. Install the **Plugin Builder** plugin
#. Create a new plugin using the Plugin Builder. When the Plugin Builder asks you for
   the template to use, select "Processing provider".
#. The created plugin contains a provider with a single algorithm. Both the provider
   file and the algorithm file are fully commented and contain information about how to
   modify the provider and add additional algorithms. Refer to them for more information.

Updating a plugin
=================

If you want to add your existing plugin to Processing, you need to add some code.

#. In your :file:`metadata.txt` file, you need to add a variable:

   .. code-block:: ini

    hasProcessingProvider=yes

#. In the Python file where your plugin is setup with the ``initGui`` method,
   you need to adapt some lines like this:

   .. testcode:: processing
        :skipif: True

        from qgis.core import QgsApplication
        from processing_provider.provider import Provider

        class YourPluginName():

            def __init__(self):
                self.provider = None

            def initProcessing(self):
                self.provider = Provider()
                QgsApplication.processingRegistry().addProvider(self.provider)

            def initGui(self):
                self.initProcessing()

            def unload(self):
                QgsApplication.processingRegistry().removeProvider(self.provider)

#. You can create a folder :file:`processing_provider` with three files in it:

   * :file:`__init__.py` with nothing in it. This is necessary to make a valid
     Python package.
   * :file:`provider.py` which will create the Processing provider and expose
     your algorithms.

     .. testcode:: processing
      :skipif: True

      from qgis.core import QgsProcessingProvider

      from processing_provider.example_processing_algorithm import ExampleProcessingAlgorithm


      class Provider(QgsProcessingProvider):

          def loadAlgorithms(self, *args, **kwargs):
              self.addAlgorithm(ExampleProcessingAlgorithm())
              # add additional algorithms here
              # self.addAlgorithm(MyOtherAlgorithm())

          def id(self, *args, **kwargs):
              """The ID of your plugin, used for identifying the provider.

              This string should be a unique, short, character only string,
              eg "qgis" or "gdal". This string should not be localised.
              """
              return 'yourplugin'

          def name(self, *args, **kwargs):
              """The human friendly name of your plugin in Processing.

              This string should be as short as possible (e.g. "Lastools", not
              "Lastools version 1.0.1 64-bit") and localised.
              """
              return self.tr('Your plugin')

          def icon(self):
              """Should return a QIcon which is used for your provider inside
              the Processing toolbox.
              """
              return QgsProcessingProvider.icon(self)

   * :file:`example_processing_algorithm.py` which contains the example
     algorithm file. Copy/paste the content of the :source:`script template
     file <python/plugins/processing/script/ScriptTemplate.py>` and
     update it according to your needs.

#. Now you can reload your plugin in QGIS and you should see your example
   script in the Processing toolbox and modeler.