Plugins
=======

Datasette's plugin system is currently under active development. It allows
additional features to be implemented as Python code (or, soon, JavaScript)
which can be wrapped up in a separate Python package. The underlying mechanism
uses `pluggy <https://pluggy.readthedocs.io/>`_.

You can follow the development of plugins in `issue #14 <https://github.com/simonw/datasette/issues/14>`_.

Using plugins
-------------

If a plugin has been packaged for distribution using setuptools you can use
the plugin by installing it alongside Datasette in the same virtual
environment or Docker container.

You can also define one-off per-project plugins by saving them as
``plugin_name.py`` functions in a ``plugins/`` folder and then passing that
folder to ``datasette serve``.

Writing plugins
---------------

The easiest way to write a plugin is to create a ``my_plugin.py`` file and
drop it into your ``plugins/`` directory. Here is an example plugin, which
adds a new custom SQL function called ``hello_world()`` which takes no
arguments and returns the string ``Hello world!``.

.. code-block:: python

    from datasette import hookimpl

    @hookimpl
    def prepare_connection(conn):
        conn.create_function('hello_world', 0, lambda: 'Hello world!')

If you save this in ``plugins/my_plugin.py`` you can then start Datasette like
this::

    datasette serve mydb.db --plugins-dir=plugins/

Now you can navigate to http://localhost:8001/mydb and run this SQL::

    select hello_world();

To see the output of your plugin.

Packaging a plugin
------------------

Plugins can be packaged using Python setuptools. You can see an example of a
packaged plugin at https://github.com/simonw/datasette-plugin-demos

The example consists of two files: a ``setup.py`` file that defines the plugin:

.. code-block:: python

    from setuptools import setup

    VERSION = '0.1'

    setup(
        name='datasette-plugin-demos',
        description='Examples of plugins for Datasette',
        author='Simon Willison',
        url='https://github.com/simonw/datasette-plugin-demos',
        license='Apache License, Version 2.0',
        version=VERSION,
        py_modules=['datasette_plugin_demos'],
        entry_points={
            'datasette': [
                'plugin_demos = datasette_plugin_demos'
            ]
        },
        install_requires=['datasette']
    )

And a Python module file, ``datasette_plugin_demos.py``, that implements the
plugin:

.. code-block:: python

    from datasette import hookimpl
    import random


    @hookimpl
    def prepare_jinja2_environment(env):
        env.filters['uppercase'] = lambda u: u.upper()


    @hookimpl
    def prepare_connection(conn):
        conn.create_function('random_integer', 2, random.randint)


Having built a plugin in this way you can turn it into an installable package
using the following command::

    python3 setup.py sdist

This will create a ``.tar.gz`` file in the ``dist/`` directory.

You can then install your new plugin into a Datasette virtual environment or
Docker container using ``pip``::

    pip install datasette-plugin-demos-0.1.tar.gz

To learn how to upload your plugin to `PyPI <https://pypi.org/>`_ for use by
other people, read the PyPA guide to `Packaging and distributing projects
<https://packaging.python.org/tutorials/distributing-packages/>`_.

Plugin hooks
------------

Datasette will eventually have many more plugin hooks. You can track and
contribute to their development in `issue #14
<https://github.com/simonw/datasette/issues/14>`_.

prepare_connection(conn)
~~~~~~~~~~~~~~~~~~~~~~~~

This hook is called when a new SQLite database connection is created. You can
use it to `register custom SQL functions <https://docs.python.org/2/library/sqlite3.html#sqlite3.Connection.create_function>`_,
aggregates and collations. For example:

.. code-block:: python

    from datasette import hookimpl
    import random

    @hookimpl
    def prepare_connection(conn):
        conn.create_function('random_integer', 2, random.randint)

This registers a SQL function called ``random_integer`` which takes two
arguments and can be called like this::

    select random_integer(1, 10);

prepare_jinja2_environment(env)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This hook is called with the Jinja2 environment that is used to evaluate
Datasette HTML templates. You can use it to do things like `register custom
template filters <http://jinja.pocoo.org/docs/2.10/api/#custom-filters>`_, for
example:

.. code-block:: python

    from datasette import hookimpl

    @hookimpl
    def prepare_jinja2_environment(env):
        env.filters['uppercase'] = lambda u: u.upper()

You can now use this filter in your custom templates like so::

    Table name: {{ table|uppercase }}