Initial documentation for plugins, closes #213

https://datasette.readthedocs.io/en/latest/plugins.html

README.md

@@ -117,6 +117,7 @@ http://localhost:8001/History/downloads.json?_shape=objects will return that dat
       -m, --metadata FILENAME      Path to JSON file containing license/source
                                    metadata
       --template-dir DIRECTORY     Path to directory containing custom templates
+      --plugins-dir DIRECTORY      Path to directory containing custom plugins
       --static STATIC MOUNT        mountpoint:path-to-directory for serving static
                                    files
       --help                       Show this message and exit.
@@ -167,6 +168,7 @@ This will create a docker image containing both the datasette application and th
       --force                   Pass --force option to now
       --branch TEXT             Install datasette from a GitHub branch e.g. master
       --template-dir DIRECTORY  Path to directory containing custom templates
+      --plugins-dir DIRECTORY   Path to directory containing custom plugins
       --static STATIC MOUNT     mountpoint:path-to-directory for serving static
                                 files
       --title TEXT              Title for metadata
@@ -192,6 +194,7 @@ If you have docker installed you can use `datasette package` to create a new Doc
       --extra-options TEXT      Extra options to pass to datasette serve
       --branch TEXT             Install datasette from a GitHub branch e.g. master
       --template-dir DIRECTORY  Path to directory containing custom templates
+      --plugins-dir DIRECTORY   Path to directory containing custom plugins
       --static STATIC MOUNT     mountpoint:path-to-directory for serving static
                                 files
       --title TEXT              Title for metadata

docs/getting_started.rst

@@ -110,6 +110,7 @@ datasette serve options
       -m, --metadata FILENAME      Path to JSON file containing license/source
                                    metadata
       --template-dir DIRECTORY     Path to directory containing custom templates
+      --plugins-dir DIRECTORY      Path to directory containing custom plugins
       --static STATIC MOUNT        mountpoint:path-to-directory for serving static
                                    files
       --help                       Show this message and exit.

docs/index.rst

@@ -21,6 +21,7 @@ Contents
    sql_queries
    metadata
    custom_templates
+   plugins
    changelog
 
 .. _Zeit Now: https://zeit.co/now

docs/plugins.rst

@@ -0,0 +1,156 @@
+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.
+
+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()