Start of the plugin system, based on pluggy (#210)

Uses https://pluggy.readthedocs.io/ originally created for the py.test project

We're starting with two plugin hooks:

prepare_connection(conn)

This is called when a new SQLite connection is created. It can be used to register custom SQL functions.

prepare_jinja2_environment(env)

This is called with the Jinja2 environment. It can be used to register custom template tags and filters.

An example plugin which uses these two hooks can be found at https://github.com/simonw/datasette-plugin-demos or installed using `pip install datasette-plugin-demos`

Refs #14

datasette/__init__.py

@@ -1 +1,3 @@
 from datasette.version import __version_info__, __version__  # noqa
+from .hookspecs import hookimpl # noqa
+from .hookspecs import hookspec # noqa

datasette/app.py

@@ -17,6 +17,7 @@ import jinja2
 import hashlib
 import time
 import pint
+import pluggy
 import traceback
 from .utils import (
     Filters,
@@ -38,6 +39,7 @@ from .utils import (
     urlsafe_components,
     validate_sql_select,
 )
+from . import hookspecs
 from .version import __version__
 
 app_root = Path(__file__).parent.parent
@@ -49,6 +51,11 @@ connections = threading.local()
 ureg = pint.UnitRegistry()
 
 
+pm = pluggy.PluginManager('datasette')
+pm.add_hookspecs(hookspecs)
+pm.load_setuptools_entrypoints('datasette')
+
+
 class DatasetteError(Exception):
     def __init__(self, message, title=None, error_dict=None, status=500, template=None):
         self.message = message
@@ -1100,6 +1107,7 @@ class Datasette:
             conn.enable_load_extension(True)
             for extension in self.sqlite_extensions:
                 conn.execute("SELECT load_extension('{}')".format(extension))
+        pm.hook.prepare_connection(conn=conn)
 
     def inspect(self):
         if not self._inspect:
@@ -1226,6 +1234,7 @@ class Datasette:
         self.jinja_env.filters['quote_plus'] = lambda u: urllib.parse.quote_plus(u)
         self.jinja_env.filters['escape_sqlite'] = escape_sqlite
         self.jinja_env.filters['to_css_class'] = to_css_class
+        pm.hook.prepare_jinja2_environment(env=self.jinja_env)
         app.add_route(IndexView.as_view(self), '/<as_json:(\.jsono?)?$>')
         # TODO: /favicon.ico and /-/static/ deserve far-future cache expires
         app.add_route(favicon, '/favicon.ico')

datasette/hookspecs.py

@@ -0,0 +1,15 @@
+from pluggy import HookimplMarker
+from pluggy import HookspecMarker
+
+hookspec = HookspecMarker('datasette')
+hookimpl = HookimplMarker('datasette')
+
+
+@hookspec
+def prepare_connection(conn):
+    "Modify SQLite connection in some way e.g. register custom SQL functions"
+
+
+@hookspec
+def prepare_jinja2_environment(env):
+    "Modify Jinja2 template environment e.g. register custom template tags"

setup.py

@@ -1,5 +1,5 @@
 from setuptools import setup, find_packages
-from datasette import __version__
+from datasette.version import __version__
 import os
 
 
@@ -28,7 +28,8 @@ setup(
         'Sanic==0.7.0',
         'Jinja2==2.10',
         'hupper==1.0',
-        'pint==0.8.1'
+        'pint==0.8.1',
+        'pluggy>=0.1.0,<1.0',
     ],
     entry_points='''
         [console_scripts]