Removed plugin_demo.py

Running `from datasette import __version__` in `setup.py` was throwing
an error `ModuleNotFoundError: No module named 'pluggy'`

See https://packaging.python.org/guides/single-sourcing-package-version/

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]