New plugin hooks: extra_css_urls() and extra_js_urls()

Closes #214

datasette/app.py

@@ -12,6 +12,7 @@ import asyncio
 import os
 import threading
 import urllib.parse
+import itertools
 import json
 import jinja2
 import hashlib
@@ -1126,7 +1127,12 @@ class Datasette:
             }
 
     def asset_urls(self, key):
-        for url_or_dict in (self.metadata.get(key) or []):
+        urls_or_dicts = (self.metadata.get(key) or [])
+        # Flatten list-of-lists from plugins:
+        urls_or_dicts += list(
+            itertools.chain.from_iterable(getattr(pm.hook, key)())
+        )
+        for url_or_dict in urls_or_dicts:
             if isinstance(url_or_dict, dict):
                 yield {
                     'url': url_or_dict['url'],
@@ -1297,10 +1303,14 @@ class Datasette:
             app.static(path, dirname)
         # Mount any plugin static/ directories
         for plugin_module in pm.get_plugins():
-            if pkg_resources.resource_isdir(plugin_module.__name__, 'static'):
-                modpath = '/-/static-plugins/{}/'.format(plugin_module.__name__)
-                dirpath = pkg_resources.resource_filename(plugin_module.__name__, 'static')
-                app.static(modpath, dirpath)
+            try:
+                if pkg_resources.resource_isdir(plugin_module.__name__, 'static'):
+                    modpath = '/-/static-plugins/{}/'.format(plugin_module.__name__)
+                    dirpath = pkg_resources.resource_filename(plugin_module.__name__, 'static')
+                    app.static(modpath, dirpath)
+            except ModuleNotFoundError:
+                # Caused by --plugins_dir= plugins
+                pass
         app.add_route(
             DatabaseView.as_view(self),
             '/<db_name:[^/\.]+?><as_json:(\.jsono?)?$>'

datasette/hookspecs.py

@@ -13,3 +13,13 @@ def prepare_connection(conn):
 @hookspec
 def prepare_jinja2_environment(env):
     "Modify Jinja2 template environment e.g. register custom template tags"
+
+
+@hookspec
+def extra_css_urls():
+    "Extra CSS URLs added by this plugin"
+
+
+@hookspec
+def extra_js_urls():
+    "Extra JavaScript URLs added by this plugin"

docs/custom_templates.rst

@@ -1,5 +1,7 @@
-Customizing Datasette's appearance
-==================================
+.. _customization:
+
+Customization
+=============
 
 Datasette provides a number of ways of customizing the way data is displayed.
 

docs/plugins.rst

@@ -169,3 +169,65 @@ example:
 You can now use this filter in your custom templates like so::
 
     Table name: {{ table|uppercase }}
+
+extra_css_urls()
+~~~~~~~~~~~~~~~~
+
+Return a list of extra CSS URLs that should be included on every page. These can
+take advantage of the CSS class hooks described in :ref:`customization`.
+
+This can be a list of URLs:
+
+.. code-block:: python
+
+    from datasette import hookimpl
+
+    @hookimpl
+    def extra_css_urls():
+        return [
+            'https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css'
+        ]
+
+Or a list of dictionaries defining both a URL and an
+`SRI hash <https://www.srihash.org/>`_:
+
+.. code-block:: python
+
+    from datasette import hookimpl
+
+    @hookimpl
+    def extra_css_urls():
+        return [{
+            'url': 'https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css',
+            'sri': 'sha384-9gVQ4dYFwwWSjIDZnLEWnxCjeSWFphJiwGPXr1jddIhOegiu1FwO5qRGvFXOdJZ4',
+        }]
+
+extra_js_urls()
+~~~~~~~~~~~~~~~
+
+This works in the same way as ``extra_css_urls()`` but for JavaScript. You can
+return either a list of URLs or a list of dictionaries:
+
+.. code-block:: python
+
+    from datasette import hookimpl
+
+    @hookimpl
+    def extra_js_urls():
+        return [{
+            'url': 'https://code.jquery.com/jquery-3.3.1.slim.min.js',
+            'sri': 'sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo',
+        }]
+
+You can also return URLs to files from your plugin's ``static/`` directory, if
+you have one:
+
+.. code-block:: python
+
+    from datasette import hookimpl
+
+    @hookimpl
+    def extra_js_urls():
+        return [
+            '/-/static-plugins/your_plugin/app.js'
+        ]

tests/fixtures.py

@@ -113,6 +113,19 @@ def prepare_connection(conn):
         "select convert_units(100, 'm', 'ft');"
         return (amount * ureg(from_)).to(to_).to_tuple()[0]
     conn.create_function('convert_units', 3, convert_units)
+
+
+@hookimpl
+def extra_css_urls():
+    return ['https://example.com/app.css']
+
+
+@hookimpl
+def extra_js_urls():
+    return [{
+        'url': 'https://example.com/app.js',
+        'sri': 'SRIHASH',
+    }]
 '''
 
 TABLES = '''

tests/test_html.py

@@ -333,6 +333,19 @@ def test_view_html(app_client):
     assert expected == [[str(td) for td in tr.select('td')] for tr in table.select('tbody tr')]
 
 
+def test_plugin_extra_css_urls(app_client):
+    response = app_client.get('/', gather_request=False)
+    assert b'<link rel="stylesheet" href="https://example.com/app.css">' in response.body
+
+
+def test_plugin_extra_js_urls(app_client):
+    response = app_client.get('/', gather_request=False)
+    assert (
+        b'<script src="https://example.com/app.js" integrity="SRIHASH" crossorigin="anonymous"></script>'
+        in response.body
+    )
+
+
 def test_index_metadata(app_client):
     response = app_client.get('/', gather_request=False)
     assert response.status == 200