Render templates using Jinja async mode

Closes #628

datasette/app.py

@@ -583,7 +583,9 @@ class Datasette:
                 ),
             ]
         )
-        self.jinja_env = Environment(loader=template_loader, autoescape=True)
+        self.jinja_env = Environment(
+            loader=template_loader, autoescape=True, enable_async=True
+        )
         self.jinja_env.filters["escape_css_string"] = escape_css_string
         self.jinja_env.filters["quote_plus"] = lambda u: urllib.parse.quote_plus(u)
         self.jinja_env.filters["escape_sqlite"] = escape_sqlite
@@ -730,5 +732,5 @@ class DatasetteRouter(AsgiRouter):
         else:
             template = self.ds.jinja_env.select_template(templates)
             await asgi_send_html(
-                send, template.render(info), status=status, headers=headers
+                send, await template.render_async(info), status=status, headers=headers
             )

datasette/views/base.py

@@ -139,7 +139,7 @@ class BaseView(AsgiView):
             extra_template_vars.update(extra_vars)
 
         return Response.html(
-            template.render(
+            await template.render_async(
                 {
                     **context,
                     **{

docs/plugins.rst

@@ -629,7 +629,9 @@ Function that returns a dictionary
     If you return a function it will be executed. If it returns a dictionary those values will will be merged into the template context.
 
 Function that returns an awaitable function that returns a dictionary
-    You can also return a function which returns an awaitable function which returns a dictionary. This means you can execute additional SQL queries using ``datasette.execute()``.
+    You can also return a function which returns an awaitable function which returns a dictionary.
+
+Datasette runs Jinja2 in `async mode <https://jinja.palletsprojects.com/en/2.10.x/api/#async-support>`__, which means you can add awaitable functions to the template scope and they will be automatically awaited when they are rendered by the template.
 
 Here's an example plugin that returns an authentication object from the ASGI scope:
 
@@ -641,20 +643,19 @@ Here's an example plugin that returns an authentication object from the ASGI sco
             "auth": request.scope.get("auth")
         }
 
-And here's an example which returns the current version of SQLite:
+And here's an example which adds a ``sql_first(sql_query)`` function which executes a SQL statement and returns the first column of the first row of results:
 
 .. code-block:: python
 
     @hookimpl
-    def extra_template_vars(datasette):
-        async def inner():
-            first_db = list(datasette.databases.keys())[0]
-            return {
-                "sqlite_version": (
-                    await datasette.execute(first_db, "select sqlite_version()")
-                ).rows[0][0]
-            }
-        return inner
+    def extra_template_vars(datasette, database):
+        async def sql_first(sql, dbname=None):
+            dbname = dbname or database or next(iter(datasette.databases.keys()))
+            return (await datasette.execute(dbname, sql)).rows[0][0]
+
+You can then use the new function in a template like so::
+
+    SQLite version: {{ sql_first("select sqlite_version()") }}
 
 .. _plugin_register_output_renderer:
 

tests/fixtures.py

@@ -446,13 +446,19 @@ def render_cell(value, database):
 
 @hookimpl
 def extra_template_vars(template, database, table, view_name, request, datasette):
+    async def query_database(sql):
+        first_db = list(datasette.databases.keys())[0]
+        return (
+            await datasette.execute(first_db, sql)
+        ).rows[0][0]
     async def inner():
         return {
             "extra_template_vars_from_awaitable": json.dumps({
                 "template": template,
                 "scope_path": request.scope["path"],
                 "awaitable": True,
-            }, default=lambda b: b.decode("utf8"))
+            }, default=lambda b: b.decode("utf8")),
+            "query_database": query_database,
         }
     return inner
 

tests/test_plugins.py

@@ -1,5 +1,6 @@
 from bs4 import BeautifulSoup as Soup
 from .fixtures import app_client, make_app_client, TEMP_PLUGIN_SECRET_FILE  # noqa
+from datasette.utils import sqlite3
 import base64
 import json
 import os
@@ -214,3 +215,20 @@ def test_plugins_extra_template_vars(restore_working_directory):
             "awaitable": True,
             "scope_path": "/-/metadata",
         } == extra_template_vars_from_awaitable
+
+
+def test_plugins_async_template_function(restore_working_directory):
+    for client in make_app_client(
+        template_dir=str(pathlib.Path(__file__).parent / "test_templates")
+    ):
+        response = client.get("/-/metadata")
+        assert response.status == 200
+        extra_from_awaitable_function = (
+            Soup(response.body, "html.parser")
+            .select("pre.extra_from_awaitable_function")[0]
+            .text
+        )
+        expected = (
+            sqlite3.connect(":memory:").execute("select sqlite_version()").fetchone()[0]
+        )
+        assert expected == extra_from_awaitable_function

tests/test_templates/show_json.html

@@ -5,4 +5,5 @@
 Test data for extra_template_vars:
 <pre class="extra_template_vars">{{ extra_template_vars|safe }}</pre>
 <pre class="extra_template_vars_from_awaitable">{{ extra_template_vars_from_awaitable|safe }}</pre>
+<pre class="extra_from_awaitable_function">{{ query_database("select sqlite_version();") }}</pre>
 {% endblock %}