extra_template_vars plugin hook (#542)

* extra_template_vars plugin hook

Closes #541

* Workaround for cwd bug

Based on https://github.com/pytest-dev/pytest/issues/1235#issuecomment-175295691

tests/conftest.py

@@ -1,3 +1,7 @@
+import os
+import pytest
+
+
 def pytest_configure(config):
     import sys
 
@@ -22,3 +26,14 @@ def move_to_front(items, test_name):
     test = [fn for fn in items if fn.name == test_name]
     if test:
         items.insert(0, items.pop(items.index(test[0])))
+
+
+@pytest.fixture
+def restore_working_directory(tmpdir, request):
+    previous_cwd = os.getcwd()
+    tmpdir.chdir()
+
+    def return_to_previous():
+        os.chdir(previous_cwd)
+
+    request.addfinalizer(return_to_previous)

tests/fixtures.py

@@ -376,6 +376,16 @@ def render_cell(value, column, table, database, datasette):
             table=table,
         )
     })
+
+
+@hookimpl
+def extra_template_vars(template, database, table, view_name, request, datasette):
+    return {
+        "extra_template_vars": json.dumps({
+            "template": template,
+            "scope_path": request.scope["path"]
+        }, default=lambda b: b.decode("utf8"))
+    }
 """
 
 PLUGIN2 = """
@@ -424,6 +434,19 @@ def render_cell(value, database):
     )
 
 
+@hookimpl
+def extra_template_vars(template, database, table, view_name, request, datasette):
+    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"))
+        }
+    return inner
+
+
 @hookimpl
 def asgi_wrapper(datasette):
     def wrap_with_databases_header(app):

tests/test_plugins.py

@@ -3,6 +3,7 @@ from .fixtures import app_client, make_app_client, TEMP_PLUGIN_SECRET_FILE  # no
 import base64
 import json
 import os
+import pathlib
 import re
 import pytest
 import urllib
@@ -188,3 +189,28 @@ def test_plugins_extra_body_script(app_client, path, expected_extra_body_script)
 def test_plugins_asgi_wrapper(app_client):
     response = app_client.get("/fixtures")
     assert "fixtures" == response.headers["x-databases"]
+
+
+def test_plugins_extra_template_vars(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_template_vars = json.loads(
+            Soup(response.body, "html.parser").select("pre.extra_template_vars")[0].text
+        )
+        assert {
+            "template": "show_json.html",
+            "scope_path": "/-/metadata",
+        } == extra_template_vars
+        extra_template_vars_from_awaitable = json.loads(
+            Soup(response.body, "html.parser")
+            .select("pre.extra_template_vars_from_awaitable")[0]
+            .text
+        )
+        assert {
+            "template": "show_json.html",
+            "awaitable": True,
+            "scope_path": "/-/metadata",
+        } == extra_template_vars_from_awaitable

tests/test_templates/show_json.html

@@ -0,0 +1,8 @@
+{% extends "base.html" %}
+
+{% block content %}
+{{ super() }}
+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>
+{% endblock %}