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

datasette/hookspecs.py

@@ -35,6 +35,11 @@ def extra_body_script(template, database, table, view_name, datasette):
     "Extra JavaScript code to be included in <script> at bottom of body"
 
 
+@hookspec
+def extra_template_vars(template, database, table, view_name, request, datasette):
+    "Extra template variables to be made available to the template - can return dict or callable or awaitable"
+
+
 @hookspec
 def publish_subcommand(publish):
     "Subcommands for 'datasette publish'"

datasette/views/base.py

@@ -102,7 +102,7 @@ class BaseView(AsgiView):
     def database_color(self, database):
         return "ff0000"
 
-    def render(self, templates, **context):
+    async def render(self, templates, request, context):
         template = self.ds.jinja_env.select_template(templates)
         select_templates = [
             "{}{}".format("*" if template_name == template.name else "", template_name)
@@ -118,6 +118,26 @@ class BaseView(AsgiView):
             datasette=self.ds,
         ):
             body_scripts.append(jinja2.Markup(script))
+
+        extra_template_vars = {}
+        # pylint: disable=no-member
+        for extra_vars in pm.hook.extra_template_vars(
+            template=template.name,
+            database=context.get("database"),
+            table=context.get("table"),
+            view_name=self.name,
+            request=request,
+            datasette=self.ds,
+        ):
+            if callable(extra_vars):
+                extra_vars = extra_vars()
+            if asyncio.iscoroutine(extra_vars):
+                extra_vars = await extra_vars
+            assert isinstance(extra_vars, dict), "extra_vars is of type {}".format(
+                type(extra_vars)
+            )
+            extra_template_vars.update(extra_vars)
+
         return Response.html(
             template.render(
                 {
@@ -137,6 +157,7 @@ class BaseView(AsgiView):
                         "database_url": self.database_url,
                         "database_color": self.database_color,
                     },
+                    **extra_template_vars,
                 }
             )
         )
@@ -471,7 +492,7 @@ class DataView(BaseView):
             }
             if "metadata" not in context:
                 context["metadata"] = self.ds.metadata
-            r = self.render(templates, **context)
+            r = await self.render(templates, request=request, context=context)
             r.status = status_code
 
         ttl = request.args.get("_ttl", None)

datasette/views/index.py

@@ -109,9 +109,12 @@ class IndexView(BaseView):
                 headers=headers,
             )
         else:
-            return self.render(
+            return await self.render(
                 ["index.html"],
-                databases=databases,
-                metadata=self.ds.metadata(),
-                datasette_version=__version__,
+                request=request,
+                context={
+                    "databases": databases,
+                    "metadata": self.ds.metadata(),
+                    "datasette_version": __version__,
+                },
             )

datasette/views/special.py

@@ -24,4 +24,8 @@ class JsonDataView(BaseView):
             )
 
         else:
-            return self.render(["show_json.html"], filename=self.filename, data=data)
+            return await self.render(
+                ["show_json.html"],
+                request=request,
+                context={"filename": self.filename, "data": data},
+            )