HTML comment showing which templates were considered for a page

Closes #171

datasette/app.py

@@ -45,10 +45,16 @@ connections = threading.local()
 
 class RenderMixin(HTTPMethodView):
     def render(self, templates, **context):
+        template = self.jinja_env.select_template(templates)
+        select_templates = ['{}{}'.format(
+            '*' if template_name == template.name else '',
+            template_name
+        ) for template_name in templates]
         return response.html(
-            self.jinja_env.select_template(templates).render({
+            template.render({
                 **context, **{
                     'app_css_hash': self.ds.app_css_hash(),
+                    'select_templates': select_templates,
                 }
             })
         )

datasette/templates/base.html

@@ -33,6 +33,6 @@
         {% endif %}{{ metadata.source or metadata.source_url }}{% if metadata.source_url %}</a>{% endif %}
     {% endif %}
 </div>
-
+{% if select_templates %}<!-- Templates considered: {{ select_templates|join(", ") }} -->{% endif %}
 </body>
 </html>

docs/custom_templates.rst

@@ -143,6 +143,18 @@ templates::
     table-mydatabase-Food-Trucks-399138.html
     table.html
 
+You can find out which templates were considered for a specific page by viewing
+source on that page and looking for an HTML comment at the bottom. The comment
+will look something like this::
+
+    <!-- Templates considered: *query-mydb-tz.html, query-mydb.html, query.html -->
+
+This example is from the canned query page for a query called "tz" in the
+database called "mydb". The asterisk shows which template was selected - so in
+this case, Datasette found a template file called ``query-mydb-tz.html`` and
+used that - but if that template had not been found, it would have tried for
+``query-mydb.html`` or the default ``query.html``.
+
 It is possible to extend the default templates using Jinja template
 inheritance. If you want to customize EVERY row template with some additional
 content you can do so by creating a ``row.html`` template like this::