Template debug mode that outputs template context

[simonw]

It would make writing templates (including custom templates) easier if there was an option to dump out the full template context - maybe ?_context=1

[simonw]

Proof of concept:

diff --git a/datasette/views/base.py b/datasette/views/base.py
index 5182479..39d1f77 100644
--- a/datasette/views/base.py
+++ b/datasette/views/base.py
@@ -1,6 +1,7 @@
 import asyncio
 import csv
 import itertools
+import json
 import re
 import time
 import urllib
@@ -138,28 +139,31 @@ class BaseView(AsgiView):
             )
             extra_template_vars.update(extra_vars)
 
+        template_context = {
+            **context,
+            **{
+                "app_css_hash": self.ds.app_css_hash(),
+                "select_templates": select_templates,
+                "zip": zip,
+                "body_scripts": body_scripts,
+                "extra_css_urls": self._asset_urls(
+                    "extra_css_urls", template, context
+                ),
+                "extra_js_urls": self._asset_urls(
+                    "extra_js_urls", template, context
+                ),
+                "format_bytes": format_bytes,
+                "database_url": self.database_url,
+                "database_color": self.database_color,
+            },
+            **extra_template_vars,
+        }
+        if request.args.get("_context"):
+            return Response.html("<pre>{}</pre>".format(
+                    escape(json.dumps(template_context, default=repr, indent=4))
+            ))
         return Response.html(
-            await template.render_async(
-                {
-                    **context,
-                    **{
-                        "app_css_hash": self.ds.app_css_hash(),
-                        "select_templates": select_templates,
-                        "zip": zip,
-                        "body_scripts": body_scripts,
-                        "extra_css_urls": self._asset_urls(
-                            "extra_css_urls", template, context
-                        ),
-                        "extra_js_urls": self._asset_urls(
-                            "extra_js_urls", template, context
-                        ),
-                        "format_bytes": format_bytes,
-                        "database_url": self.database_url,
-                        "database_color": self.database_color,
-                    },
-                    **extra_template_vars,
-                }
-            )
+            await template.render_async(template_context)
         )

[simonw]

One problem with this: what if secrets end up being dumped out in this debug view?

This won't happen with default Datasette but could potentially happen with a plugin.

This feature should be opt-in - maybe a template_debug:1 config setting.

[simonw]

Documentation: https://datasette.readthedocs.io/en/latest/config.html#template-debug

Demos:

• https://latest.datasette.io/?_context=1
• https://latest.datasette.io/fixtures?_context=1
• https://latest.datasette.io/fixtures/roadside_attractions?_context=1