Added template_debug setting, closes #654

.travis.yml

@@ -27,7 +27,7 @@ jobs:
         - npm install -g now
         - python tests/fixtures.py fixtures.db fixtures.json
         - export ALIAS=`echo $TRAVIS_COMMIT | cut -c 1-7`
-        - datasette publish nowv1 fixtures.db -m fixtures.json --token=$NOW_TOKEN --branch=$TRAVIS_COMMIT --version-note=$TRAVIS_COMMIT --name=datasette-latest-$ALIAS --alias=latest.datasette.io --alias=$ALIAS.datasette.io
+        - datasette publish nowv1 fixtures.db -m fixtures.json --token=$NOW_TOKEN --branch=$TRAVIS_COMMIT --version-note=$TRAVIS_COMMIT --name=datasette-latest-$ALIAS --alias=latest.datasette.io --alias=$ALIAS.datasette.io --extra-options='--config template_debug:1'
     - stage: release tagged version
       if: tag IS present
       python: 3.6

datasette/app.py

@@ -126,6 +126,11 @@ CONFIG_OPTIONS = (
         False,
         "Force URLs in API output to always use https:// protocol",
     ),
+    ConfigOption(
+        "template_debug",
+        False,
+        "Allow display of template debug information with ?_context=1",
+    ),
 )
 DEFAULT_CONFIG = {option.name: option.default for option in CONFIG_OPTIONS}
 

datasette/views/base.py

@@ -1,6 +1,7 @@
 import asyncio
 import csv
 import itertools
+import json
 import re
 import time
 import urllib
@@ -138,29 +139,28 @@ class BaseView(AsgiView):
             )
             extra_template_vars.update(extra_vars)
 
-        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,
-                }
+        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") and self.ds.config("template_debug"):
+            return Response.html(
+                "<pre>{}</pre>".format(
+                    escape(json.dumps(template_context, default=repr, indent=4))
+                )
             )
-        )
+        return Response.html(await template.render_async(template_context))
 
 
 class DataView(BaseView):

docs/config.rst

@@ -209,3 +209,22 @@ itself will result in new, uncachcacheed URL paths.
 ::
 
     datasette mydatabase.db --config hash_urls:1
+
+.. _config_template_debug:
+
+template_debug
+--------------
+
+This setting enables template context debug mode, which is useful to help understand what variables are available to custom templates when you are writing them.
+
+Enable it like this::
+
+    datasette mydatabase.db --config template_debug:1
+
+Now you can add ``?_context=1`` or ``&_context=1`` to any Datasette page to see the context that was passed to that template.
+
+Some examples:
+
+* https://latest.datasette.io/?_context=1
+* https://latest.datasette.io/fixtures?_context=1
+* https://latest.datasette.io/fixtures/roadside_attractions?_context=1

tests/test_api.py

@@ -1287,6 +1287,7 @@ def test_config_json(app_client):
         "truncate_cells_html": 2048,
         "force_https_urls": False,
         "hash_urls": False,
+        "template_debug": False,
     } == response.json
 
 

tests/test_html.py

@@ -1073,3 +1073,16 @@ def test_zero_results(app_client, path):
     soup = Soup(response.text, "html.parser")
     assert 0 == len(soup.select("table"))
     assert 1 == len(soup.select("p.zero-results"))
+
+
+def test_config_template_debug_on():
+    for client in make_app_client(config={"template_debug": True}):
+        response = client.get("/fixtures/facetable?_context=1")
+        assert response.status == 200
+        assert response.text.startswith("<pre>{")
+
+
+def test_config_template_debug_off(app_client):
+    response = app_client.get("/fixtures/facetable?_context=1")
+    assert response.status == 200
+    assert not response.text.startswith("<pre>{")