panels(templates): avoid evaluating LazyObject

LazyObject is typically used for something expensive to evaluate, so avoid evaluating it just for showing it in the debug toolbar.

Author: nijel

debug_toolbar/panels/templates/panel.py

@@ -133,10 +133,15 @@ def process_context_list(self, context_layers):
             if pformatted is None:
                 temp_layer = {}
                 for key, value in context_layer.items():
+                    # Do not force evaluating LazyObject
+                    if hasattr(value, "_wrapped"):
+                        # SimpleLazyObject has __repr__ which includes actual value
+                        # if it has been already evaluated
+                        temp_layer[key] = repr(value)
                     # Replace any request elements - they have a large
                     # Unicode representation and the request data is
                     # already made available from the Request panel.
-                    if isinstance(value, http.HttpRequest):
+                    elif isinstance(value, http.HttpRequest):
                         temp_layer[key] = "<<request>>"
                     # Replace the debugging sql_queries element. The SQL
                     # data is already made available from the SQL panel.

docs/changes.rst

@@ -7,6 +7,7 @@ Pending
 * Removed outdated third-party panels from the list.
 * Postponed context process in templates panel to include lazy evaluated
   content.
+* Fixed template panel to avoid evaluating ``LazyObject`` when not already evaluated.
 
 4.2.0 (2023-08-10)
 ------------------

tests/panels/test_template.py

@@ -2,6 +2,7 @@
 from django.contrib.auth.models import User
 from django.template import Context, RequestContext, Template
 from django.test import override_settings
+from django.utils.functional import SimpleLazyObject
 
 from ..base import BaseTestCase, IntegrationTestCase
 from ..forms import TemplateReprForm
@@ -113,6 +114,22 @@ def test_empty_context(self):
             ["{'False': False, 'None': None, 'True': True}"],
         )
 
+    def test_lazyobject(self):
+        response = self.panel.process_request(self.request)
+        t = Template("")
+        c = Context({"lazy": SimpleLazyObject(lambda: "lazy_value")})
+        t.render(c)
+        self.panel.generate_stats(self.request, response)
+        self.assertNotIn("lazy_value", self.panel.content)
+
+    def test_lazyobject_eval(self):
+        response = self.panel.process_request(self.request)
+        t = Template("{{lazy}}")
+        c = Context({"lazy": SimpleLazyObject(lambda: "lazy_value")})
+        self.assertEqual(t.render(c), "lazy_value")
+        self.panel.generate_stats(self.request, response)
+        self.assertIn("lazy_value", self.panel.content)
+
 
 @override_settings(
     DEBUG=True, DEBUG_TOOLBAR_PANELS=["debug_toolbar.panels.templates.TemplatesPanel"]