Don't render a StructValue's template on calls to str(value)

This is liable to cause infinite loops on debug / logging code that attempts to log the fact that it's rendered a template with this value in the context.

Fixes #2874, https://github.com/jazzband/django-debug-toolbar/issues/950

CHANGELOG.txt

@@ -17,6 +17,7 @@ Changelog
  * Fix: Prevent explorer view from crashing when page model definitions are missing, allowing the offending pages to be deleted (Matt Westcott)
  * Fix: Hide the userbar from printed page representation (Eugene Morozov)
  * Fix: Prevent the page editor footer content from collapsing into two lines unnecessarily (Jack Paine)
+ * Fix: StructBlock values no longer render HTML templates as their `str` representation, to prevent infinite loops in debugging / logging tools (Matt Westcott)
 
 
 1.11.1 (07.07.2017)

docs/releases/1.12.rst

@@ -37,6 +37,7 @@ Bug fixes
  * Prevent explorer view from crashing when page model definitions are missing, allowing the offending pages to be deleted (Matt Westcott)
  * Hide the userbar from printed page representation (Eugene Morozov)
  * Prevent the page editor footer content from collapsing into two lines unnecessarily (Jack Paine)
+ * StructBlock values no longer render HTML templates as their ``str`` representation, to prevent infinite loops in debugging / logging tools (Matt Westcott)
 
 Upgrade considerations
 ======================

wagtail/wagtailcore/blocks/struct_block.py

@@ -9,7 +9,6 @@ from django.forms.utils import ErrorList
 from django.template.loader import render_to_string
 # Must be imported from Django so we get the new implementation of with_metaclass
 from django.utils import six
-from django.utils.encoding import python_2_unicode_compatible
 from django.utils.functional import cached_property
 from django.utils.html import format_html, format_html_join
 
@@ -194,13 +193,12 @@ class StructBlock(six.with_metaclass(DeclarativeSubBlocksMetaclass, BaseStructBl
     pass
 
 
-@python_2_unicode_compatible  # provide equivalent __unicode__ and __str__ methods on Py2
 class StructValue(collections.OrderedDict):
     def __init__(self, block, *args):
         super(StructValue, self).__init__(*args)
         self.block = block
 
-    def __str__(self):
+    def __html__(self):
         return self.block.render(self)
 
     def render_as_block(self, context=None):

wagtail/wagtailcore/tests/test_blocks.py

@@ -1316,16 +1316,35 @@ class TestStructBlock(SimpleTestCase):
 
     def test_render_structvalue(self):
         """
-        The string representation of a StructValue should use the block's template
+        The HTML representation of a StructValue should use the block's template
+        """
+        block = SectionBlock()
+        value = block.to_python({'title': 'Hello', 'body': '<i>italic</i> world'})
+        result = value.__html__()
+        self.assertEqual(result, """<h1>Hello</h1><div class="rich-text"><i>italic</i> world</div>""")
+
+        # value.render_as_block() should be equivalent to value.__html__()
+        result = value.render_as_block()
+        self.assertEqual(result, """<h1>Hello</h1><div class="rich-text"><i>italic</i> world</div>""")
+
+    def test_str_structvalue(self):
+        """
+        The str() representation of a StructValue should NOT render the template, as that's liable
+        to cause an infinite loop if any debugging / logging code attempts to log the fact that
+        it rendered a template with this object in the context:
+        https://github.com/wagtail/wagtail/issues/2874
+        https://github.com/jazzband/django-debug-toolbar/issues/950
         """
         block = SectionBlock()
         value = block.to_python({'title': 'Hello', 'body': '<i>italic</i> world'})
         result = str(value)
-        self.assertEqual(result, """<h1>Hello</h1><div class="rich-text"><i>italic</i> world</div>""")
-
-        # value.render_as_block() should be equivalent to str(value)
-        result = value.render_as_block()
-        self.assertEqual(result, """<h1>Hello</h1><div class="rich-text"><i>italic</i> world</div>""")
+        self.assertNotIn('<h1>', result)
+        # The expected rendering should correspond to the native representation of an OrderedDict:
+        # "StructValue([('title', u'Hello'), ('body', <wagtail.wagtailcore.rich_text.RichText object at 0xb12d5eed>)])"
+        # - give or take some quoting differences between Python versions
+        self.assertIn('StructValue', result)
+        self.assertIn('title', result)
+        self.assertIn('Hello', result)
 
     def test_render_structvalue_with_extra_context(self):
         block = SectionBlock()