Fixed #34883 -- Allowed template tags to set extra data on templates.

By setting a value in the `parser.extra_data` mapping, template tags
pass additional data out of the parsing context.

Any extra data set is exposed on the template via the matching
`.extra_data` attribute.

Library authors should use a key to namespace extra data. The 'django'
namespace is reserved for internal use.

django/template/base.py

@@ -193,7 +193,9 @@ class Template:
         )
 
         try:
-            return parser.parse()
+            nodelist = parser.parse()
+            self.extra_data = parser.extra_data
+            return nodelist
         except Exception as e:
             if self.engine.debug:
                 e.template_debug = self.get_exception_info(e, e.token)
@@ -439,6 +441,12 @@ class Parser:
         self.filters = {}
         self.command_stack = []
 
+        # Custom template tags may store additional data on the parser that
+        # will be made available on the template instance. Library authors
+        # should use a key to namespace any added data. The 'django' namespace
+        # is reserved for internal use.
+        self.extra_data = {}
+
         if libraries is None:
             libraries = {}
         if builtins is None:

docs/releases/5.1.txt

@@ -194,7 +194,9 @@ Signals
 Templates
 ~~~~~~~~~
 
-* ...
+* Custom tags may now set extra data on the ``Parser`` object that will later
+  be made available on the ``Template`` instance. Such data may be used, for
+  example, by the template loader, or other template clients.
 
 Tests
 ~~~~~

tests/template_tests/templatetags/custom.py

@@ -1,4 +1,5 @@
 from django import template
+from django.template.base import TextNode
 from django.template.defaultfilters import stringfilter
 from django.utils.html import escape, format_html
 from django.utils.safestring import mark_safe
@@ -216,3 +217,9 @@ class CounterNode(template.Node):
         count = self.count
         self.count = count + 1
         return str(count)
+
+
+@register.tag("extra_data")
+def do_extra_data(parser, token):
+    parser.extra_data["extra_data"] = "CUSTOM_DATA"
+    return TextNode("")

tests/template_tests/tests.py

@@ -140,6 +140,15 @@ class TemplateTestMixin:
         if self.debug_engine:
             self.assertEqual(e.exception.template_debug["during"], "{% badtag %}")
 
+    def test_compile_tag_extra_data(self):
+        """Custom tags can pass extra data back to template."""
+        engine = self._engine(
+            app_dirs=True,
+            libraries={"custom": "template_tests.templatetags.custom"},
+        )
+        t = engine.from_string("{% load custom %}{% extra_data %}")
+        self.assertEqual(t.extra_data["extra_data"], "CUSTOM_DATA")
+
     def test_render_tag_error_in_extended_block(self):
         """Errors in extended block are displayed correctly."""
         e = self._engine(app_dirs=True)