Fixed #36410 -- Added support for Template Partials to the Django Template Language.

Introduced `{% partialdef %}` and `{% partial %}` template tags to
define and render reusable named fragments within a template file.
Partials can also be accessed using the `template_name#partial_name`
syntax via `get_template()`, `render()`, `{% include %}`, and other
template-loading tools.

Adjusted `get_template()` behavior to support partial resolution, with
appropriate error handling for invalid names and edge cases. Introduced
`PartialTemplate` to encapsulate partial rendering behavior.

Includes tests and internal refactors to support partial context
binding, exception reporting, and tag validation.

Co-authored-by: Carlton Gibson <carlton@noumenal.es>
Co-authored-by: Natalia <124304+nessita@users.noreply.github.com>
Co-authored-by: Nick Pope <nick@nickpope.me.uk>

1 files changed, 18 insertions, 0 deletions

diff --git a/django/utils/datastructures.py b/django/utils/datastructures.py
index 7c8669a350..7118c814b5 100644
--- a/django/utils/datastructures.py
+++ b/django/utils/datastructures.py
@@ -345,3 +345,21 @@ class CaseInsensitiveMapping(Mapping):
                     "Element key %r invalid, only strings are allowed" % elem[0]
                 )
             yield elem
+
+
+class DeferredSubDict:
+    """
+    Wrap a dict, allowing deferred access to a sub-dict under a given key.
+
+    The value at ``deferred_key`` must itself be a dict. Accessing
+    ``DeferredSubDict(parent_dict, deferred_key)[key]`` retrieves
+    ``parent_dict[deferred_key][key]`` at access time, so updates to
+    the parent dict are reflected.
+    """
+
+    def __init__(self, parent_dict, deferred_key):
+        self.parent_dict = parent_dict
+        self.deferred_key = deferred_key
+
+    def __getitem__(self, key):
+        return self.parent_dict[self.deferred_key][key]