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>

16 files changed, 1587 insertions, 2 deletions

diff --git a/django/template/__init__.py b/django/template/__init__.py
index adb431c00d..92568da793 100644
--- a/django/template/__init__.py
+++ b/django/template/__init__.py
@@ -61,6 +61,7 @@ from .base import (  # NOQA isort:skip
     Node,
     NodeList,
     Origin,
+    PartialTemplate,
     Template,
     Variable,
 )
diff --git a/django/template/base.py b/django/template/base.py
index 3e8a59fbe7..74b3987410 100644
--- a/django/template/base.py
+++ b/django/template/base.py
@@ -88,6 +88,11 @@ UNKNOWN_SOURCE = "<unknown source>"
 # than instantiating SimpleLazyObject with _lazy_re_compile().
 tag_re = re.compile(r"({%.*?%}|{{.*?}}|{#.*?#})")
 
+combined_partial_re = re.compile(
+    r"{%\s*partialdef\s+(?P<name>[\w-]+)(?:\s+inline)?\s*%}"
+    r"|{%\s*endpartialdef(?:\s+[\w-]+)?\s*%}"
+)
+
 logger = logging.getLogger("django.template")
 
 
@@ -288,6 +293,57 @@ class Template:
         }
 
 
+class PartialTemplate:
+    """
+    A lightweight Template lookalike used for template partials.
+
+    Wraps nodelist as a partial, in order to be able to bind context.
+    """
+
+    def __init__(self, nodelist, origin, name):
+        self.nodelist = nodelist
+        self.origin = origin
+        self.name = name
+
+    def get_exception_info(self, exception, token):
+        template = self.origin.loader.get_template(self.origin.template_name)
+        return template.get_exception_info(exception, token)
+
+    def find_partial_source(self, full_source, partial_name):
+        start_match = None
+        nesting = 0
+
+        for match in combined_partial_re.finditer(full_source):
+            if name := match["name"]:  # Opening tag.
+                if start_match is None and name == partial_name:
+                    start_match = match
+                if start_match is not None:
+                    nesting += 1
+            elif start_match is not None:
+                nesting -= 1
+                if nesting == 0:
+                    return full_source[start_match.start() : match.end()]
+
+        return ""
+
+    @property
+    def source(self):
+        template = self.origin.loader.get_template(self.origin.template_name)
+        return self.find_partial_source(template.source, self.name)
+
+    def _render(self, context):
+        return self.nodelist.render(context)
+
+    def render(self, context):
+        with context.render_context.push_state(self):
+            if context.template is None:
+                with context.bind_template(self):
+                    context.template_name = self.name
+                    return self._render(context)
+            else:
+                return self._render(context)
+
+
 def linebreak_iter(template_source):
     yield 0
     p = template_source.find("\n")
diff --git a/django/template/defaulttags.py b/django/template/defaulttags.py
index a20598152c..4cbaf852e1 100644
--- a/django/template/defaulttags.py
+++ b/django/template/defaulttags.py
@@ -12,6 +12,7 @@ from itertools import groupby
 from django.conf import settings
 from django.http import QueryDict
 from django.utils import timezone
+from django.utils.datastructures import DeferredSubDict
 from django.utils.html import conditional_escape, escape, format_html
 from django.utils.lorem_ipsum import paragraphs, words
 from django.utils.safestring import mark_safe
@@ -29,6 +30,7 @@ from .base import (
     VARIABLE_TAG_START,
     Node,
     NodeList,
+    PartialTemplate,
     TemplateSyntaxError,
     VariableDoesNotExist,
     kwarg_re,
@@ -408,6 +410,31 @@ class NowNode(Node):
             return formatted
 
 
+class PartialDefNode(Node):
+    def __init__(self, partial_name, inline, nodelist):
+        self.partial_name = partial_name
+        self.inline = inline
+        self.nodelist = nodelist
+
+    def render(self, context):
+        return self.nodelist.render(context) if self.inline else ""
+
+
+class PartialNode(Node):
+    def __init__(self, partial_name, partial_mapping):
+        # Defer lookup in `partial_mapping` and nodelist to runtime.
+        self.partial_name = partial_name
+        self.partial_mapping = partial_mapping
+
+    def render(self, context):
+        try:
+            return self.partial_mapping[self.partial_name].render(context)
+        except KeyError:
+            raise TemplateSyntaxError(
+                f"Partial '{self.partial_name}' is not defined in the current template."
+            )
+
+
 class ResetCycleNode(Node):
     def __init__(self, node):
         self.node = node
@@ -1174,6 +1201,75 @@ def now(parser, token):
     return NowNode(format_string, asvar)
 
 
+@register.tag(name="partialdef")
+def partialdef_func(parser, token):
+    """
+    Declare a partial that can be used in the template.
+
+    Usage::
+
+        {% partialdef partial_name %}
+        Content goes here.
+        {% endpartialdef %}
+
+    Store the nodelist in the context under the key "partials". It can be
+    retrieved using the ``{% partial %}`` tag.
+
+    The optional ``inline`` argument renders the partial's contents
+    immediately, at the point where it is defined.
+    """
+    match token.split_contents():
+        case "partialdef", partial_name, "inline":
+            inline = True
+        case "partialdef", partial_name, _:
+            raise TemplateSyntaxError(
+                "The 'inline' argument does not have any parameters; either use "
+                "'inline' or remove it completely."
+            )
+        case "partialdef", partial_name:
+            inline = False
+        case ["partialdef"]:
+            raise TemplateSyntaxError("'partialdef' tag requires a name")
+        case _:
+            raise TemplateSyntaxError("'partialdef' tag takes at most 2 arguments")
+
+    # Parse the content until the end tag.
+    valid_endpartials = ("endpartialdef", f"endpartialdef {partial_name}")
+    nodelist = parser.parse(valid_endpartials)
+    endpartial = parser.next_token()
+    if endpartial.contents not in valid_endpartials:
+        parser.invalid_block_tag(endpartial, "endpartialdef", valid_endpartials)
+
+    # Store the partial nodelist in the parser.extra_data attribute.
+    partials = parser.extra_data.setdefault("partials", {})
+    if partial_name in partials:
+        raise TemplateSyntaxError(
+            f"Partial '{partial_name}' is already defined in the "
+            f"'{parser.origin.name}' template."
+        )
+    partials[partial_name] = PartialTemplate(nodelist, parser.origin, partial_name)
+
+    return PartialDefNode(partial_name, inline, nodelist)
+
+
+@register.tag(name="partial")
+def partial_func(parser, token):
+    """
+    Render a partial previously declared with the ``{% partialdef %}`` tag.
+
+    Usage::
+
+        {% partial partial_name %}
+    """
+    match token.split_contents():
+        case "partial", partial_name:
+            extra_data = parser.extra_data
+            partial_mapping = DeferredSubDict(extra_data, "partials")
+            return PartialNode(partial_name, partial_mapping=partial_mapping)
+        case _:
+            raise TemplateSyntaxError("'partial' tag requires a single argument")
+
+
 @register.simple_tag(name="querystring", takes_context=True)
 def querystring(context, *args, **kwargs):
     """
diff --git a/django/template/engine.py b/django/template/engine.py
index 9882d3a16d..df5c8316b9 100644
--- a/django/template/engine.py
+++ b/django/template/engine.py
@@ -174,11 +174,31 @@ class Engine:
         Return a compiled Template object for the given template name,
         handling template inheritance recursively.
         """
+        original_name = template_name
+        try:
+            template_name, _, partial_name = template_name.partition("#")
+        except AttributeError:
+            raise TemplateDoesNotExist(original_name)
+
+        if not template_name:
+            raise TemplateDoesNotExist(original_name)
+
         template, origin = self.find_template(template_name)
         if not hasattr(template, "render"):
             # template needs to be compiled
             template = Template(template, origin, template_name, engine=self)
-        return template
+
+        if not partial_name:
+            return template
+
+        extra_data = getattr(template, "extra_data", {})
+        try:
+            partial = extra_data["partials"][partial_name]
+        except (KeyError, TypeError):
+            raise TemplateDoesNotExist(partial_name, tried=[template_name])
+        partial.engine = self
+
+        return partial
 
     def render_to_string(self, template_name, context=None):
         """
diff --git a/django/test/utils.py b/django/test/utils.py
index ea39794e1a..3661010463 100644
--- a/django/test/utils.py
+++ b/django/test/utils.py
@@ -24,7 +24,7 @@ from django.core.exceptions import ImproperlyConfigured
 from django.core.signals import request_started, setting_changed
 from django.db import DEFAULT_DB_ALIAS, connections, reset_queries
 from django.db.models.options import Options
-from django.template import Template
+from django.template import PartialTemplate, Template
 from django.test.signals import template_rendered
 from django.urls import get_script_prefix, set_script_prefix
 from django.utils.translation import deactivate
@@ -147,7 +147,9 @@ def setup_test_environment(debug=None):
     settings.EMAIL_BACKEND = "django.core.mail.backends.locmem.EmailBackend"
 
     saved_data.template_render = Template._render
+    saved_data.partial_template_render = PartialTemplate._render
     Template._render = instrumented_test_render
+    PartialTemplate._render = instrumented_test_render
 
     mail.outbox = []
 
@@ -165,6 +167,7 @@ def teardown_test_environment():
     settings.DEBUG = saved_data.debug
     settings.EMAIL_BACKEND = saved_data.email_backend
     Template._render = saved_data.template_render
+    PartialTemplate._render = saved_data.partial_template_render
 
     del _TestState.saved_data
     del mail.outbox
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]
diff --git a/docs/ref/templates/builtins.txt b/docs/ref/templates/builtins.txt
index 1eb391d8c0..464c4d83ac 100644
--- a/docs/ref/templates/builtins.txt
+++ b/docs/ref/templates/builtins.txt
@@ -957,6 +957,80 @@ output (as a string) inside a variable. This is useful if you want to use
     {% now "Y" as current_year %}
     {% blocktranslate %}Copyright {{ current_year }}{% endblocktranslate %}
 
+.. templatetag:: partial
+
+``partial``
+-----------
+
+.. versionadded:: 6.0
+
+Renders a template fragment that was defined with :ttag:`partialdef`, inserting
+the matching partial at this location.
+
+Usage:
+
+.. code-block:: html+django
+
+    {% partial partial_name %}
+
+The ``partial_name`` argument is the name of the template fragment to render.
+
+In the following example, a partial named ``button`` is defined and then
+rendered three times:
+
+.. code-block:: html+django
+
+    {% partialdef button %}
+        <button>Submit</button>
+    {% endpartialdef %}
+
+    {% partial button %}
+    {% partial button %}
+    {% partial button %}
+
+.. templatetag:: partialdef
+
+``partialdef``
+--------------
+
+.. versionadded:: 6.0
+
+Defines a reusable template fragment that can be rendered multiple times within
+the same template or accessed directly via :ref:`template loading or inclusion
+<template-partials-direct-access>`.
+
+Usage:
+
+.. code-block:: html+django
+
+    {% partialdef partial_name %}
+        {# Reusable content. #}
+    {% endpartialdef %}
+
+The ``partial_name`` argument is required and must be a valid template
+identifier.
+
+In the following example, a new fragment named ``card`` is defined:
+
+.. code-block:: html+django
+
+    {% partialdef card %}
+        <div class="card">
+            <h3>{{ title }}</h3>
+            <p>{{ content }}</p>
+        </div>
+    {% endpartialdef %}
+
+This partial can then be rendered using the :ttag:`partial` tag:
+
+.. code-block:: html+django
+
+    {% partial card %}
+    {% partial card %}
+
+To :ref:`render a fragment immediately in place  <template-partials-inline>`,
+use the ``inline`` option. The partial is still stored and can be reused later.
+
 .. templatetag:: querystring
 
 ``querystring``
diff --git a/docs/ref/templates/language.txt b/docs/ref/templates/language.txt
index 16a3682086..e8812666fa 100644
--- a/docs/ref/templates/language.txt
+++ b/docs/ref/templates/language.txt
@@ -475,6 +475,150 @@ it also defines the content that fills the hole in the *parent*. If there were
 two similarly-named :ttag:`block` tags in a template, that template's parent
 wouldn't know which one of the blocks' content to use.
 
+.. _template-partials:
+
+Template partials
+=================
+
+.. versionadded:: 6.0
+
+Template partials define reusable template fragments within a template file.
+They are self-contained components that can be rendered multiple times within
+the same template, helping to avoid repetition and maintain consistent output.
+
+Basic syntax
+------------
+
+A partial can be defined using the :ttag:`partialdef` tag:
+
+.. code-block:: html+django
+    :caption: ``authors.html``
+
+    {% partialdef user-info %}
+        <div id="user-info-{{ user.username }}">
+            <h3>{{ user.name }}</h3>
+            <p>{{ user.bio }}</p>
+        </div>
+    {% endpartialdef %}
+
+For extra readability, the name can be included in the ``{% endpartialdef %}``
+closing tag:
+
+.. code-block:: html+django
+
+    {% partialdef user-info %}
+        {# ... #}
+    {% endpartialdef user-info %}
+
+The template fragment can be rendered using the :ttag:`partial` tag:
+
+.. code-block:: html+django
+
+    {% partial user-info %}
+
+Fragment reuse
+--------------
+
+A template fragment can be reused multiple times:
+
+.. code-block:: html+django
+    :caption: ``authors.html``
+
+    {% block content %}
+        <h2>Authors</h2>
+        {% for user in authors %}
+            {% partial user-info %}
+        {% endfor %}
+
+        <h2>Editors</h2>
+        {% for user in editors %}
+            {% partial user-info %}
+        {% endfor %}
+    {% endblock %}
+
+The partial content is rendered each time the named partial is used, with the
+current template context.
+
+.. _template-partials-inline:
+
+Inline partials
+---------------
+
+A template fragment can be defined and rendered in-place using the ``inline``
+argument. This defines the partial for later reuse while also rendering it
+immediately at its definition:
+
+.. code-block:: html+django
+
+    {# Define and render immediately. #}
+    {% partialdef user-info inline %}
+        <div id="user-info-{{ user.username }}">
+            <h3>{{ user.name }}</h3>
+            <p>{{ user.bio }}</p>
+        </div>
+    {% endpartialdef %}
+
+    {# Other page content here. #}
+
+    {# Reuse later elsewhere in the template. #}
+    <section class="featured-authors">
+        <h2>Featured Authors</h2>
+        {% for user in featured %}
+            {% partial user-info %}
+        {% endfor %}
+    </section>
+
+.. _template-partials-direct-access:
+
+Accessing partials directly
+---------------------------
+
+Template fragments defined with :ttag:`partialdef` can be accessed directly via
+template loading or inclusion using the ``template.html#partial_name`` syntax.
+
+For example, using the :func:`~django.shortcuts.render` shortcut, the following
+code renders only the partial named ``user-info`` defined in the
+``authors.html`` template::
+
+    from django.contrib.auth.models import User
+    from django.shortcuts import get_object_or_404, render
+
+
+    def user_info_partial(request, user_id):
+        user = get_object_or_404(User, id=user_id)
+        return render(request, "authors.html#user-info", {"user": user})
+
+This approach is particularly useful for AJAX-style requests that update only
+specific portions of a page with the rendered template fragment.
+
+Template partials can also be included using the :ttag:`include` template tag
+with the same ``#`` directive:
+
+.. code-block:: html+django
+
+    {% include "authors.html#user-info" %}
+
+Context handling
+----------------
+
+Template partials are rendered with the current template context. They work as
+expected in loops and with context variables:
+
+.. code-block:: html+django
+
+    {% for user in users %}
+        {% partial user-info %}
+    {% endfor %}
+
+Context variables can be adjusted using the :ttag:`with` tag:
+
+.. code-block:: html+django
+
+    {% with user=featured_author %}
+        <h2>Featured Author of the Month</h2>
+        {% partial user-info %}
+    {% endwith %}
+
 .. _next section: #automatic-html-escaping
 .. _automatic-html-escaping:
 
diff --git a/docs/releases/6.0.txt b/docs/releases/6.0.txt
index 0c1a2c87eb..3e250706ec 100644
--- a/docs/releases/6.0.txt
+++ b/docs/releases/6.0.txt
@@ -91,6 +91,21 @@ Notably, the return type of the :class:`EmailMessage.message()
 previous ``SafeMIMEText`` and ``SafeMIMEMultipart`` return types, but is not an
 instance of those now-deprecated classes.
 
+Template Partials
+-----------------
+
+The :ref:`Django Template Language <template-language-intro>` now supports
+:ref:`template partials <template-partials>` , making it easier to encapsulate
+and reuse small named fragments within a template file. The new tags
+:ttag:`{% partialdef %} <partialdef>` and :ttag:`{% partial %} <partial>`
+define a partial and render it, respectively.
+
+Partials can also be referenced using the ``template_name#partial_name`` syntax
+with :func:`~django.template.Engine.get_template`,
+:func:`~django.shortcuts.render`, :ttag:`{% include %}<include>`, and other
+template-loading tools, enabling more modular and maintainable templates
+without needing to split components into separate files.
+
 Minor features
 --------------
 
diff --git a/docs/topics/templates.txt b/docs/topics/templates.txt
index df55cccb06..1b5559db5a 100644
--- a/docs/topics/templates.txt
+++ b/docs/topics/templates.txt
@@ -306,6 +306,33 @@ The ``django.template.loader`` module defines two functions to load templates.
     If you want to restrict the search to a particular template engine, pass
     the engine's :setting:`NAME <TEMPLATES-NAME>` in the ``using`` argument.
 
+    **Partial loading:**
+
+    When using the :class:`~django.template.backends.django.DjangoTemplates`
+    backend, a specific fragment from a template can be loaded by name. This
+    fragment should have been previously defined using the :ttag:`partialdef`
+    template tag::
+
+        from django.template.loader import get_template
+
+        # Load an entire template.
+        template = get_template("template.html")
+
+        # Load a specific fragment from a template.
+        partial = get_template("template.html#partial_name")
+
+    When loading a partial, the returned object behaves like a regular
+    ``Template`` but contains only the partial's content.
+
+    See :ref:`template-partials` for more information about defining and using
+    template fragments.
+
+    .. versionchanged:: 6.0
+
+        Support for loading template partials when using the
+        :class:`~django.template.backends.django.DjangoTemplates` backend was
+        added.
+
 .. function:: select_template(template_name_list, using=None)
 
     ``select_template()`` is just like ``get_template()``, except it takes a
diff --git a/tests/template_tests/syntax_tests/test_partials.py b/tests/template_tests/syntax_tests/test_partials.py
new file mode 100644
index 0000000000..a2cd3ae96a
--- /dev/null
+++ b/tests/template_tests/syntax_tests/test_partials.py
@@ -0,0 +1,652 @@
+from django.template import (
+    Context,
+    TemplateDoesNotExist,
+    TemplateSyntaxError,
+    VariableDoesNotExist,
+)
+from django.template.base import Token, TokenType
+from django.test import SimpleTestCase
+from django.views.debug import ExceptionReporter
+
+from ..utils import setup
+
+partial_templates = {
+    "partial_base.html": (
+        "<main>{% block main %}Default main content.{% endblock main %}</main>"
+    ),
+    "partial_included.html": (
+        "INCLUDED TEMPLATE START\n"
+        "{% partialdef included-partial %}\n"
+        "THIS IS CONTENT FROM THE INCLUDED PARTIAL\n"
+        "{% endpartialdef %}\n\n"
+        "Now using the partial: {% partial included-partial %}\n"
+        "INCLUDED TEMPLATE END\n"
+    ),
+}
+
+valid_partialdef_names = (
+    "dot.in.name",
+    "'space in name'",
+    "exclamation!",
+    "@at",
+    "slash/something",
+    "inline",
+    "inline-inline",
+    "INLINE" "with+plus",
+    "with&amp",
+    "with%percent",
+    "with,comma",
+    "with:colon",
+    "with;semicolon",
+    "[brackets]",
+    "(parens)",
+    "{curly}",
+)
+
+
+def gen_partial_template(name, *args, **kwargs):
+    if args or kwargs:
+        extra = " ".join((args, *("{k}={v}" for k, v in kwargs.items()))) + " "
+    else:
+        extra = ""
+    return (
+        f"{{% partialdef {name} {extra}%}}TEST with {name}!{{% endpartialdef %}}"
+        f"{{% partial {name} %}}"
+    )
+
+
+class PartialTagTests(SimpleTestCase):
+    libraries = {"bad_tag": "template_tests.templatetags.bad_tag"}
+
+    @setup({name: gen_partial_template(name) for name in valid_partialdef_names})
+    def test_valid_partialdef_names(self):
+        for template_name in valid_partialdef_names:
+            with self.subTest(template_name=template_name):
+                output = self.engine.render_to_string(template_name)
+                self.assertEqual(output, f"TEST with {template_name}!")
+
+    @setup(
+        {
+            "basic": (
+                "{% partialdef testing-name %}"
+                "HERE IS THE TEST CONTENT"
+                "{% endpartialdef %}"
+                "{% partial testing-name %}"
+            ),
+            "basic_inline": (
+                "{% partialdef testing-name inline %}"
+                "HERE IS THE TEST CONTENT"
+                "{% endpartialdef %}"
+            ),
+            "inline_inline": (
+                "{% partialdef inline inline %}"
+                "HERE IS THE TEST CONTENT"
+                "{% endpartialdef %}"
+            ),
+            "with_newlines": (
+                "{% partialdef testing-name %}\n"
+                "HERE IS THE TEST CONTENT\n"
+                "{% endpartialdef testing-name %}\n"
+                "{% partial testing-name %}"
+            ),
+        }
+    )
+    def test_basic_usage(self):
+        for template_name in (
+            "basic",
+            "basic_inline",
+            "inline_inline",
+            "with_newlines",
+        ):
+            with self.subTest(template_name=template_name):
+                output = self.engine.render_to_string(template_name)
+                self.assertEqual(output.strip(), "HERE IS THE TEST CONTENT")
+
+    @setup(
+        {
+            "inline_partial_with_context": (
+                "BEFORE\n"
+                "{% partialdef testing-name inline %}"
+                "HERE IS THE TEST CONTENT"
+                "{% endpartialdef %}\n"
+                "AFTER"
+            )
+        }
+    )
+    def test_partial_inline_only_with_before_and_after_content(self):
+        output = self.engine.render_to_string("inline_partial_with_context")
+        self.assertEqual(output.strip(), "BEFORE\nHERE IS THE TEST CONTENT\nAFTER")
+
+    @setup(
+        {
+            "inline_partial_explicit_end": (
+                "{% partialdef testing-name inline %}"
+                "HERE IS THE TEST CONTENT"
+                "{% endpartialdef testing-name %}\n"
+                "{% partial testing-name %}"
+            )
+        }
+    )
+    def test_partial_inline_and_used_once(self):
+        output = self.engine.render_to_string("inline_partial_explicit_end")
+        self.assertEqual(output, "HERE IS THE TEST CONTENT\nHERE IS THE TEST CONTENT")
+
+    @setup(
+        {
+            "inline_partial_with_usage": (
+                "BEFORE\n"
+                "{% partialdef content_snippet inline %}"
+                "HERE IS THE TEST CONTENT"
+                "{% endpartialdef %}\n"
+                "AFTER\n"
+                "{% partial content_snippet %}"
+            )
+        }
+    )
+    def test_partial_inline_and_used_once_with_before_and_after_content(self):
+        output = self.engine.render_to_string("inline_partial_with_usage")
+        self.assertEqual(
+            output.strip(),
+            "BEFORE\nHERE IS THE TEST CONTENT\nAFTER\nHERE IS THE TEST CONTENT",
+        )
+
+    @setup(
+        {
+            "partial_used_before_definition": (
+                "TEMPLATE START\n"
+                "{% partial testing-name %}\n"
+                "MIDDLE CONTENT\n"
+                "{% partialdef testing-name %}\n"
+                "THIS IS THE PARTIAL CONTENT\n"
+                "{% endpartialdef %}\n"
+                "TEMPLATE END"
+            ),
+        }
+    )
+    def test_partial_used_before_definition(self):
+        output = self.engine.render_to_string("partial_used_before_definition")
+        expected = (
+            "TEMPLATE START\n\nTHIS IS THE PARTIAL CONTENT\n\n"
+            "MIDDLE CONTENT\n\nTEMPLATE END"
+        )
+        self.assertEqual(output, expected)
+
+    @setup(
+        {
+            "partial_with_extends": (
+                "{% extends 'partial_base.html' %}"
+                "{% partialdef testing-name %}Inside Content{% endpartialdef %}"
+                "{% block main %}"
+                "Main content with {% partial testing-name %}"
+                "{% endblock %}"
+            ),
+        },
+        partial_templates,
+    )
+    def test_partial_defined_outside_main_block(self):
+        output = self.engine.render_to_string("partial_with_extends")
+        self.assertIn("<main>Main content with Inside Content</main>", output)
+
+    @setup(
+        {
+            "partial_with_extends_and_block_super": (
+                "{% extends 'partial_base.html' %}"
+                "{% partialdef testing-name %}Inside Content{% endpartialdef %}"
+                "{% block main %}{{ block.super }} "
+                "Main content with {% partial testing-name %}"
+                "{% endblock %}"
+            ),
+        },
+        partial_templates,
+    )
+    def test_partial_used_with_block_super(self):
+        output = self.engine.render_to_string("partial_with_extends_and_block_super")
+        self.assertIn(
+            "<main>Default main content. Main content with Inside Content</main>",
+            output,
+        )
+
+    @setup(
+        {
+            "partial_with_include": (
+                "MAIN TEMPLATE START\n"
+                "{% include 'partial_included.html' %}\n"
+                "MAIN TEMPLATE END"
+            )
+        },
+        partial_templates,
+    )
+    def test_partial_in_included_template(self):
+        output = self.engine.render_to_string("partial_with_include")
+        expected = (
+            "MAIN TEMPLATE START\nINCLUDED TEMPLATE START\n\n\n"
+            "Now using the partial: \n"
+            "THIS IS CONTENT FROM THE INCLUDED PARTIAL\n\n"
+            "INCLUDED TEMPLATE END\n\nMAIN TEMPLATE END"
+        )
+        self.assertEqual(output, expected)
+
+    @setup(
+        {
+            "partial_as_include_in_other_template": (
+                "MAIN TEMPLATE START\n"
+                "{% include 'partial_included.html#included-partial' %}\n"
+                "MAIN TEMPLATE END"
+            )
+        },
+        partial_templates,
+    )
+    def test_partial_as_include_in_template(self):
+        output = self.engine.render_to_string("partial_as_include_in_other_template")
+        expected = (
+            "MAIN TEMPLATE START\n\n"
+            "THIS IS CONTENT FROM THE INCLUDED PARTIAL\n\n"
+            "MAIN TEMPLATE END"
+        )
+        self.assertEqual(output, expected)
+
+    @setup(
+        {
+            "nested_simple": (
+                "{% extends 'base.html' %}"
+                "{% block content %}"
+                "This is my main page."
+                "{% partialdef outer inline %}"
+                "    It hosts a couple of partials.\n"
+                "    {% partialdef inner inline %}"
+                "        And an inner one."
+                "    {% endpartialdef inner %}"
+                "{% endpartialdef outer %}"
+                "{% endblock content %}"
+            ),
+            "use_outer": "{% include 'nested_simple#outer' %}",
+            "use_inner": "{% include 'nested_simple#inner' %}",
+        }
+    )
+    def test_nested_partials(self):
+        with self.subTest(template_name="use_outer"):
+            output = self.engine.render_to_string("use_outer")
+            self.assertEqual(
+                [line.strip() for line in output.split("\n")],
+                ["It hosts a couple of partials.", "And an inner one."],
+            )
+        with self.subTest(template_name="use_inner"):
+            output = self.engine.render_to_string("use_inner")
+            self.assertEqual(output.strip(), "And an inner one.")
+
+    @setup(
+        {
+            "partial_undefined_name": "{% partial undefined %}",
+            "partial_missing_name": "{% partial %}",
+            "partial_closing_tag": (
+                "{% partialdef testing-name %}TEST{% endpartialdef %}"
+                "{% partial testing-name %}{% endpartial %}"
+            ),
+            "partialdef_missing_name": "{% partialdef %}{% endpartialdef %}",
+            "partialdef_missing_close_tag": "{% partialdef name %}TEST",
+            "partialdef_opening_closing_name_mismatch": (
+                "{% partialdef testing-name %}TEST{% endpartialdef invalid %}"
+            ),
+            "partialdef_invalid_name": gen_partial_template("with\nnewline"),
+            "partialdef_extra_params": (
+                "{% partialdef testing-name inline extra %}TEST{% endpartialdef %}"
+            ),
+            "partialdef_duplicated_names": (
+                "{% partialdef testing-name %}TEST{% endpartialdef %}"
+                "{% partialdef testing-name %}TEST{% endpartialdef %}"
+                "{% partial testing-name %}"
+            ),
+            "partialdef_duplicated_nested_names": (
+                "{% partialdef testing-name %}"
+                "TEST"
+                "{% partialdef testing-name %}TEST{% endpartialdef %}"
+                "{% endpartialdef %}"
+                "{% partial testing-name %}"
+            ),
+        },
+    )
+    def test_basic_parse_errors(self):
+        for template_name, error_msg in (
+            (
+                "partial_undefined_name",
+                "Partial 'undefined' is not defined in the current template.",
+            ),
+            ("partial_missing_name", "'partial' tag requires a single argument"),
+            ("partial_closing_tag", "Invalid block tag on line 1: 'endpartial'"),
+            ("partialdef_missing_name", "'partialdef' tag requires a name"),
+            ("partialdef_missing_close_tag", "Unclosed tag on line 1: 'partialdef'"),
+            (
+                "partialdef_opening_closing_name_mismatch",
+                "expected 'endpartialdef' or 'endpartialdef testing-name'.",
+            ),
+            ("partialdef_invalid_name", "Invalid block tag on line 3: 'endpartialdef'"),
+            ("partialdef_extra_params", "'partialdef' tag takes at most 2 arguments"),
+            (
+                "partialdef_duplicated_names",
+                "Partial 'testing-name' is already defined in the "
+                "'partialdef_duplicated_names' template.",
+            ),
+            (
+                "partialdef_duplicated_nested_names",
+                "Partial 'testing-name' is already defined in the "
+                "'partialdef_duplicated_nested_names' template.",
+            ),
+        ):
+            with (
+                self.subTest(template_name=template_name),
+                self.assertRaisesMessage(TemplateSyntaxError, error_msg),
+            ):
+                self.engine.render_to_string(template_name)
+
+    @setup(
+        {
+            "with_params": (
+                "{% partialdef testing-name inline=true %}TEST{% endpartialdef %}"
+            ),
+            "uppercase": "{% partialdef testing-name INLINE %}TEST{% endpartialdef %}",
+        }
+    )
+    def test_partialdef_invalid_inline(self):
+        error_msg = "The 'inline' argument does not have any parameters"
+        for template_name in ("with_params", "uppercase"):
+            with (
+                self.subTest(template_name=template_name),
+                self.assertRaisesMessage(TemplateSyntaxError, error_msg),
+            ):
+                self.engine.render_to_string(template_name)
+
+    @setup(
+        {
+            "partial_broken_unclosed": (
+                "<div>Before partial</div>"
+                "{% partialdef unclosed_partial %}"
+                "<p>This partial has no closing tag</p>"
+                "<div>After partial content</div>"
+            )
+        }
+    )
+    def test_broken_partial_unclosed_exception_info(self):
+        with self.assertRaises(TemplateSyntaxError) as cm:
+            self.engine.get_template("partial_broken_unclosed")
+
+        self.assertIn("endpartialdef", str(cm.exception))
+        self.assertIn("Unclosed tag", str(cm.exception))
+
+        reporter = ExceptionReporter(None, cm.exception.__class__, cm.exception, None)
+        traceback_data = reporter.get_traceback_data()
+
+        exception_value = str(traceback_data.get("exception_value", ""))
+        self.assertIn("Unclosed tag", exception_value)
+
+    @setup(
+        {
+            "partial_with_variable_error": (
+                "<h1>Title</h1>\n"
+                "{% partialdef testing-name %}\n"
+                "<p>{{ nonexistent|default:alsonotthere }}</p>\n"
+                "{% endpartialdef %}\n"
+                "<h2>Sub Title</h2>\n"
+                "{% partial testing-name %}\n"
+            ),
+        }
+    )
+    def test_partial_runtime_exception_has_debug_info(self):
+        template = self.engine.get_template("partial_with_variable_error")
+        context = Context({})
+
+        if hasattr(self.engine, "string_if_invalid") and self.engine.string_if_invalid:
+            output = template.render(context)
+            # The variable should be replaced with INVALID
+            self.assertIn("INVALID", output)
+        else:
+            with self.assertRaises(VariableDoesNotExist) as cm:
+                template.render(context)
+
+            if self.engine.debug:
+                exc_info = cm.exception.template_debug
+
+                self.assertEqual(
+                    exc_info["during"], "{{ nonexistent|default:alsonotthere }}"
+                )
+                self.assertEqual(exc_info["line"], 3)
+                self.assertEqual(exc_info["name"], "partial_with_variable_error")
+                self.assertIn("Failed lookup", exc_info["message"])
+
+    @setup(
+        {
+            "partial_exception_info_test": (
+                "<h1>Title</h1>\n"
+                "{% partialdef testing-name %}\n"
+                "<p>Content</p>\n"
+                "{% endpartialdef %}\n"
+            ),
+        }
+    )
+    def test_partial_template_get_exception_info_delegation(self):
+        if self.engine.debug:
+            template = self.engine.get_template("partial_exception_info_test")
+
+            partial_template = template.extra_data["partials"]["testing-name"]
+
+            test_exc = Exception("Test exception")
+            token = Token(
+                token_type=TokenType.VAR,
+                contents="test",
+                position=(0, 4),
+            )
+
+            exc_info = partial_template.get_exception_info(test_exc, token)
+            self.assertIn("message", exc_info)
+            self.assertIn("line", exc_info)
+            self.assertIn("name", exc_info)
+            self.assertEqual(exc_info["name"], "partial_exception_info_test")
+            self.assertEqual(exc_info["message"], "Test exception")
+
+    @setup(
+        {
+            "partial_with_undefined_reference": (
+                "<h1>Header</h1>\n"
+                "{% partial undefined %}\n"
+                "<p>After undefined partial</p>\n"
+            ),
+        }
+    )
+    def test_undefined_partial_exception_info(self):
+        template = self.engine.get_template("partial_with_undefined_reference")
+        with self.assertRaises(TemplateSyntaxError) as cm:
+            template.render(Context())
+
+        self.assertIn("undefined", str(cm.exception))
+        self.assertIn("is not defined", str(cm.exception))
+
+        if self.engine.debug:
+            exc_debug = cm.exception.template_debug
+
+            self.assertEqual(exc_debug["during"], "{% partial undefined %}")
+            self.assertEqual(exc_debug["line"], 2)
+            self.assertEqual(exc_debug["name"], "partial_with_undefined_reference")
+            self.assertIn("undefined", exc_debug["message"])
+
+    @setup(
+        {
+            "existing_template": (
+                "<h1>Header</h1><p>This template has no partials defined</p>"
+            ),
+        }
+    )
+    def test_undefined_partial_exception_info_template_does_not_exist(self):
+        with self.assertRaises(TemplateDoesNotExist) as cm:
+            self.engine.get_template("existing_template#undefined")
+
+        self.assertIn("undefined", str(cm.exception))
+
+    @setup(
+        {
+            "partial_with_syntax_error": (
+                "<h1>Title</h1>\n"
+                "{% partialdef syntax_error_partial %}\n"
+                "    {% if user %}\n"
+                "        <p>User: {{ user.name }}</p>\n"
+                "    {% endif\n"
+                "    <p>Missing closing tag above</p>\n"
+                "{% endpartialdef %}\n"
+                "{% partial syntax_error_partial %}\n"
+            ),
+        }
+    )
+    def test_partial_with_syntax_error_exception_info(self):
+        with self.assertRaises(TemplateSyntaxError) as cm:
+            self.engine.get_template("partial_with_syntax_error")
+
+        self.assertIn("endif", str(cm.exception).lower())
+
+        if self.engine.debug:
+            exc_debug = cm.exception.template_debug
+
+            self.assertIn("endpartialdef", exc_debug["during"])
+            self.assertEqual(exc_debug["name"], "partial_with_syntax_error")
+            self.assertIn("endif", exc_debug["message"].lower())
+
+    @setup(
+        {
+            "partial_with_runtime_error": (
+                "<h1>Title</h1>\n"
+                "{% load bad_tag %}\n"
+                "{% partialdef runtime_error_partial %}\n"
+                "    <p>This will raise an error:</p>\n"
+                "    {% badsimpletag %}\n"
+                "{% endpartialdef %}\n"
+                "{% partial runtime_error_partial %}\n"
+            ),
+        }
+    )
+    def test_partial_runtime_error_exception_info(self):
+        template = self.engine.get_template("partial_with_runtime_error")
+        context = Context()
+
+        with self.assertRaises(RuntimeError) as cm:
+            template.render(context)
+
+        if self.engine.debug:
+            exc_debug = cm.exception.template_debug
+
+            self.assertIn("badsimpletag", exc_debug["during"])
+            self.assertEqual(exc_debug["line"], 5)  # Line 5 is where badsimpletag is
+            self.assertEqual(exc_debug["name"], "partial_with_runtime_error")
+            self.assertIn("bad simpletag", exc_debug["message"])
+
+    @setup(
+        {
+            "nested_partial_with_undefined_var": (
+                "<h1>Title</h1>\n"
+                "{% partialdef outer_partial %}\n"
+                '    <div class="outer">\n'
+                "        {% partialdef inner_partial %}\n"
+                "            <p>{{ undefined_var }}</p>\n"
+                "        {% endpartialdef %}\n"
+                "        {% partial inner_partial %}\n"
+                "    </div>\n"
+                "{% endpartialdef %}\n"
+                "{% partial outer_partial %}\n"
+            ),
+        }
+    )
+    def test_nested_partial_error_exception_info(self):
+        template = self.engine.get_template("nested_partial_with_undefined_var")
+        context = Context()
+        output = template.render(context)
+
+        # When string_if_invalid is set, it will show INVALID
+        # When not set, undefined variables just render as empty string
+        if hasattr(self.engine, "string_if_invalid") and self.engine.string_if_invalid:
+            self.assertIn("INVALID", output)
+        else:
+            self.assertIn("<p>", output)
+            self.assertIn("</p>", output)
+
+    @setup(
+        {
+            "parent.html": (
+                "<!DOCTYPE html>\n"
+                "<html>\n"
+                "<head>{% block title %}Default Title{% endblock %}</head>\n"
+                "<body>\n"
+                "    {% block content %}{% endblock %}\n"
+                "</body>\n"
+                "</html>\n"
+            ),
+            "child.html": (
+                "{% extends 'parent.html' %}\n"
+                "{% block content %}\n"
+                "    {% partialdef content_partial %}\n"
+                "        <p>{{ missing_variable|undefined_filter }}</p>\n"
+                "    {% endpartialdef %}\n"
+                "    {% partial content_partial %}\n"
+                "{% endblock %}\n"
+            ),
+        }
+    )
+    def test_partial_in_extended_template_error(self):
+        with self.assertRaises(TemplateSyntaxError) as cm:
+            self.engine.get_template("child.html")
+
+        self.assertIn("undefined_filter", str(cm.exception))
+
+        if self.engine.debug:
+            exc_debug = cm.exception.template_debug
+
+            self.assertIn("undefined_filter", exc_debug["during"])
+            self.assertEqual(exc_debug["name"], "child.html")
+            self.assertIn("undefined_filter", exc_debug["message"])
+
+    @setup(
+        {
+            "partial_broken_nesting": (
+                "<div>Before partial</div>\n"
+                "{% partialdef outer %}\n"
+                "{% partialdef inner %}...{% endpartialdef outer %}\n"
+                "{% endpartialdef inner %}\n"
+                "<div>After partial content</div>"
+            )
+        }
+    )
+    def test_broken_partial_nesting(self):
+        with self.assertRaises(TemplateSyntaxError) as cm:
+            self.engine.get_template("partial_broken_nesting")
+
+        self.assertIn("endpartialdef", str(cm.exception))
+        self.assertIn("Invalid block tag", str(cm.exception))
+        self.assertIn("'endpartialdef inner'", str(cm.exception))
+
+        reporter = ExceptionReporter(None, cm.exception.__class__, cm.exception, None)
+        traceback_data = reporter.get_traceback_data()
+
+        exception_value = str(traceback_data.get("exception_value", ""))
+        self.assertIn("Invalid block tag", exception_value)
+        self.assertIn("'endpartialdef inner'", str(cm.exception))
+
+    @setup(
+        {
+            "partial_broken_nesting_mixed": (
+                "<div>Before partial</div>\n"
+                "{% partialdef outer %}\n"
+                "{% partialdef inner %}...{% endpartialdef %}\n"
+                "{% endpartialdef inner %}\n"
+                "<div>After partial content</div>"
+            )
+        }
+    )
+    def test_broken_partial_nesting_mixed(self):
+        with self.assertRaises(TemplateSyntaxError) as cm:
+            self.engine.get_template("partial_broken_nesting_mixed")
+
+        self.assertIn("endpartialdef", str(cm.exception))
+        self.assertIn("Invalid block tag", str(cm.exception))
+        self.assertIn("'endpartialdef outer'", str(cm.exception))
+
+        reporter = ExceptionReporter(None, cm.exception.__class__, cm.exception, None)
+        traceback_data = reporter.get_traceback_data()
+
+        exception_value = str(traceback_data.get("exception_value", ""))
+        self.assertIn("Invalid block tag", exception_value)
+        self.assertIn("'endpartialdef outer'", str(cm.exception))
diff --git a/tests/template_tests/templates/partial_base.html b/tests/template_tests/templates/partial_base.html
new file mode 100644
index 0000000000..c23da046f5
--- /dev/null
+++ b/tests/template_tests/templates/partial_base.html
@@ -0,0 +1,2 @@
+{% block main %}
+{% endblock main %}
diff --git a/tests/template_tests/templates/partial_child.html b/tests/template_tests/templates/partial_child.html
new file mode 100644
index 0000000000..ac059dc87e
--- /dev/null
+++ b/tests/template_tests/templates/partial_child.html
@@ -0,0 +1,9 @@
+{% extends 'partial_base.html' %}
+
+{% partialdef extra-content %}
+Extra Content
+{% endpartialdef %}
+
+{% block main %}
+Main Content
+{% endblock %}
diff --git a/tests/template_tests/templates/partial_examples.html b/tests/template_tests/templates/partial_examples.html
new file mode 100644
index 0000000000..61ffff4005
--- /dev/null
+++ b/tests/template_tests/templates/partial_examples.html
@@ -0,0 +1,15 @@
+{% partialdef test-partial %}
+TEST-PARTIAL-CONTENT
+{% endpartialdef %}
+
+{% block main %}
+BEGINNING
+{% partial test-partial %}
+MIDDLE
+{% partial test-partial %}
+END
+{% endblock main %}
+
+{% partialdef inline-partial inline %}
+INLINE-CONTENT
+{% endpartialdef %}
diff --git a/tests/template_tests/test_partials.py b/tests/template_tests/test_partials.py
new file mode 100644
index 0000000000..8f1a74b2ec
--- /dev/null
+++ b/tests/template_tests/test_partials.py
@@ -0,0 +1,413 @@
+import os
+from unittest import mock
+
+from django.http import HttpResponse
+from django.template import (
+    Context,
+    Origin,
+    Template,
+    TemplateDoesNotExist,
+    TemplateSyntaxError,
+    engines,
+)
+from django.template.backends.django import DjangoTemplates
+from django.template.loader import render_to_string
+from django.test import TestCase, override_settings
+from django.urls import path, reverse
+
+engine = engines["django"]
+
+
+class PartialTagsTests(TestCase):
+
+    def test_invalid_template_name_raises_template_does_not_exist(self):
+        for template_name in [123, None, "", "#", "#name"]:
+            with (
+                self.subTest(template_name=template_name),
+                self.assertRaisesMessage(TemplateDoesNotExist, str(template_name)),
+            ):
+                engine.get_template(template_name)
+
+    def test_template_source_is_correct(self):
+        partial = engine.get_template("partial_examples.html#test-partial")
+        self.assertEqual(
+            partial.template.source,
+            "{% partialdef test-partial %}\nTEST-PARTIAL-CONTENT\n{% endpartialdef %}",
+        )
+
+    def test_template_source_inline_is_correct(self):
+        partial = engine.get_template("partial_examples.html#inline-partial")
+        self.assertEqual(
+            partial.template.source,
+            "{% partialdef inline-partial inline %}\nINLINE-CONTENT\n"
+            "{% endpartialdef %}",
+        )
+
+    def test_full_template_from_loader(self):
+        template = engine.get_template("partial_examples.html")
+        rendered = template.render({})
+
+        # Check the partial was rendered twice
+        self.assertEqual(2, rendered.count("TEST-PARTIAL-CONTENT"))
+        self.assertEqual(1, rendered.count("INLINE-CONTENT"))
+
+    def test_chained_exception_forwarded(self):
+        with self.assertRaises(TemplateDoesNotExist) as ctx:
+            engine.get_template("not_there.html#not-a-partial")
+
+        exception = ctx.exception
+        self.assertGreater(len(exception.tried), 0)
+        origin, _ = exception.tried[0]
+        self.assertEqual(origin.template_name, "not_there.html")
+
+    def test_partials_use_cached_loader_when_configured(self):
+        template_dir = os.path.join(os.path.dirname(__file__), "templates")
+        backend = DjangoTemplates(
+            {
+                "NAME": "django",
+                "DIRS": [template_dir],
+                "APP_DIRS": False,
+                "OPTIONS": {
+                    "loaders": [
+                        (
+                            "django.template.loaders.cached.Loader",
+                            ["django.template.loaders.filesystem.Loader"],
+                        ),
+                    ],
+                },
+            }
+        )
+        cached_loader = backend.engine.template_loaders[0]
+        filesystem_loader = cached_loader.loaders[0]
+
+        with mock.patch.object(
+            filesystem_loader, "get_contents", wraps=filesystem_loader.get_contents
+        ) as mock_get_contents:
+            full_template = backend.get_template("partial_examples.html")
+            self.assertIn("TEST-PARTIAL-CONTENT", full_template.render({}))
+
+            partial_template = backend.get_template(
+                "partial_examples.html#test-partial"
+            )
+            self.assertEqual(
+                "TEST-PARTIAL-CONTENT", partial_template.render({}).strip()
+            )
+
+            mock_get_contents.assert_called_once()
+
+    def test_context_available_in_response_for_partial_template(self):
+        def sample_view(request):
+            return HttpResponse(
+                render_to_string("partial_examples.html#test-partial", {"foo": "bar"})
+            )
+
+        class PartialUrls:
+            urlpatterns = [path("sample/", sample_view, name="sample-view")]
+
+        with override_settings(ROOT_URLCONF=PartialUrls):
+            response = self.client.get(reverse("sample-view"))
+
+        self.assertContains(response, "TEST-PARTIAL-CONTENT")
+        self.assertEqual(response.context.get("foo"), "bar")
+
+    def test_response_with_multiple_parts(self):
+        context = {}
+        template_partials = ["partial_child.html", "partial_child.html#extra-content"]
+
+        response_whole_content_at_once = HttpResponse(
+            "".join(
+                render_to_string(template_name, context)
+                for template_name in template_partials
+            )
+        )
+
+        response_with_multiple_writes = HttpResponse()
+        for template_name in template_partials:
+            response_with_multiple_writes.write(
+                render_to_string(template_name, context)
+            )
+
+        response_with_generator = HttpResponse(
+            render_to_string(template_name, context)
+            for template_name in template_partials
+        )
+
+        for label, response in [
+            ("response_whole_content_at_once", response_whole_content_at_once),
+            ("response_with_multiple_writes", response_with_multiple_writes),
+            ("response_with_generator", response_with_generator),
+        ]:
+            with self.subTest(response=label):
+                self.assertIn(b"Main Content", response.content)
+                self.assertIn(b"Extra Content", response.content)
+
+    def test_partial_engine_assignment_with_real_template(self):
+        template_with_partial = engine.get_template(
+            "partial_examples.html#test-partial"
+        )
+        self.assertEqual(template_with_partial.template.engine, engine.engine)
+        rendered_content = template_with_partial.render({})
+        self.assertEqual("TEST-PARTIAL-CONTENT", rendered_content.strip())
+
+
+class RobustPartialHandlingTests(TestCase):
+
+    def override_get_template(self, **kwargs):
+        class TemplateWithCustomAttrs:
+            def __init__(self, **kwargs):
+                for k, v in kwargs.items():
+                    setattr(self, k, v)
+
+            def render(self, context):
+                return "rendered content"
+
+        template = TemplateWithCustomAttrs(**kwargs)
+        origin = self.id()
+        return mock.patch.object(
+            engine.engine,
+            "find_template",
+            return_value=(template, origin),
+        )
+
+    def test_template_without_extra_data_attribute(self):
+        partial_name = "some_partial_name"
+        with (
+            self.override_get_template(),
+            self.assertRaisesMessage(TemplateDoesNotExist, partial_name),
+        ):
+            engine.get_template(f"some_template.html#{partial_name}")
+
+    def test_template_extract_extra_data_robust(self):
+        partial_name = "some_partial_name"
+        for extra_data in (
+            None,
+            0,
+            [],
+            {},
+            {"wrong-key": {}},
+            {"partials": None},
+            {"partials": {}},
+            {"partials": []},
+            {"partials": 0},
+        ):
+            with (
+                self.subTest(extra_data=extra_data),
+                self.override_get_template(extra_data=extra_data),
+                self.assertRaisesMessage(TemplateDoesNotExist, partial_name),
+            ):
+                engine.get_template(f"template.html#{partial_name}")
+
+    def test_nested_partials_rendering_with_context(self):
+        template_source = """
+        {% partialdef outer inline %}
+            Hello {{ name }}!
+            {% partialdef inner inline %}
+                Your age is {{ age }}.
+            {% endpartialdef inner %}
+            Nice to meet you.
+        {% endpartialdef outer %}
+        """
+        template = Template(template_source, origin=Origin(name="template.html"))
+
+        context = Context({"name": "Alice", "age": 25})
+        rendered = template.render(context)
+
+        self.assertIn("Hello Alice!", rendered)
+        self.assertIn("Your age is 25.", rendered)
+        self.assertIn("Nice to meet you.", rendered)
+
+
+class FindPartialSourceTests(TestCase):
+
+    def test_find_partial_source_success(self):
+        template = engine.get_template("partial_examples.html").template
+        partial_proxy = template.extra_data["partials"]["test-partial"]
+
+        expected = """{% partialdef test-partial %}
+TEST-PARTIAL-CONTENT
+{% endpartialdef %}"""
+        self.assertEqual(partial_proxy.source.strip(), expected.strip())
+
+    def test_find_partial_source_with_inline(self):
+        template = engine.get_template("partial_examples.html").template
+        partial_proxy = template.extra_data["partials"]["inline-partial"]
+
+        expected = """{% partialdef inline-partial inline %}
+INLINE-CONTENT
+{% endpartialdef %}"""
+        self.assertEqual(partial_proxy.source.strip(), expected.strip())
+
+    def test_find_partial_source_nonexistent_partial(self):
+        template = engine.get_template("partial_examples.html").template
+        partial_proxy = template.extra_data["partials"]["test-partial"]
+
+        result = partial_proxy.find_partial_source(
+            template.source, "nonexistent-partial"
+        )
+        self.assertEqual(result, "")
+
+    def test_find_partial_source_empty_partial(self):
+        template_source = "{% partialdef empty %}{% endpartialdef %}"
+        template = Template(template_source)
+        partial_proxy = template.extra_data["partials"]["empty"]
+
+        result = partial_proxy.find_partial_source(template_source, "empty")
+        self.assertEqual(result, "{% partialdef empty %}{% endpartialdef %}")
+
+    def test_find_partial_source_multiple_consecutive_partials(self):
+
+        template_source = (
+            "{% partialdef empty %}{% endpartialdef %}"
+            "{% partialdef other %}...{% endpartialdef %}"
+        )
+        template = Template(template_source)
+
+        empty_proxy = template.extra_data["partials"]["empty"]
+        other_proxy = template.extra_data["partials"]["other"]
+
+        empty_result = empty_proxy.find_partial_source(template_source, "empty")
+        self.assertEqual(empty_result, "{% partialdef empty %}{% endpartialdef %}")
+
+        other_result = other_proxy.find_partial_source(template_source, "other")
+        self.assertEqual(other_result, "{% partialdef other %}...{% endpartialdef %}")
+
+    def test_partials_with_duplicate_names(self):
+        test_cases = [
+            (
+                "nested",
+                """
+                {% partialdef duplicate %}{% partialdef duplicate %}
+                CONTENT
+                {% endpartialdef %}{% endpartialdef %}
+                """,
+            ),
+            (
+                "conditional",
+                """
+                {% if ... %}
+                  {% partialdef duplicate %}
+                  CONTENT
+                  {% endpartialdef %}
+                {% else %}
+                  {% partialdef duplicate %}
+                  OTHER-CONTENT
+                  {% endpartialdef %}
+                {% endif %}
+                """,
+            ),
+        ]
+
+        for test_name, template_source in test_cases:
+            with self.subTest(test_name=test_name):
+                with self.assertRaisesMessage(
+                    TemplateSyntaxError,
+                    "Partial 'duplicate' is already defined in the "
+                    "'template.html' template.",
+                ):
+                    Template(template_source, origin=Origin(name="template.html"))
+
+    def test_find_partial_source_supports_named_end_tag(self):
+        template_source = "{% partialdef thing %}CONTENT{% endpartialdef thing %}"
+        template = Template(template_source)
+        partial_proxy = template.extra_data["partials"]["thing"]
+
+        result = partial_proxy.find_partial_source(template_source, "thing")
+        self.assertEqual(
+            result, "{% partialdef thing %}CONTENT{% endpartialdef thing %}"
+        )
+
+    def test_find_partial_source_supports_nested_partials(self):
+        template_source = (
+            "{% partialdef outer %}"
+            "{% partialdef inner %}...{% endpartialdef %}"
+            "{% endpartialdef %}"
+        )
+        template = Template(template_source)
+
+        empty_proxy = template.extra_data["partials"]["outer"]
+        other_proxy = template.extra_data["partials"]["inner"]
+
+        outer_result = empty_proxy.find_partial_source(template_source, "outer")
+        self.assertEqual(
+            outer_result,
+            (
+                "{% partialdef outer %}{% partialdef inner %}"
+                "...{% endpartialdef %}{% endpartialdef %}"
+            ),
+        )
+
+        inner_result = other_proxy.find_partial_source(template_source, "inner")
+        self.assertEqual(inner_result, "{% partialdef inner %}...{% endpartialdef %}")
+
+    def test_find_partial_source_supports_nested_partials_and_named_end_tags(self):
+        template_source = (
+            "{% partialdef outer %}"
+            "{% partialdef inner %}...{% endpartialdef inner %}"
+            "{% endpartialdef outer %}"
+        )
+        template = Template(template_source)
+
+        empty_proxy = template.extra_data["partials"]["outer"]
+        other_proxy = template.extra_data["partials"]["inner"]
+
+        outer_result = empty_proxy.find_partial_source(template_source, "outer")
+        self.assertEqual(
+            outer_result,
+            (
+                "{% partialdef outer %}{% partialdef inner %}"
+                "...{% endpartialdef inner %}{% endpartialdef outer %}"
+            ),
+        )
+
+        inner_result = other_proxy.find_partial_source(template_source, "inner")
+        self.assertEqual(
+            inner_result, "{% partialdef inner %}...{% endpartialdef inner %}"
+        )
+
+    def test_find_partial_source_supports_nested_partials_and_mixed_end_tags_1(self):
+        template_source = (
+            "{% partialdef outer %}"
+            "{% partialdef inner %}...{% endpartialdef %}"
+            "{% endpartialdef outer %}"
+        )
+        template = Template(template_source)
+
+        empty_proxy = template.extra_data["partials"]["outer"]
+        other_proxy = template.extra_data["partials"]["inner"]
+
+        outer_result = empty_proxy.find_partial_source(template_source, "outer")
+        self.assertEqual(
+            outer_result,
+            (
+                "{% partialdef outer %}{% partialdef inner %}"
+                "...{% endpartialdef %}{% endpartialdef outer %}"
+            ),
+        )
+
+        inner_result = other_proxy.find_partial_source(template_source, "inner")
+        self.assertEqual(inner_result, "{% partialdef inner %}...{% endpartialdef %}")
+
+    def test_find_partial_source_supports_nested_partials_and_mixed_end_tags_2(self):
+        template_source = (
+            "{% partialdef outer %}"
+            "{% partialdef inner %}...{% endpartialdef inner %}"
+            "{% endpartialdef %}"
+        )
+        template = Template(template_source)
+
+        empty_proxy = template.extra_data["partials"]["outer"]
+        other_proxy = template.extra_data["partials"]["inner"]
+
+        outer_result = empty_proxy.find_partial_source(template_source, "outer")
+        self.assertEqual(
+            outer_result,
+            (
+                "{% partialdef outer %}{% partialdef inner %}"
+                "...{% endpartialdef inner %}{% endpartialdef %}"
+            ),
+        )
+
+        inner_result = other_proxy.find_partial_source(template_source, "inner")
+        self.assertEqual(
+            inner_result, "{% partialdef inner %}...{% endpartialdef inner %}"
+        )
diff --git a/tests/utils_tests/test_datastructures.py b/tests/utils_tests/test_datastructures.py
index d5d90b68fc..460ff0042e 100644
--- a/tests/utils_tests/test_datastructures.py
+++ b/tests/utils_tests/test_datastructures.py
@@ -9,6 +9,7 @@ import pickle
 from django.test import SimpleTestCase
 from django.utils.datastructures import (
     CaseInsensitiveMapping,
+    DeferredSubDict,
     DictWrapper,
     ImmutableList,
     MultiValueDict,
@@ -367,3 +368,42 @@ class CaseInsensitiveMappingTests(SimpleTestCase):
         with self.assertRaisesMessage(TypeError, msg):
             self.dict1["New Key"] = 1
         self.assertEqual(len(self.dict1), 2)
+
+
+class DeferredSubDictTests(SimpleTestCase):
+    def test_basic(self):
+        parent = {
+            "settings": {"theme": "dark", "language": "en"},
+            "config": {"enabled": True, "timeout": 30},
+        }
+        sub = DeferredSubDict(parent, "settings")
+        self.assertEqual(sub["theme"], "dark")
+        self.assertEqual(sub["language"], "en")
+        with self.assertRaises(KeyError):
+            sub["enabled"]
+
+    def test_reflects_changes_in_parent(self):
+        parent = {"settings": {"theme": "dark"}}
+        sub = DeferredSubDict(parent, "settings")
+        parent["settings"]["theme"] = "light"
+        self.assertEqual(sub["theme"], "light")
+        parent["settings"]["mode"] = "tight"
+        self.assertEqual(sub["mode"], "tight")
+
+    def test_missing_deferred_key_raises_keyerror(self):
+        parent = {"settings": {"theme": "dark"}}
+        sub = DeferredSubDict(parent, "nonexistent")
+        with self.assertRaises(KeyError):
+            sub["anything"]
+
+    def test_missing_child_key_raises_keyerror(self):
+        parent = {"settings": {"theme": "dark"}}
+        sub = DeferredSubDict(parent, "settings")
+        with self.assertRaises(KeyError):
+            sub["nonexistent"]
+
+    def test_child_not_a_dict_raises_typeerror(self):
+        parent = {"bad": "not_a_dict"}
+        sub = DeferredSubDict(parent, "bad")
+        with self.assertRaises(TypeError):
+            sub["any_key"]