From 1e62a6420230c512b78433265ae8003f27c6eaeb Mon Sep 17 00:00:00 2001
From: Nick Pope <nick@nickpope.me.uk>
Date: Mon, 8 Mar 2021 13:27:19 +0000
Subject: [PATCH] Refs #32528 -- Simplified Media.merge().

This avoids building up a second datastructure for the duplicate files
warning case and simply flatten and strip duplicates if that case ever
arises.
---
 django/forms/widgets.py               | 17 +++++------------
 tests/forms_tests/tests/test_media.py |  2 +-
 2 files changed, 6 insertions(+), 13 deletions(-)

diff --git a/django/forms/widgets.py b/django/forms/widgets.py
index a1c67bb863..a53569c5a3 100644
--- a/django/forms/widgets.py
+++ b/django/forms/widgets.py
@@ -12,7 +12,6 @@ from itertools import chain
 from django.forms.utils import to_current_timezone
 from django.templatetags.static import static
 from django.utils import formats
-from django.utils.datastructures import OrderedSet
 from django.utils.dates import MONTHS
 from django.utils.formats import get_format
 from django.utils.html import format_html, html_safe
@@ -152,16 +151,10 @@ class Media:
         global or in CSS you might want to override a style.
         """
         ts = TopologicalSorter()
-        all_items = OrderedSet()
-        for list_ in filter(None, lists):
-            head = list_[0]
-            # The first items depend on nothing but have to be part of the
-            # dependency graph to be included in the result.
-            ts.add(head)
-            for item in list_:
-                all_items.add(item)
-                # No self dependencies
-                if head != item:
+        for head, *tail in filter(None, lists):
+            ts.add(head)  # Ensure that the first items are included.
+            for item in tail:
+                if head != item:  # Avoid circular dependency to self.
                     ts.add(item, head)
                 head = item
         try:
@@ -173,7 +166,7 @@ class Media:
                 ),
                 MediaOrderConflictWarning,
             )
-            return list(all_items)
+            return list(dict.fromkeys(chain.from_iterable(filter(None, lists))))
 
     def __add__(self, other):
         combined = Media()
diff --git a/tests/forms_tests/tests/test_media.py b/tests/forms_tests/tests/test_media.py
index aa508cb415..061adc09ca 100644
--- a/tests/forms_tests/tests/test_media.py
+++ b/tests/forms_tests/tests/test_media.py
@@ -606,7 +606,7 @@ class FormsMediaTestCase(SimpleTestCase):
     def test_merge_warning(self):
         msg = "Detected duplicate Media files in an opposite order: [1, 2], [2, 1]"
         with self.assertWarnsMessage(RuntimeWarning, msg):
-            self.assertEqual(Media.merge([1, 2], [2, 1]), [1, 2])
+            self.assertEqual(Media.merge([1, 2], [2, 1], None), [1, 2])
 
     def test_merge_js_three_way(self):
         """