From 6ebf8f9057893b802fa85599573886081f32edc9 Mon Sep 17 00:00:00 2001
From: Adam Chainz <me@adamj.eu>
Date: Fri, 23 Dec 2016 14:58:22 +0000
Subject: [PATCH] Refs #27624 -- Made QuerySet._prefetch_related_lookups
 immutable.

---
 django/db/models/query.py | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/django/db/models/query.py b/django/db/models/query.py
index f96a5f13ff..6a0b1ca0ac 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -165,9 +165,9 @@ class QuerySet(object):
         self._result_cache = None
         self._sticky_filter = False
         self._for_write = False
-        self._prefetch_related_lookups = []
+        self._prefetch_related_lookups = ()
         self._prefetch_done = False
-        self._known_related_objects = {}  # {rel_field, {pk: rel_obj}}
+        self._known_related_objects = {}  # {rel_field: {pk: rel_obj}}
         self._iterable_class = ModelIterable
         self._fields = None
 
@@ -863,9 +863,9 @@ class QuerySet(object):
         """
         clone = self._clone()
         if lookups == (None,):
-            clone._prefetch_related_lookups = []
+            clone._prefetch_related_lookups = ()
         else:
-            clone._prefetch_related_lookups.extend(lookups)
+            clone._prefetch_related_lookups = clone._prefetch_related_lookups + lookups
         return clone
 
     def annotate(self, *args, **kwargs):
@@ -1061,7 +1061,7 @@ class QuerySet(object):
             query.filter_is_sticky = True
         clone = self.__class__(model=self.model, query=query, using=self._db, hints=self._hints)
         clone._for_write = self._for_write
-        clone._prefetch_related_lookups = self._prefetch_related_lookups[:]
+        clone._prefetch_related_lookups = self._prefetch_related_lookups
         clone._known_related_objects = self._known_related_objects
         clone._iterable_class = self._iterable_class
         clone._fields = self._fields
@@ -1543,13 +1543,13 @@ def prefetch_one_level(instances, prefetcher, lookup, level):
     # later (happens in nested prefetch_related).
     additional_lookups = [
         copy.copy(additional_lookup) for additional_lookup
-        in getattr(rel_qs, '_prefetch_related_lookups', [])
+        in getattr(rel_qs, '_prefetch_related_lookups', ())
     ]
     if additional_lookups:
         # Don't need to clone because the manager should have given us a fresh
         # instance, so we access an internal instead of using public interface
         # for performance reasons.
-        rel_qs._prefetch_related_lookups = []
+        rel_qs._prefetch_related_lookups = ()
 
     all_related_objects = list(rel_qs)