fix: preserve custom CookiePolicy in Session.prepare_request

src/requests/sessions.py

@@ -17,6 +17,7 @@
 from .compat import Mapping, cookielib, urljoin, urlparse
 from .cookies import (
     RequestsCookieJar,
+    _copy_cookie_jar,
     cookiejar_from_dict,
     extract_cookies_to_jar,
     merge_cookies,
@@ -472,7 +473,7 @@ def prepare_request(self, request):
 
         # Merge with session cookies
         merged_cookies = merge_cookies(
-            merge_cookies(RequestsCookieJar(), self.cookies), cookies
+            _copy_cookie_jar(self.cookies), cookies
         )
 
         # Set environment's basic authentication if not explicitly set.

tests/test_requests.py

@@ -469,6 +469,17 @@ def test_cookielib_cookiejar_on_redirect(self, httpbin):
         assert cookies["foo"] == "bar"
         assert cookies["cookie"] == "tasty"
 
+    def test_cookie_policy_preserved_on_prepare_request(self):
+        class MyCookiePolicy(cookielib.DefaultCookiePolicy):
+            pass
+
+        s = requests.Session()
+        s.cookies.set_policy(MyCookiePolicy())
+
+        req = requests.Request("GET", "https://example.com/")
+        prepared = s.prepare_request(req)
+        assert isinstance(prepared._cookies.get_policy(), MyCookiePolicy)
+
     def test_requests_in_history_are_not_overridden(self, httpbin):
         resp = requests.get(httpbin("redirect/3"))
         urls = [r.url for r in resp.history]