Fix --skip-existing for twine (#3522)

Author: di

tests/unit/forklift/test_legacy.py

@@ -1835,10 +1835,7 @@ def test_upload_fails_with_existing_filename_diff_content(self,
             pretend.call('help', _anchor='file-name-reuse')
         ]
         assert resp.status_code == 400
-        assert resp.status == (
-            "400 The filename or contents already exist. "
-            "See /the/help/url/"
-        )
+        assert resp.status == "400 File already exists. See /the/help/url/"
 
     def test_upload_fails_with_diff_filename_same_blake2(self,
                                                          pyramid_config,
@@ -1897,10 +1894,7 @@ def test_upload_fails_with_diff_filename_same_blake2(self,
             pretend.call('help', _anchor='file-name-reuse')
         ]
         assert resp.status_code == 400
-        assert resp.status == (
-            "400 The filename or contents already exist. "
-            "See /the/help/url/"
-        )
+        assert resp.status == "400 File already exists. See /the/help/url/"
 
     def test_upload_fails_with_wrong_filename(self, pyramid_config,
                                               db_request):

warehouse/forklift/legacy.py

@@ -1097,11 +1097,16 @@ def file_upload(request):
             return Response()
         elif is_duplicate is not None:
             raise _exc_with_message(
-                HTTPBadRequest, "The filename or contents already exist. "
-                                "See " +
-                                request.route_url(
-                                    'help', _anchor='file-name-reuse'
-                                )
+                HTTPBadRequest,
+                # Note: Changing this error message to something that doesn't
+                # start with "File already exists" will break the
+                # --skip-existing functionality in twine
+                # ref: https://github.com/pypa/warehouse/issues/3482
+                # ref: https://github.com/pypa/twine/issues/332
+                "File already exists. See " +
+                request.route_url(
+                    'help', _anchor='file-name-reuse'
+                )
             )
 
         # Check to see if the file that was uploaded exists in our filename log