Stop silently accepting obsolete slaves and is_slave connection arguments

docs/changelog.rst

@@ -29,6 +29,7 @@ Changes in 1.0.0
 - Fixed stacklevel of many warnings (to point places emitting the warning more accurately)
 - Add support for collation/hint/comment to delete/update and aggregate #2842
 - BREAKING CHANGE: Remove LongField as it's equivalent to IntField since we drop support to Python2 long time ago (User should simply switch to IntField) #2309
+- BREAKING CHANGE: The obsolete ``slaves`` and ``is_slave`` connection options were silently ignored since 2014 and will now raise ``ConnectionFailure`` if provided #2920.
 - BugFix - Calling .clear on a ListField wasn't being marked as changed (and flushed to db upon .save()) #2858
 - Improve error message in case a document assigned to a ReferenceField wasn't saved yet #1955
 - BugFix - Take `where()` into account when using `.modify()`, as in MyDocument.objects().where("this[field] >= this[otherfield]").modify(field='new') #2044

mongoengine/connection.py

@@ -203,10 +203,6 @@ def _get_connection_settings(
             resolved_hosts.append(entity)
     conn_settings["host"] = resolved_hosts
 
-    # Deprecated parameters that should not be passed on
-    kwargs.pop("slaves", None)
-    kwargs.pop("is_slave", None)
-
     keys = {
         key.lower() for key in kwargs.keys()
     }  # pymongo options are case insensitive

tests/test_connection.py

@@ -173,6 +173,17 @@ def test_connect_fails_if_connect_2_times_with_custom_alias(self):
             == str(exc_info.value)
         )
 
+    def test_connect__obsolete_slave_options__raises_connection_failure(self):
+        obsolete_options = ({"slaves": []}, {"is_slave": True})
+
+        for options in obsolete_options:
+            option_name = next(iter(options))
+            with self.subTest(options=options), pytest.raises(
+                ConnectionFailure
+            ) as exc_info:
+                connect(alias=random_str(), **options)
+            assert option_name in str(exc_info.value)
+
     def test_connect_fails_if_similar_connection_settings_arent_defined_the_same_way(
         self,
     ):