Addressed PR comments and added a test.

py/server/deephaven/table.py

@@ -3596,14 +3596,16 @@ def __init__(self, table: Table, on: Union[str, Sequence[str]], joins: Union[str
 
 
 class MultiJoinTable(JObjectWrapper):
-    """A MultiJoinTable represents the result of a multi-table natural join. """
+    """A MultiJoinTable is an object that contains the result of a multi-table natural join. To retrieve the underlying
+    result Table, use the table() method. """
     j_object_type = _JMultiJoinTable
 
     @property
     def j_object(self) -> jpy.JType:
         return self.j_multijointable
 
     def table(self) -> Table:
+        """Returns the Table containing the multi-table natural join output. """
         return Table(j_table=self.j_multijointable.table())
 
     def __init__(self, input: Union[Table, Sequence[Table], MultiJoinInput, Sequence[MultiJoinInput]],
@@ -3614,8 +3616,8 @@ def __init__(self, input: Union[Table, Sequence[Table], MultiJoinInput, Sequence
             input (Union[Table, Sequence[Table], MultiJoinInput, Sequence[MultiJoinInput]]): the input objects
                 specifying the tables and columns to include in the join.
             on (Union[str, Sequence[str]], optional): the column(s) to match, can be a common name or an equality
-                expression that matches every input table, i.e. "col_a = col_b" to rename output column names.  When
-                using MultiJoinInput objects, this parameter is ignored.
+                expression that matches every input table, i.e. "col_a = col_b" to rename output column names. Note:
+                When MultiJoinInput objects are supplied, this parameter must be omitted.
 
         Raises:
             DHError
@@ -3627,6 +3629,8 @@ def __init__(self, input: Union[Table, Sequence[Table], MultiJoinInput, Sequence
                     j_tables = to_sequence(input)
                     self.j_multijointable = _JMultiJoinFactory.of(on, *j_tables)
             elif isinstance(input, MultiJoinInput) or (isinstance(input, Sequence) and all(isinstance(ji, MultiJoinInput) for ji in input)):
+                if on is not None:
+                    raise DHError(message="on parameter is not permitted when MultiJoinInput objects are provided.")
                 wrapped_input = to_sequence(input, wrapped=True)
                 tables = [ji.table for ji in wrapped_input]
                 with auto_locking_ctx(*tables):
@@ -3650,8 +3654,8 @@ def multi_join(input: Union[Table, Sequence[Table], MultiJoinInput, Sequence[Mul
         input (Union[Table, Sequence[Table], MultiJoinInput, Sequence[MultiJoinInput]]): the input objects specifying the
             tables and columns to include in the join.
         on (Union[str, Sequence[str]], optional): the column(s) to match, can be a common name or an equality expression
-            that matches every input table, i.e. "col_a = col_b" to rename output column names.  When using MultiJoinInput
-            objects, this parameter is ignored.
+            that matches every input table, i.e. "col_a = col_b" to rename output column names. Note: When
+            MultiJoinInput objects are supplied, this parameter must be omitted.
 
     Returns:
         MultiJoinTable: the result of the multi-table natural join operation. To access the underlying Table, use the

py/server/tests/test_multijoin.py

@@ -4,7 +4,7 @@
 
 import unittest
 
-from deephaven import read_csv, time_table, update_graph
+from deephaven import read_csv, time_table, update_graph, DHError
 from deephaven.table import MultiJoinInput, MultiJoinTable, multi_join
 from tests.testbase import BaseTestCase
 from deephaven.execution_context import get_exec_ctx
@@ -115,6 +115,15 @@ def test_ticking(self):
         with update_graph.exclusive_lock(self.test_update_graph):
             self.assertEqual(mj_table.table().size, self.ticking_tableA.size)
 
+    def test_errors(self):
+        # Assert the exception is raised when providing MultiJoinInput and the on parameter is not None (omitted).
+        mj_input = [
+            MultiJoinInput(table=self.ticking_tableA, on=["key1=a","key2=b"], joins=["c1","e1"]),
+            MultiJoinInput(table=self.ticking_tableB, on=["key1=a","key2=b"], joins=["d2"])
+        ]
+        with self.assertRaises(DHError):
+            mj_table = multi_join(mj_input, on=["key1=a","key2=b"])
+
 
 if __name__ == '__main__':
     unittest.main()