BUG: Raise TypeError when joining with non-DataFrame using 'on=' (GH#61434)

Author: iabhi4

doc/source/whatsnew/v3.0.0.rst

@@ -880,6 +880,7 @@ Other
 - Bug in :meth:`DataFrame.apply` where passing ``engine="numba"`` ignored ``args`` passed to the applied function (:issue:`58712`)
 - Bug in :meth:`DataFrame.eval` and :meth:`DataFrame.query` which caused an exception when using NumPy attributes via ``@`` notation, e.g., ``df.eval("@np.floor(a)")``. (:issue:`58041`)
 - Bug in :meth:`DataFrame.eval` and :meth:`DataFrame.query` which did not allow to use ``tan`` function. (:issue:`55091`)
+- Bug in :meth:`DataFrame.join` where passing a non-pandas object like a ``polars.DataFrame`` with the ``on=`` parameter raised a misleading error message instead of a ``TypeError``. (:issue:`61434`)
 - Bug in :meth:`DataFrame.query` where using duplicate column names led to a ``TypeError``. (:issue:`59950`)
 - Bug in :meth:`DataFrame.query` which raised an exception or produced incorrect results when expressions contained backtick-quoted column names containing the hash character ``#``, backticks, or characters that fall outside the ASCII range (U+0001..U+007F). (:issue:`59285`) (:issue:`49633`)
 - Bug in :meth:`DataFrame.query` which raised an exception when querying integer column names using backticks. (:issue:`60494`)

pandas/core/frame.py

@@ -10885,6 +10885,12 @@ def join(
                 raise ValueError("Other Series must have a name")
             other = DataFrame({other.name: other})
 
+        if on is not None and not isinstance(other, (DataFrame, Series)):
+            raise TypeError(
+                f"Join with 'on=...' requires a pandas DataFrame or Series as 'other', "
+                f"got {type(other).__name__} instead."
+            )
+
         if isinstance(other, DataFrame):
             if how == "cross":
                 return merge(

pandas/tests/frame/methods/test_join.py

@@ -418,6 +418,29 @@ def test_suppress_future_warning_with_sort_kw(sort):
     tm.assert_frame_equal(result, expected)
 
 
+def test_join_with_non_pandas_object_raises_typeerror():
+    # GH#61434
+    df1 = DataFrame(
+        {
+            "Column2": [10, 20, 30],
+            "Column3": ["A", "B", "C"],
+            "Column4": ["Lala", "YesYes", "NoNo"],
+        }
+    )
+
+    class FakeOther:
+        def __init__(self):
+            self.Column2 = [10, 20, 30]
+            self.Column3 = ["A", "B", "C"]
+
+    other = FakeOther()
+
+    with pytest.raises(
+        TypeError, match="Join with 'on=.*' requires a pandas DataFrame or Series"
+    ):
+        df1.join(other, on=["Column2", "Column3"], how="inner")
+
+
 class TestDataFrameJoin:
     def test_join(self, multiindex_dataframe_random_data):
         frame = multiindex_dataframe_random_data