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

doc/source/whatsnew/v3.0.0.rst

@@ -846,6 +846,7 @@ Reshaping
 - Bug in :meth:`DataFrame.stack` with the new implementation where ``ValueError`` is raised when ``level=[]`` (:issue:`60740`)
 - Bug in :meth:`DataFrame.unstack` producing incorrect results when manipulating empty :class:`DataFrame` with an :class:`ExtentionDtype` (:issue:`59123`)
 - Bug in :meth:`concat` where concatenating DataFrame and Series with ``ignore_index = True`` drops the series name (:issue:`60723`, :issue:`56257`)
+- 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`)
 
 Sparse
 ^^^^^^

pandas/core/frame.py

@@ -10885,6 +10885,23 @@ def join(
                 raise ValueError("Other Series must have a name")
             other = DataFrame({other.name: other})
 
+        if on is not None:
+            if isinstance(other, Iterable) and not isinstance(
+                other, (DataFrame, Series, str, bytes, bytearray)
+            ):
+                if not all(isinstance(obj, (DataFrame, Series)) for obj in other):
+                    raise TypeError(
+                        f"Join with 'on={on}' requires a pandas DataFrame or Series, "
+                        "or an iterable of such objects as 'other'. "
+                        f"Got {type(other).__name__} with invalid elements."
+                    )
+            elif not isinstance(other, (DataFrame, Series)):
+                raise TypeError(
+                    f"Join with 'on={on}' requires a pandas DataFrame or Series as "
+                    "'other'. Got "
+                    f"{type(other).__name__} instead."
+                )
+
         if isinstance(other, DataFrame):
             if how == "cross":
                 return merge(

pandas/tests/frame/methods/test_join.py

@@ -418,6 +418,35 @@ def test_suppress_future_warning_with_sort_kw(sort):
     tm.assert_frame_equal(result, expected)
 
 
+def test_join_with_invalid_non_pandas_objects_raises_typeerror():
+    # GH#61434
+    # case - 'other' is an invalid non-pandas object
+    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"]
+
+    invalid_other = FakeOther()
+
+    with pytest.raises(TypeError, match="requires a pandas DataFrame or Series"):
+        df1.join(invalid_other, on=["Column2", "Column3"], how="inner")
+
+    # 'other' is an iterable with mixed types
+    df2 = DataFrame({"Column2": [10, 20, 30], "Column3": ["A", "B", "C"]})
+    mixed_iterable = [df2, 42]
+
+    with pytest.raises(TypeError, match="requires a pandas DataFrame or Series"):
+        df1.join(mixed_iterable, on=["Column2", "Column3"], how="inner")
+
+
 class TestDataFrameJoin:
     def test_join(self, multiindex_dataframe_random_data):
         frame = multiindex_dataframe_random_data