Add tests for transform sum with series (#58012)

* Add tests for transform sum with series

* Refactor to seperate tests

---------

Co-authored-by: Kei <[email protected]>

Author: undermyumbrella1

pandas/tests/groupby/transform/test_transform.py

@@ -1491,3 +1491,47 @@ def test_idxmin_idxmax_transform_args(how, skipna, numeric_only):
         msg = f"DataFrameGroupBy.{how} with skipna=False encountered an NA value"
         with pytest.raises(ValueError, match=msg):
             gb.transform(how, skipna, numeric_only)
+
+
+def test_transform_sum_one_column_no_matching_labels():
+    df = DataFrame({"X": [1.0]})
+    series = Series(["Y"])
+    result = df.groupby(series, as_index=False).transform("sum")
+    expected = DataFrame({"X": [1.0]})
+    tm.assert_frame_equal(result, expected)
+
+
+def test_transform_sum_no_matching_labels():
+    df = DataFrame({"X": [1.0, -93204, 4935]})
+    series = Series(["A", "B", "C"])
+
+    result = df.groupby(series, as_index=False).transform("sum")
+    expected = DataFrame({"X": [1.0, -93204, 4935]})
+    tm.assert_frame_equal(result, expected)
+
+
+def test_transform_sum_one_column_with_matching_labels():
+    df = DataFrame({"X": [1.0, -93204, 4935]})
+    series = Series(["A", "B", "A"])
+
+    result = df.groupby(series, as_index=False).transform("sum")
+    expected = DataFrame({"X": [4936.0, -93204, 4936.0]})
+    tm.assert_frame_equal(result, expected)
+
+
+def test_transform_sum_one_column_with_missing_labels():
+    df = DataFrame({"X": [1.0, -93204, 4935]})
+    series = Series(["A", "C"])
+
+    result = df.groupby(series, as_index=False).transform("sum")
+    expected = DataFrame({"X": [1.0, -93204, np.nan]})
+    tm.assert_frame_equal(result, expected)
+
+
+def test_transform_sum_one_column_with_matching_labels_and_missing_labels():
+    df = DataFrame({"X": [1.0, -93204, 4935]})
+    series = Series(["A", "A"])
+
+    result = df.groupby(series, as_index=False).transform("sum")
+    expected = DataFrame({"X": [-93203.0, -93203.0, np.nan]})
+    tm.assert_frame_equal(result, expected)