TST: Adding unit tests for named indexes resulting from empty datafra…

pandas/tests/groupby/test_groupby_shift_emptydf.py

@@ -0,0 +1,45 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    DataFrame   
+)
+
+import pandas._testing as tm
+
+def test_empty_shift_with_fill():
+    #This test is designed to replicate the extra named indexes in 41264 caused by having a fill_value parameter
+
+    empty_df = DataFrame(columns=["a", "b", "c"])
+
+    shifted = empty_df.groupby(['a']).shift(1)
+    shifted_with_fill = empty_df.groupby(['a']).shift(1, fill_value=0)
+
+    tm.assert_frame_equal(shifted, shifted_with_fill)
+    tm.assert_index_equal(shifted.index, shifted_with_fill.index)
+
+def test_multindex_empty_shift_with_fill():
+    #This test is designed to replicate the extra named indexes in 41264 caused by having a fill_value parameter for multindexes
+
+    empty_df = DataFrame(columns=["a", "b", "c"])
+
+    shifted = empty_df.groupby(['a','b']).shift(1)
+    shifted_with_fill = empty_df.groupby(['a','b']).shift(1, fill_value=0)
+
+    tm.assert_frame_equal(shifted, shifted_with_fill)
+    tm.assert_index_equal(shifted.index, shifted_with_fill.index)
+
+def test_multindex_shift_with_fill():
+    #This test is designed to replicate the extra named indexes in 41264. 
+    #Both numbered dataframes and empty dataframes should return FrozenList([None])
+
+    num_df = DataFrame({'a':[1,2,3], 'b':[4,5,6], 'c':[7,8,9]})
+    empty_df = DataFrame(columns=["a", "b", "c"])
+
+    shifted = empty_df.groupby(['a','b']).shift(1)
+    shifted_with_fill = num_df.groupby(['a','b']).shift(1, fill_value=0)
+
+    tm.assert_class_equal(shifted.index.names, shifted_with_fill.index.names)
+
+
+