TST: Test nLargest with MI grouper (#29423)

Closes #21411

Author: gfyoung

pandas/tests/groupby/test_function.py

@@ -607,6 +607,51 @@ def test_nlargest():
     tm.assert_series_equal(gb.nlargest(3, keep="last"), e)
 
 
+def test_nlargest_mi_grouper():
+    # see gh-21411
+    npr = np.random.RandomState(123456789)
+
+    dts = date_range("20180101", periods=10)
+    iterables = [dts, ["one", "two"]]
+
+    idx = MultiIndex.from_product(iterables, names=["first", "second"])
+    s = Series(npr.randn(20), index=idx)
+
+    result = s.groupby("first").nlargest(1)
+
+    exp_idx = MultiIndex.from_tuples(
+        [
+            (dts[0], dts[0], "one"),
+            (dts[1], dts[1], "one"),
+            (dts[2], dts[2], "one"),
+            (dts[3], dts[3], "two"),
+            (dts[4], dts[4], "one"),
+            (dts[5], dts[5], "one"),
+            (dts[6], dts[6], "one"),
+            (dts[7], dts[7], "one"),
+            (dts[8], dts[8], "two"),
+            (dts[9], dts[9], "one"),
+        ],
+        names=["first", "first", "second"],
+    )
+
+    exp_values = [
+        2.2129019979039612,
+        1.8417114045748335,
+        0.858963679564603,
+        1.3759151378258088,
+        0.9430284594687134,
+        0.5296914208183142,
+        0.8318045593815487,
+        -0.8476703342910327,
+        0.3804446884133735,
+        -0.8028845810770998,
+    ]
+
+    expected = Series(exp_values, index=exp_idx)
+    tm.assert_series_equal(result, expected, check_exact=False, check_less_precise=True)
+
+
 def test_nsmallest():
     a = Series([1, 3, 5, 7, 2, 9, 0, 4, 6, 10])
     b = Series(list("a" * 5 + "b" * 5))