BUG: Fix bug in SeriesGroupBy.value_counts when DataFrame has one row (#42618)

Author: neelmraman

pandas/core/groupby/generic.py

@@ -758,7 +758,7 @@ def apply_series_value_counts():
         # new values are where sorted labels change
         lchanges = llab(lab, slice(1, None)) != llab(lab, slice(None, -1))
         inc = np.r_[True, lchanges]
-        if not len(lchanges):
+        if not len(val):
             inc = lchanges
         inc[idx] = True  # group boundaries are also new values
         out = np.diff(np.nonzero(np.r_[inc, True])[0])  # value counts

pandas/tests/groupby/test_value_counts.py

@@ -143,6 +143,25 @@ def test_series_groupby_value_counts_empty():
     tm.assert_series_equal(result, expected)
 
 
+def test_series_groupby_value_counts_one_row():
+    # GH42618
+    df = DataFrame([[1, 2]], columns=["A", "B"])
+    dfg = df.groupby("A")
+
+    result = dfg["B"].value_counts()
+    expected = df.value_counts()
+
+    tm.assert_series_equal(result, expected, check_names=False)
+
+    df = DataFrame([[1, 2, 3]], columns=["A", "B", "C"])
+    dfg = df.groupby(["A", "B"])
+
+    result = dfg["C"].value_counts()
+    expected = df.value_counts()
+
+    tm.assert_series_equal(result, expected, check_names=False)
+
+
 def test_series_groupby_value_counts_on_categorical():
     # GH38672