BUG: stack with empty level list (#60826)

* return early if set_levels is empty * add test * add whatsnew * check empty before make set

Author: quangngd

doc/source/whatsnew/v3.0.0.rst

@@ -766,6 +766,7 @@ Reshaping
 - Bug in :meth:`DataFrame.unstack` producing incorrect results when ``sort=False`` (:issue:`54987`, :issue:`55516`)
 - Bug in :meth:`DataFrame.merge` when merging two :class:`DataFrame` on ``intc`` or ``uintc`` types on Windows (:issue:`60091`, :issue:`58713`)
 - Bug in :meth:`DataFrame.pivot_table` incorrectly subaggregating results when called without an ``index`` argument (:issue:`58722`)
+- 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`)
 
 Sparse

pandas/core/reshape/reshape.py

@@ -929,6 +929,8 @@ def _reorder_for_extension_array_stack(
 def stack_v3(frame: DataFrame, level: list[int]) -> Series | DataFrame:
  if frame.columns.nunique() != len(frame.columns):
  raise ValueError("Columns with duplicate values are not supported in stack")
+ if not len(level):
+ return frame
  set_levels = set(level)
  stack_cols = frame.columns._drop_level_numbers(
  [k for k in range(frame.columns.nlevels - 1, -1, -1) if k not in set_levels]

pandas/tests/frame/test_stack_unstack.py

@@ -1452,6 +1452,25 @@ def test_stack_empty_frame(dropna, future_stack):
  tm.assert_series_equal(result, expected)
 
 
+@pytest.mark.filterwarnings("ignore:The previous implementation of stack is deprecated")
+@pytest.mark.parametrize("dropna", [True, False, lib.no_default])
+def test_stack_empty_level(dropna, future_stack, int_frame):
+ # GH 60740
+ if future_stack and dropna is not lib.no_default:
+ with pytest.raises(ValueError, match="dropna must be unspecified"):
+ DataFrame(dtype=np.int64).stack(dropna=dropna, future_stack=future_stack)
+ else:
+ expected = int_frame
+ result = int_frame.copy().stack(
+ level=[], dropna=dropna, future_stack=future_stack
+ )
+ tm.assert_frame_equal(result, expected)
+
+ expected = DataFrame()
+ result = DataFrame().stack(level=[], dropna=dropna, future_stack=future_stack)
+ tm.assert_frame_equal(result, expected)
+
+
 @pytest.mark.filterwarnings("ignore:The previous implementation of stack is deprecated")
 @pytest.mark.parametrize("dropna", [True, False, lib.no_default])
 @pytest.mark.parametrize("fill_value", [None, 0])