API: Avoid returning same object for various methods (#51032)

Author: phofl

doc/source/whatsnew/v2.0.0.rst

@@ -624,7 +624,7 @@ Other API changes
 - Loading a JSON file with duplicate columns using ``read_json(orient='split')`` renames columns to avoid duplicates, as :func:`read_csv` and the other readers do (:issue:`50370`)
 - The levels of the index of the :class:`Series` returned from ``Series.sparse.from_coo`` now always have dtype ``int32``. Previously they had dtype ``int64`` (:issue:`50926`)
 - :func:`to_datetime` with ``unit`` of either "Y" or "M" will now raise if a sequence contains a non-round ``float`` value, matching the ``Timestamp`` behavior (:issue:`50301`)
--
+- The methods :meth:`Series.round`, :meth:`DataFrame.__invert__`, :meth:`Series.__invert__`, :meth:`DataFrame.swapaxes`, :meth:`DataFrame.first`, :meth:`DataFrame.last`, :meth:`Series.first`, :meth:`Series.last` and :meth:`DataFrame.align` will now always return new objects (:issue:`51032`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_200.deprecations:

pandas/core/frame.py

@@ -9945,7 +9945,7 @@ def _series_round(ser: Series, decimals: int):
                 concat(new_cols, axis=1), index=self.index, columns=self.columns
             ).__finalize__(self, method="round")
         else:
-            return self
+            return self.copy(deep=False)
 
     # ----------------------------------------------------------------------
     # Statistical methods, etc.

pandas/core/generic.py

@@ -780,8 +780,6 @@ def swapaxes(
         j = self._get_axis_number(axis2)
 
         if i == j:
-            if copy is False and not using_copy_on_write():
-                return self
             return self.copy(deep=copy)
 
         mapping = {i: j, j: i}
@@ -1487,7 +1485,7 @@ def blk_func(values: ArrayLike):
     def __invert__(self: NDFrameT) -> NDFrameT:
         if not self.size:
             # inv fails with 0 len
-            return self
+            return self.copy(deep=False)
 
         new_data = self._mgr.apply(operator.invert)
         return self._constructor(new_data).__finalize__(self, method="__invert__")
@@ -8900,7 +8898,7 @@ def first(self: NDFrameT, offset) -> NDFrameT:
             raise TypeError("'first' only supports a DatetimeIndex index")
 
         if len(self.index) == 0:
-            return self
+            return self.copy(deep=False)
 
         offset = to_offset(offset)
         if not isinstance(offset, Tick) and offset.is_on_offset(self.index[0]):
@@ -8973,7 +8971,7 @@ def last(self: NDFrameT, offset) -> NDFrameT:
             raise TypeError("'last' only supports a DatetimeIndex index")
 
         if len(self.index) == 0:
-            return self
+            return self.copy(deep=False)
 
         offset = to_offset(offset)
 
@@ -9481,8 +9479,6 @@ def _align_series(
         limit=None,
         fill_axis: Axis = 0,
     ):
-        uses_cow = using_copy_on_write()
-
         is_series = isinstance(self, ABCSeries)
 
         if (not is_series and axis is None) or axis not in [None, 0, 1]:
@@ -9505,10 +9501,7 @@ def _align_series(
             if is_series:
                 left = self._reindex_indexer(join_index, lidx, copy)
             elif lidx is None or join_index is None:
-                if uses_cow:
-                    left = self.copy(deep=copy)
-                else:
-                    left = self.copy(deep=copy) if copy or copy is None else self
+                left = self.copy(deep=copy)
             else:
                 left = self._constructor(
                     self._mgr.reindex_indexer(join_index, lidx, axis=1, copy=copy)
@@ -9537,10 +9530,7 @@ def _align_series(
             left = self._constructor(fdata)
 
             if ridx is None:
-                if uses_cow:
-                    right = other.copy(deep=copy)
-                else:
-                    right = other.copy(deep=copy) if copy or copy is None else other
+                right = other.copy(deep=copy)
             else:
                 right = other.reindex(join_index, level=level)
 

pandas/tests/frame/methods/test_align.py

@@ -414,3 +414,23 @@ def test_align_series_check_copy(self):
         result, other = df.align(ser, axis=1)
         ser.iloc[0] = 100
         tm.assert_series_equal(other, expected)
+
+    def test_align_identical_different_object(self):
+        # GH#51032
+        df = DataFrame({"a": [1, 2]})
+        ser = Series([3, 4])
+        result, result2 = df.align(ser, axis=0)
+        tm.assert_frame_equal(result, df)
+        tm.assert_series_equal(result2, ser)
+        assert df is not result
+        assert ser is not result2
+
+    def test_align_identical_different_object_columns(self):
+        # GH#51032
+        df = DataFrame({"a": [1, 2]})
+        ser = Series([1], index=["a"])
+        result, result2 = df.align(ser, axis=1)
+        tm.assert_frame_equal(result, df)
+        tm.assert_series_equal(result2, ser)
+        assert df is not result
+        assert ser is not result2

pandas/tests/frame/methods/test_first_and_last.py

@@ -3,6 +3,7 @@
 """
 import pytest
 
+import pandas as pd
 from pandas import (
     DataFrame,
     bdate_range,
@@ -86,3 +87,11 @@ def test_first_with_first_day_end_of_frq_n_greater_one(self, frame_or_series):
             [1] * 23, index=bdate_range("2010-03-31", "2010-04-30")
         )
         tm.assert_equal(result, expected)
+
+    @pytest.mark.parametrize("func", ["first", "last"])
+    def test_empty_not_input(self, func):
+        # GH#51032
+        df = DataFrame(index=pd.DatetimeIndex([]))
+        result = getattr(df, func)(offset=1)
+        tm.assert_frame_equal(df, result)
+        assert df is not result

pandas/tests/frame/methods/test_round.py

@@ -216,3 +216,10 @@ def test_round_interval_category_columns(self):
         result = df.round()
         expected = DataFrame([[1.0, 1.0], [0.0, 0.0]], columns=columns)
         tm.assert_frame_equal(result, expected)
+
+    def test_round_empty_not_input(self):
+        # GH#51032
+        df = DataFrame()
+        result = df.round()
+        tm.assert_frame_equal(df, result)
+        assert df is not result

pandas/tests/frame/methods/test_swapaxes.py

@@ -20,3 +20,10 @@ def test_swapaxes_invalid_axis(self):
         msg = "No axis named 2 for object type DataFrame"
         with pytest.raises(ValueError, match=msg):
             df.swapaxes(2, 5)
+
+    def test_round_empty_not_input(self):
+        # GH#51032
+        df = DataFrame({"a": [1, 2]})
+        result = df.swapaxes("index", "index")
+        tm.assert_frame_equal(df, result)
+        assert df is not result

pandas/tests/frame/test_unary.py

@@ -84,6 +84,13 @@ def test_invert_mixed(self):
         )
         tm.assert_frame_equal(result, expected)
 
+    def test_invert_empy_not_input(self):
+        # GH#51032
+        df = pd.DataFrame()
+        result = ~df
+        tm.assert_frame_equal(df, result)
+        assert df is not result
+
     @pytest.mark.parametrize(
         "df",
         [