API / CoW: Copy NumPy arrays by default in DataFrame constructor

pandas/core/construction.py

@@ -18,6 +18,8 @@
 import numpy as np
 from numpy import ma
 
+from pandas._config import using_copy_on_write
+
 from pandas._libs import lib
 from pandas._libs.tslibs.period import Period
 from pandas._typing import (
@@ -762,6 +764,10 @@ def _try_cast(
 
         subarr = maybe_cast_to_integer_array(arr, dtype)
     else:
-        subarr = np.array(arr, dtype=dtype, copy=copy)
+        if using_copy_on_write():
+            # switch to same memory layout as with blk.copy()
+            subarr = np.array(arr, dtype=dtype, copy=copy, order="F")
+        else:
+            subarr = np.array(arr, dtype=dtype, copy=copy)
 
     return subarr

pandas/core/dtypes/cast.py

@@ -19,6 +19,8 @@
 
 import numpy as np
 
+from pandas._config import using_copy_on_write
+
 from pandas._libs import lib
 from pandas._libs.missing import (
     NA,
@@ -1614,11 +1616,19 @@ def maybe_cast_to_integer_array(arr: list | np.ndarray, dtype: np.dtype) -> np.n
                     "NumPy will stop allowing conversion of out-of-bound Python int",
                     DeprecationWarning,
                 )
-                casted = np.array(arr, dtype=dtype, copy=False)
+                if using_copy_on_write():
+                    # switch to same memory layout as with blk.copy()
+                    casted = np.array(arr, dtype=dtype, copy=False, order="F")
+                else:
+                    casted = np.array(arr, dtype=dtype, copy=False)
         else:
             with warnings.catch_warnings():
                 warnings.filterwarnings("ignore", category=RuntimeWarning)
-                casted = arr.astype(dtype, copy=False)
+                if using_copy_on_write():
+                    # switch to same memory layout as with blk.copy()
+                    casted = arr.astype(dtype, copy=False, order="F")
+                else:
+                    casted = arr.astype(dtype, copy=False)
     except OverflowError as err:
         raise OverflowError(
             "The elements provided in the data cannot all be "

pandas/core/frame.py

@@ -687,6 +687,10 @@ def __init__(
                 # INFO(ArrayManager) by default copy the 2D input array to get
                 # contiguous 1D arrays
                 copy = True
+            elif using_copy_on_write() and not isinstance(
+                data, (Index, DataFrame, Series)
+            ):
+                copy = True
             else:
                 copy = False
 

pandas/core/internals/construction.py

@@ -15,6 +15,8 @@
 import numpy as np
 from numpy import ma
 
+from pandas._config import using_copy_on_write
+
 from pandas._libs import lib
 
 from pandas.core.dtypes.cast import (
@@ -291,6 +293,16 @@ def ndarray_to_mgr(
         if values.ndim == 1:
             values = values.reshape(-1, 1)
 
+    elif (
+        using_copy_on_write()
+        and isinstance(values, np.ndarray)
+        and (dtype is None or is_dtype_equal(values.dtype, dtype))
+        and copy_on_sanitize
+    ):
+        # switch to same memory layout as with blk.copy()
+        values = np.array(values, order="F", copy=copy_on_sanitize)
+        values = _ensure_2d(values)
+
     elif isinstance(values, (np.ndarray, ExtensionArray, ABCSeries, Index)):
         # drop subclass info
         values = np.array(values, copy=copy_on_sanitize)

pandas/tests/frame/methods/test_fillna.py

@@ -47,8 +47,9 @@ def test_fillna_dict_inplace_nonunique_columns(self, using_copy_on_write):
     def test_fillna_on_column_view(self, using_copy_on_write):
         # GH#46149 avoid unnecessary copies
         arr = np.full((40, 50), np.nan)
-        df = DataFrame(arr)
+        df = DataFrame(arr, copy=False)
 
+        # TODO(CoW): This should raise a chained assignment error
         df[0].fillna(-1, inplace=True)
         if using_copy_on_write:
             assert np.isnan(arr[:, 0]).all()

pandas/tests/frame/methods/test_to_numpy.py

@@ -23,11 +23,15 @@ def test_to_numpy_dtype(self):
         tm.assert_numpy_array_equal(result, expected)
 
     @td.skip_array_manager_invalid_test
-    def test_to_numpy_copy(self):
+    def test_to_numpy_copy(self, using_copy_on_write):
         arr = np.random.randn(4, 3)
         df = DataFrame(arr)
-        assert df.values.base is arr
-        assert df.to_numpy(copy=False).base is arr
+        if using_copy_on_write:
+            assert df.values.base is not arr
+            assert df.to_numpy(copy=False).base is not arr
+        else:
+            assert df.values.base is arr
+            assert df.to_numpy(copy=False).base is arr
         assert df.to_numpy(copy=True).base is not arr
 
     def test_to_numpy_mixed_dtype_to_str(self):

pandas/tests/frame/methods/test_transpose.py

@@ -119,7 +119,7 @@ def test_transpose_get_view(self, float_frame):
         assert (float_frame.values[5:10] == 5).all()
 
     @td.skip_array_manager_invalid_test
-    def test_transpose_get_view_dt64tzget_view(self):
+    def test_transpose_get_view_dt64tzget_view(self, using_copy_on_write):
         dti = date_range("2016-01-01", periods=6, tz="US/Pacific")
         arr = dti._data.reshape(3, 2)
         df = DataFrame(arr)
@@ -129,7 +129,10 @@ def test_transpose_get_view_dt64tzget_view(self):
         assert result._mgr.nblocks == 1
 
         rtrip = result._mgr.blocks[0].values
-        assert np.shares_memory(arr._ndarray, rtrip._ndarray)
+        if using_copy_on_write:
+            assert not np.shares_memory(arr._ndarray, rtrip._ndarray)
+        else:
+            assert np.shares_memory(arr._ndarray, rtrip._ndarray)
 
     def test_transpose_not_inferring_dt(self):
         # GH#51546

pandas/tests/frame/methods/test_values.py

@@ -225,29 +225,35 @@ def test_values_lcd(self, mixed_float_frame, mixed_int_frame):
 
 class TestPrivateValues:
     @td.skip_array_manager_invalid_test
-    def test_private_values_dt64tz(self):
+    def test_private_values_dt64tz(self, using_copy_on_write):
         dta = date_range("2000", periods=4, tz="US/Central")._data.reshape(-1, 1)
 
         df = DataFrame(dta, columns=["A"])
         tm.assert_equal(df._values, dta)
 
-        # we have a view
-        assert np.shares_memory(df._values._ndarray, dta._ndarray)
+        if using_copy_on_write:
+            assert not np.shares_memory(df._values._ndarray, dta._ndarray)
+        else:
+            # we have a view
+            assert np.shares_memory(df._values._ndarray, dta._ndarray)
 
         # TimedeltaArray
         tda = dta - dta
         df2 = df - df
         tm.assert_equal(df2._values, tda)
 
     @td.skip_array_manager_invalid_test
-    def test_private_values_dt64tz_multicol(self):
+    def test_private_values_dt64tz_multicol(self, using_copy_on_write):
         dta = date_range("2000", periods=8, tz="US/Central")._data.reshape(-1, 2)
 
         df = DataFrame(dta, columns=["A", "B"])
         tm.assert_equal(df._values, dta)
 
-        # we have a view
-        assert np.shares_memory(df._values._ndarray, dta._ndarray)
+        if using_copy_on_write:
+            assert not np.shares_memory(df._values._ndarray, dta._ndarray)
+        else:
+            # we have a view
+            assert np.shares_memory(df._values._ndarray, dta._ndarray)
 
         # TimedeltaArray
         tda = dta - dta

pandas/tests/frame/test_constructors.py

@@ -306,18 +306,24 @@ def test_constructor_dtype_nocast_view_2d_array(
             assert df2._mgr.arrays[0].flags.c_contiguous
 
     @td.skip_array_manager_invalid_test
-    def test_1d_object_array_does_not_copy(self):
+    def test_1d_object_array_does_not_copy(self, using_copy_on_write):
         # https://github.com/pandas-dev/pandas/issues/39272
         arr = np.array(["a", "b"], dtype="object")
         df = DataFrame(arr)
-        assert np.shares_memory(df.values, arr)
+        if using_copy_on_write:
+            assert not np.shares_memory(df.values, arr)
+        else:
+            assert np.shares_memory(df.values, arr)
 
     @td.skip_array_manager_invalid_test
-    def test_2d_object_array_does_not_copy(self):
+    def test_2d_object_array_does_not_copy(self, using_copy_on_write):
         # https://github.com/pandas-dev/pandas/issues/39272
         arr = np.array([["a", "b"], ["c", "d"]], dtype="object")
         df = DataFrame(arr)
-        assert np.shares_memory(df.values, arr)
+        if using_copy_on_write:
+            assert not np.shares_memory(df.values, arr)
+        else:
+            assert np.shares_memory(df.values, arr)
 
     def test_constructor_dtype_list_data(self):
         df = DataFrame([[1, "2"], [None, "a"]], dtype=object)
@@ -2107,13 +2113,18 @@ def test_constructor_frame_shallow_copy(self, float_frame):
         cop.index = np.arange(len(cop))
         tm.assert_frame_equal(float_frame, orig)
 
-    def test_constructor_ndarray_copy(self, float_frame, using_array_manager):
+    def test_constructor_ndarray_copy(
+        self, float_frame, using_array_manager, using_copy_on_write
+    ):
         if not using_array_manager:
             arr = float_frame.values.copy()
             df = DataFrame(arr)
 
             arr[5] = 5
-            assert (df.values[5] == 5).all()
+            if using_copy_on_write:
+                assert not (df.values[5] == 5).all()
+            else:
+                assert (df.values[5] == 5).all()
 
             df = DataFrame(arr, copy=True)
             arr[6] = 6

pandas/tests/indexing/multiindex/test_setitem.py

@@ -482,12 +482,17 @@ def test_setitem_new_column_all_na(self):
 
 @td.skip_array_manager_invalid_test  # df["foo"] select multiple columns -> .values
 # is not a view
-def test_frame_setitem_view_direct(multiindex_dataframe_random_data):
+def test_frame_setitem_view_direct(
+    multiindex_dataframe_random_data, using_copy_on_write
+):
     # this works because we are modifying the underlying array
     # really a no-no
     df = multiindex_dataframe_random_data.T
     df["foo"].values[:] = 0
-    assert (df["foo"].values == 0).all()
+    if using_copy_on_write:
+        assert not (df["foo"].values == 0).all()
+    else:
+        assert (df["foo"].values == 0).all()
 
 
 def test_frame_setitem_copy_raises(