API / CoW: constructing DataFrame from DataFrame/BlockManager creates lazy copy

doc/source/whatsnew/v2.0.0.rst

@@ -238,6 +238,9 @@ Copy-on-Write improvements
   a modification to the data happens) when constructing a Series from an existing
   Series with the default of ``copy=False`` (:issue:`50471`)
 
+- The :class:`DataFrame` constructor now will keep track of references when called
+  with another :class:`DataFrame` or ``BlockManager``.
+
 - Trying to set values using chained assignment (for example, ``df["a"][1:3] = 0``)
   will now always raise an exception when Copy-on-Write is enabled. In this mode,
   chained assignment can never work because we are always setting into a temporary

pandas/core/frame.py

@@ -654,6 +654,8 @@ def __init__(
             data = data._mgr
 
         if isinstance(data, (BlockManager, ArrayManager)):
+            if using_copy_on_write():
+                data = data.copy(deep=False)
             # first check if a Manager is passed without any other arguments
             # -> use fastpath (without checking Manager type)
             if index is None and columns is None and dtype is None and not copy:

pandas/core/reshape/concat.py

@@ -548,7 +548,7 @@ def __init__(
                     # mypy needs to know sample is not an NDFrame
                     sample = cast("DataFrame | Series", sample)
                     obj = sample._constructor(obj, columns=[name], copy=False)
-                    if using_copy_on_write():
+                    if isinstance(original_obj, ABCSeries) and using_copy_on_write():
                         # TODO(CoW): Remove when ref tracking in constructors works
                         for i, block in enumerate(original_obj._mgr.blocks):  # type: ignore[union-attr]  # noqa
                             obj._mgr.blocks[i].refs = block.refs  # type: ignore[union-attr]  # noqa

pandas/tests/copy_view/test_constructors.py

@@ -1,6 +1,12 @@
 import numpy as np
+import pytest
 
-from pandas import Series
+from pandas import (
+    DataFrame,
+    Series,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
 
 # -----------------------------------------------------------------------------
 # Copy/view behaviour for Series / DataFrame constructors
@@ -73,3 +79,22 @@ def test_series_from_series_with_reindex(using_copy_on_write):
     assert not np.shares_memory(ser.values, result.values)
     if using_copy_on_write:
         assert not result._mgr.blocks[0].refs.has_reference()
+
+
+@pytest.mark.parametrize("func", [lambda x: x, lambda x: x._mgr])
+@pytest.mark.parametrize("columns", [None, ["a"]])
+def test_dataframe_constructor_mgr_or_df(using_copy_on_write, columns, func):
+    df = DataFrame({"a": [1, 2, 3]})
+    df_orig = df.copy()
+
+    new_df = DataFrame(func(df))
+
+    assert np.shares_memory(get_array(df, "a"), get_array(new_df, "a"))
+    new_df.iloc[0] = 100
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), get_array(new_df, "a"))
+        tm.assert_frame_equal(df, df_orig)
+    else:
+        assert np.shares_memory(get_array(df, "a"), get_array(new_df, "a"))
+        tm.assert_frame_equal(df, new_df)

pandas/tests/frame/test_constructors.py

@@ -285,10 +285,8 @@ def test_constructor_dtype_nocast_view_dataframe(self, using_copy_on_write):
         df = DataFrame([[1, 2]])
         should_be_view = DataFrame(df, dtype=df[0].dtype)
         if using_copy_on_write:
-            # TODO(CoW) doesn't mutate original
             should_be_view.iloc[0, 0] = 99
-            # assert df.values[0, 0] == 1
-            assert df.values[0, 0] == 99
+            assert df.values[0, 0] == 1
         else:
             should_be_view[0][0] = 99
             assert df.values[0, 0] == 99