API / CoW: constructing DataFrame from DataFrame creates lazy copy

doc/source/whatsnew/v2.0.0.rst

@@ -951,6 +951,10 @@ Indexing
 - Bug in :class:`BusinessHour` would cause creation of :class:`DatetimeIndex` to fail when no opening hour was included in the index (:issue:`49835`)
 -
 
+Copy on write
+^^^^^^^^^^^^^
+- Bug in :class:`DataFrame` constructor not tracking reference if called with another :class:`DataFrame` (:issue:`50499`)
+
 Missing
 ^^^^^^^
 - Bug in :meth:`Index.equals` raising ``TypeError`` when :class:`Index` consists of tuples that contain ``NA`` (:issue:`48446`)

pandas/core/frame.py

@@ -205,6 +205,7 @@
     to_arrays,
     treat_as_nested,
 )
+from pandas.core.internals.managers import using_copy_on_write
 from pandas.core.reshape.melt import melt
 from pandas.core.series import Series
 from pandas.core.shared_docs import _shared_docs
@@ -637,6 +638,8 @@ def __init__(
 
         if isinstance(data, DataFrame):
             data = data._mgr
+            if not copy and using_copy_on_write():
+                data = data.copy(deep=False)
 
         if isinstance(data, (BlockManager, ArrayManager)):
             # first check if a Manager is passed without any other arguments

pandas/core/generic.py

@@ -5257,7 +5257,12 @@ def _reindex_with_indexers(
             # If we've made a copy once, no need to make another one
             copy = False
 
-        if (copy or copy is None) and new_data is self._mgr:
+        if (
+            (copy or copy is None)
+            and new_data is self._mgr
+            or not copy
+            and using_copy_on_write()
+        ):
             new_data = new_data.copy(deep=copy)
 
         return self._constructor(new_data).__finalize__(self)

pandas/tests/copy_view/test_constructors.py

@@ -0,0 +1,24 @@
+import numpy as np
+import pytest
+
+from pandas import DataFrame
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+@pytest.mark.parametrize("columns", [None, ["a"]])
+def test_dataframe_constructor_mgr(using_copy_on_write, columns):
+    df = DataFrame({"a": [1, 2, 3]})
+    df_orig = df.copy()
+
+    new_df = DataFrame(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/methods/test_align.py

@@ -11,6 +11,7 @@
     date_range,
 )
 import pandas._testing as tm
+from pandas.core.internals.managers import using_copy_on_write
 
 
 class TestDataFrameAlign:
@@ -45,7 +46,10 @@ def test_align_float(self, float_frame):
         assert af._mgr is not float_frame._mgr
 
         af, bf = float_frame.align(float_frame, copy=False)
-        assert af._mgr is float_frame._mgr
+        if using_copy_on_write():
+            assert af._mgr is not float_frame._mgr
+        else:
+            assert af._mgr is float_frame._mgr
 
         # axis = 0
         other = float_frame.iloc[:-5, :3]

pandas/tests/indexing/test_iloc.py

@@ -76,7 +76,9 @@ class TestiLocBaseIndependent:
         ],
     )
     @pytest.mark.parametrize("indexer", [tm.loc, tm.iloc])
-    def test_iloc_setitem_fullcol_categorical(self, indexer, key, using_array_manager):
+    def test_iloc_setitem_fullcol_categorical(
+        self, indexer, key, using_array_manager, using_copy_on_write
+    ):
         frame = DataFrame({0: range(3)}, dtype=object)
 
         cat = Categorical(["alpha", "beta", "gamma"])
@@ -90,7 +92,7 @@ def test_iloc_setitem_fullcol_categorical(self, indexer, key, using_array_manage
         indexer(df)[key, 0] = cat
 
         expected = DataFrame({0: cat}).astype(object)
-        if not using_array_manager:
+        if not using_array_manager and not using_copy_on_write:
             assert np.shares_memory(df[0].values, orig_vals)
 
         tm.assert_frame_equal(df, expected)