Fix merge

Author: phofl

doc/source/whatsnew/v2.0.0.rst

@@ -223,6 +223,7 @@ Copy-on-Write improvements
   - :meth:`DataFrame.to_period` / :meth:`Series.to_period`
   - :meth:`DataFrame.truncate`
   - :meth:`DataFrame.tz_convert` / :meth:`Series.tz_localize`
+  - :meth:`DataFrame.fillna` / :meth:`Series.fillna`
   - :meth:`DataFrame.interpolate` / :meth:`Series.interpolate`
   - :meth:`DataFrame.ffill` / :meth:`Series.ffill`
   - :meth:`DataFrame.bfill` / :meth:`Series.bfill`

pandas/tests/copy_view/test_interp_fillna.py

@@ -3,9 +3,11 @@
 
 from pandas import (
     DataFrame,
+    Interval,
     NaT,
     Series,
     Timestamp,
+    interval_range,
 )
 import pandas._testing as tm
 from pandas.tests.copy_view.util import get_array
@@ -162,3 +164,71 @@ def test_interpolate_downcast_reference_triggers_copy(using_copy_on_write):
         tm.assert_frame_equal(df_orig, view)
     else:
         tm.assert_frame_equal(df, view)
+
+
+def test_fillna(using_copy_on_write):
+    df = DataFrame({"a": [1.5, np.nan], "b": 1})
+    df_orig = df.copy()
+
+    df2 = df.fillna(5.5)
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+    else:
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+
+    df2.iloc[0, 1] = 100
+    tm.assert_frame_equal(df_orig, df)
+
+
+@pytest.mark.parametrize("downcast", [None, False])
+def test_fillna_inplace(using_copy_on_write, downcast):
+    df = DataFrame({"a": [1.5, np.nan], "b": 1})
+    arr_a = get_array(df, "a")
+    arr_b = get_array(df, "b")
+
+    df.fillna(5.5, inplace=True, downcast=downcast)
+    assert np.shares_memory(get_array(df, "a"), arr_a)
+    assert np.shares_memory(get_array(df, "b"), arr_b)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+        assert df._mgr._has_no_reference(1)
+
+
+def test_fillna_inplace_reference(using_copy_on_write):
+    df = DataFrame({"a": [1.5, np.nan], "b": 1})
+    df_orig = df.copy()
+    arr_a = get_array(df, "a")
+    arr_b = get_array(df, "b")
+    view = df[:]
+
+    df.fillna(5.5, inplace=True)
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), arr_a)
+        assert np.shares_memory(get_array(df, "b"), arr_b)
+        assert view._mgr._has_no_reference(0)
+        assert df._mgr._has_no_reference(0)
+        tm.assert_frame_equal(view, df_orig)
+    else:
+        assert np.shares_memory(get_array(df, "a"), arr_a)
+        assert np.shares_memory(get_array(df, "b"), arr_b)
+    expected = DataFrame({"a": [1.5, 5.5], "b": 1})
+    tm.assert_frame_equal(df, expected)
+
+
+def test_fillna_interval_inplace_reference(using_copy_on_write):
+    ser = Series(interval_range(start=0, end=5), name="a")
+    ser.iloc[1] = np.nan
+
+    ser_orig = ser.copy()
+    view = ser[:]
+    ser.fillna(value=Interval(left=0, right=5), inplace=True)
+
+    if using_copy_on_write:
+        assert not np.shares_memory(
+            get_array(ser, "a").left.values, get_array(view, "a").left.values
+        )
+        tm.assert_series_equal(view, ser_orig)
+    else:
+        assert np.shares_memory(
+            get_array(ser, "a").left.values, get_array(view, "a").left.values
+        )