API / CoW: constructing Series from Series creates lazy copy (with default copy=False) (#49524)

Author: jorisvandenbossche

doc/source/whatsnew/v2.0.0.rst

@@ -116,6 +116,10 @@ Copy-on-Write improvements
   returning multiple times an identical, cached Series object). This ensures that those
   Series objects correctly follow the Copy-on-Write rules (:issue:`49450`)
 
+- The :class:`Series` constructor will now create a lazy copy (deferring the copy until
+  a modification to the data happens) when constructing a Series from an existing
+  Series with the default of ``copy=False`` (:issue:`50471`)
+
 Copy-on-Write can be enabled through
 
 .. code-block:: python

pandas/core/series.py

@@ -426,10 +426,14 @@ def __init__(
         elif isinstance(data, Series):
             if index is None:
                 index = data.index
+                if using_copy_on_write():
+                    data = data._mgr.copy(deep=False)
+                else:
+                    data = data._mgr
             else:
                 data = data.reindex(index, copy=copy)
                 copy = False
-            data = data._mgr
+                data = data._mgr
         elif is_dict_like(data):
             data, index = self._init_dict(data, index, dtype)
             dtype = None

pandas/tests/copy_view/test_constructors.py

@@ -0,0 +1,75 @@
+import numpy as np
+
+from pandas import Series
+
+# -----------------------------------------------------------------------------
+# Copy/view behaviour for Series / DataFrame constructors
+
+
+def test_series_from_series(using_copy_on_write):
+    # Case: constructing a Series from another Series object follows CoW rules:
+    # a new object is returned and thus mutations are not propagated
+    ser = Series([1, 2, 3], name="name")
+
+    # default is copy=False -> new Series is a shallow copy / view of original
+    result = Series(ser)
+
+    # the shallow copy still shares memory
+    assert np.shares_memory(ser.values, result.values)
+
+    if using_copy_on_write:
+        assert result._mgr.refs is not None
+
+    if using_copy_on_write:
+        # mutating new series copy doesn't mutate original
+        result.iloc[0] = 0
+        assert ser.iloc[0] == 1
+        # mutating triggered a copy-on-write -> no longer shares memory
+        assert not np.shares_memory(ser.values, result.values)
+    else:
+        # mutating shallow copy does mutate original
+        result.iloc[0] = 0
+        assert ser.iloc[0] == 0
+        # and still shares memory
+        assert np.shares_memory(ser.values, result.values)
+
+    # the same when modifying the parent
+    result = Series(ser)
+
+    if using_copy_on_write:
+        # mutating original doesn't mutate new series
+        ser.iloc[0] = 0
+        assert result.iloc[0] == 1
+    else:
+        # mutating original does mutate shallow copy
+        ser.iloc[0] = 0
+        assert result.iloc[0] == 0
+
+
+def test_series_from_series_with_reindex(using_copy_on_write):
+    # Case: constructing a Series from another Series with specifying an index
+    # that potentially requires a reindex of the values
+    ser = Series([1, 2, 3], name="name")
+
+    # passing an index that doesn't actually require a reindex of the values
+    # -> without CoW we get an actual mutating view
+    for index in [
+        ser.index,
+        ser.index.copy(),
+        list(ser.index),
+        ser.index.rename("idx"),
+    ]:
+        result = Series(ser, index=index)
+        assert np.shares_memory(ser.values, result.values)
+        result.iloc[0] = 0
+        if using_copy_on_write:
+            assert ser.iloc[0] == 1
+        else:
+            assert ser.iloc[0] == 0
+
+    # ensure that if an actual reindex is needed, we don't have any refs
+    # (mutating the result wouldn't trigger CoW)
+    result = Series(ser, index=[0, 1, 2, 3])
+    assert not np.shares_memory(ser.values, result.values)
+    if using_copy_on_write:
+        assert result._mgr.refs is None or result._mgr.refs[0] is None

pandas/tests/copy_view/test_indexing.py

@@ -883,6 +883,3 @@ def test_dataframe_add_column_from_series():
     df.loc[2, "new"] = 100
     expected_s = Series([0, 11, 12])
     tm.assert_series_equal(s, expected_s)
-
-
-# TODO add tests for constructors