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API: Series and DataFrame constructors to return shallow copy (i.e. don't share index) from another Series/DataFrame #50539

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Merged
merged 7 commits into from
Feb 10, 2023
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API: Series and DataFrame constructors to return shallow copy (i.e. don't share index) from another Series/DataFrame #50539

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3c97a59
add tests
jorisvandenbossche Jan 3, 2023
1b7de43
fix implementation and add whatsnew
jorisvandenbossche Jan 3, 2023
7cf45ff
Merge remote-tracking branch 'upstream/main' into series-dataframe-co…
jorisvandenbossche Jan 20, 2023
1dfea08
Merge remote-tracking branch 'upstream/main' into series-dataframe-co…
jorisvandenbossche Jan 23, 2023
c9ca480
update note and add comment
jorisvandenbossche Jan 23, 2023
2450a94
Merge remote-tracking branch 'upstream/main' into series-dataframe-co…
jorisvandenbossche Feb 9, 2023
d355032
fixup CoW test
jorisvandenbossche Feb 10, 2023
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5 changes: 5 additions & 0 deletions doc/source/whatsnew/v2.0.0.rst
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Expand Up @@ -596,6 +596,11 @@ Other API changes
or :attr:`~DataFrame.iloc` (thus, ``df.loc[:, :]`` or ``df.iloc[:, :]``) now returns a
new DataFrame (shallow copy) instead of the original DataFrame, consistent with other
methods to get a full slice (for example ``df.loc[:]`` or ``df[:]``) (:issue:`49469`)
- The :class:`Series` and :class:`DataFrame` constructors will now return a shallow copy
(i.e. share data, but not attributes) when passed a Series and DataFrame, respectively,
and with the default of ``copy=False`` (and if no other triggers a copy). Previously,
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"and if no other triggers a copy" is there a word missing here?

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Yes, I suppose I meant "no other keyword triggers a copy", eg by passing a dtype, or by passing index/columns causing a reindex.

the new Series or DataFrame would share the index attribute (e.g. ``df.index = ...``
would also update the index of the parent or child) (:issue:`49523`)
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IIUC it is setting df.index.name = ..., not setting df.index itself?

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No, actually the index itself, see the code snippet in the issue: #49523

This is because right now the returned Series is actually sharing the same manager, and so setting df.index = .. updates the index attribute of the manager in-place, and since they share the same manager, also the other Series/DataFrame's index gets updated.

I suppose actually also after this change with doing a proper shallow copy, they still share the actual Index object, and so mutating an attribute of the index (eg df.index.name = ..) will still propagate.

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makes sense, thanks

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Just to confirm: as expected, mutating index' attributes is still propagated (since the Index object is shared, because immutable):

In [1]: s = pd.Series([1, 2, 3])
   ...: s2 = pd.Series(s)

In [2]: s2.index.name = "test!"

In [3]: s
Out[3]: 
test!
0    1
1    2
2    3
dtype: int64

We should maybe consider also "shallow copying" the index when creating a shallow copy of a Series/DataFrame, i.e. create a new Index but viewing the same data, to avoid the above. But that's a bigger change / for a separate issue.

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We should maybe consider also "shallow copying" the index when creating a shallow copy of a Series/DataFrame, i.e. create a new Index but viewing the same data, to avoid the above. But that's a bigger change / for a separate issue.

+1

- Disallow computing ``cumprod`` for :class:`Timedelta` object; previously this returned incorrect values (:issue:`50246`)
- Loading a JSON file with duplicate columns using ``read_json(orient='split')`` renames columns to avoid duplicates, as :func:`read_csv` and the other readers do (:issue:`50370`)
- :func:`to_datetime` with ``unit`` of either "Y" or "M" will now raise if a sequence contains a non-round ``float`` value, matching the ``Timestamp`` behavior (:issue:`50301`)
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2 changes: 2 additions & 0 deletions pandas/core/frame.py
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Expand Up @@ -641,6 +641,8 @@ def __init__(

if isinstance(data, DataFrame):
data = data._mgr
if not copy:
data = data.copy(deep=False)

if isinstance(data, (BlockManager, ArrayManager)):
# first check if a Manager is passed without any other arguments
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5 changes: 1 addition & 4 deletions pandas/core/series.py
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Expand Up @@ -426,10 +426,7 @@ 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
data = data._mgr.copy(deep=False)
else:
data = data.reindex(index, copy=copy)
copy = False
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11 changes: 11 additions & 0 deletions pandas/tests/frame/test_constructors.py
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Expand Up @@ -2100,6 +2100,17 @@ def test_constructor_frame_copy(self, float_frame):
assert (cop["A"] == 5).all()
assert not (float_frame["A"] == 5).all()

def test_constructor_frame_shallow_copy(self, float_frame):
# constructing a DataFrame from DataFrame with copy=False should still
# give a "shallow" copy (share data, not attributes)
# https://github.com/pandas-dev/pandas/issues/49523
orig = float_frame.copy()
cop = DataFrame(float_frame)
assert cop._mgr is not float_frame._mgr
# Overwriting index of copy doesn't change original
cop.index = np.arange(len(cop))
tm.assert_frame_equal(float_frame, orig)

def test_constructor_ndarray_copy(self, float_frame, using_array_manager):
if not using_array_manager:
df = DataFrame(float_frame.values)
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12 changes: 12 additions & 0 deletions pandas/tests/series/test_constructors.py
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Expand Up @@ -721,6 +721,18 @@ def test_constructor_limit_copies(self, index):
# we make 1 copy; this is just a smoke test here
assert s._mgr.blocks[0].values is not index

def test_constructor_shallow_copy(self):
# constructing a Series from Series with copy=False should still
# give a "shallow" copy (share data, not attributes)
# https://github.com/pandas-dev/pandas/issues/49523
s = Series([1, 2, 3])
s_orig = s.copy()
s2 = Series(s)
assert s2._mgr is not s._mgr
# Overwriting index of s2 doesn't change s
s2.index = ["a", "b", "c"]
tm.assert_series_equal(s, s_orig)

def test_constructor_pass_none(self):
s = Series(None, index=range(5))
assert s.dtype == np.float64
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