8000 API / CoW: detect and raise error for chained assignment under Copy-on-Write by jorisvandenbossche · Pull Request #49467 · pandas-dev/pandas · GitHub
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API / CoW: detect and raise error for chained assignment under Copy-on-Write #49467

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Merged
phofl merged 13 commits into from
Jan 24, 2023
Merged

API / CoW: detect and raise error for chained assignment under Copy-on-Write #49467

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16e31e9
API: detect and raise error for chained assignment under Copy-on-Write
jorisvandenbossche Nov 1, 2022
2961701
skip SPSS tests for now (to further investigate)
jorisvandenbossche Nov 2, 2022
017ed3e
use helper for option
jorisvandenbossche Jan 13, 2023
e170b6c
update error message
jorisvandenbossche Jan 13, 2023
08adb25
Merge remote-tracking branch 'upstream/main' into cow-error-chained-s…
jorisvandenbossche Jan 16, 2023
8d50d16
address feedback
jorisvandenbossche Jan 16, 2023
4521d87
add whatsnew
jorisvandenbossche Jan 16, 2023
446863a
Merge remote-tracking branch 'upstream/main' into cow-error-chained-s…
jorisvandenbossche Jan 23, 2023
dc71236
add test build for CoW using PyPy
jorisvandenbossche Jan 23, 2023
085dd70
don't try to raise on PYPY + update tests for that
jorisvandenbossche Jan 23, 2023
1c3a4bb
try fix typing
jorisvandenbossche Jan 23, 2023
a7089ba
remove PyPy with CoW build again
jorisvandenbossche Jan 23, 2023
e2bb7e1
convert raises helper to function to avoid pytest import
jorisvandenbossche Jan 24, 2023
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add whatsnew
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@jorisvandenbossche
jorisvandenbossche committed Jan 16, 2023
commit 4521d87c37b5fa7c51c5b65570d50da1e747e95e
8 changes: 8 additions & 0 deletions doc/source/whatsnew/v2.0.0.rst
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Expand Up @@ -120,6 +120,14 @@ 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`)

- 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
object that is the result of an indexing operation (getitem), which under
Copy-on-Write always behaves as a copy. Thus, assigning through a chain
can never update the original Series or DataFrame. Therefore, an informative
error is raised to the user instead of silently doing nothing (:issue:`49467`)

Copy-on-Write can be enabled through

.. code-block:: python
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