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DOC: add warning section in indexing docs. #13060

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DOC: add warning section in indexing docs. #13060

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Add warning section in indexing docs. Close #12947.
zhangxiangnick May 3, 2016
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Add warning section in indexing docs.
zhangxiangnick May 3, 2016
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23 changes: 23 additions & 0 deletions doc/source/indexing.rst
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Expand Up @@ -191,6 +191,29 @@ raised. Multiple columns can also be set in this manner:
You may find this useful for applying a transform (in-place) to a subset of the
columns.

.. warning::

pandas aligns index for setting a ``DataFrame`` from ``.loc``, ``.iloc`` and ``.ix``.
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aligns for when setting (not specific to DataFrame)

For example, the expression

.. code-block:: python

df.loc[:,['B', 'A']] = df.loc[:,['A', 'B']]

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I would show the rhs here (as well)

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What do you mean by "show here"?

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I would show df.loc[:, ['A', 'B'] (e.g. what you are assigning)

will **not** modify ``df`` because index alignment is before value assignment.

The correct way is to use raw values, and so this will work

.. code-block:: python

df.loc[:,['B', 'A']] = df.loc[:,['A', 'B']].values

This will also work because pandas ignores index alignment in column assignment

.. code-block:: python

df[['B', 'A']] = df.loc[:,['A', 'B']]

Attribute Access
----------------

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