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Doc: Add warning to treat group chunks as immutable #19175

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Doc: Add warning to treat group chunks as immutable #19175

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6 changes: 5 additions & 1 deletion doc/source/groupby.rst
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Expand Up @@ -941,11 +941,15 @@ that is itself a series, and possibly upcast the result to a DataFrame:

.. warning::

In the current implementation apply calls func twice on the
* In the current implementation ``apply`` calls func twice on the
first group to decide whether it can take a fast or slow code
path. This can lead to unexpected behavior if func has
side-effects, as they will take effect twice for the first
group.

* For predictable results and performance, func should **not** mutate
any of the group chunks. Group chunks should be treated as immutable
since changes to a group chunk may produce unexpected results.

.. ipython:: python

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