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Fix Dataframe.apply documentation to include note regarding calling f… #33880

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Fix Dataframe.apply documentation to include note regarding calling f… #33880

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8 changes: 8 additions & 0 deletions pandas/core/frame.py
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Original file line number Diff line number Diff line change
Expand Up @@ -7204,6 +7204,14 @@ def apply(self, func, axis=0, raw=False, result_type=None, args=(), **kwds):
DataFrame.aggregate: Only perform aggregating type operations.
DataFrame.transform: Only perform transforming type operations.

Notes
-----
In the current implementation apply calls `func` twice on the
first column/row 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
column/row.

Examples
--------
>>> df = pd.DataFrame([[4, 9]] * 3, columns=['A', 'B'])
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