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DOC: document CSV round-trip limitation with MultiIndex columns and unnamed index (GH#56929) #65996

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DOC: document CSV round-trip limitation with MultiIndex columns and unnamed index (GH#56929) #65996

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9 changes: 9 additions & 0 deletions doc/source/user_guide/io.rst
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Expand Up @@ -1406,6 +1406,15 @@ of multi-columns indices.
with ``df.to_csv(..., index=False)``), then any ``names`` on the columns index will
be *lost*.

.. warning::
When a ``DataFrame`` with ``MultiIndex`` columns has an *unnamed* index, the row
that would hold the index names is omitted on write. If the first data row then
consists entirely of missing values, it is written identically to that omitted
row, and on read ``read_csv`` cannot tell the two apart: it consumes the first
data row as the index names, silently dropping that row. Avoid this by writing
missing values with a non-empty ``na_rep`` (e.g. ``df.to_csv(na_rep="NaN")``) or
by giving the index a name.

.. ipython:: python
:suppress:

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