BUG: stack_multi_columns extension array downcast · Issue #45740 · pandas-dev/pandas (original) (raw)

Pandas version checks

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Reproducible Example

import pandas as pd

T = pd.core.arrays.integer.Int8Dtype()

df = pd.DataFrame({ ('a', 'x'): pd.Series(range(5), dtype=T), ('b', 'x'): pd.Series(range(5)), })

dtypes = df.stack().dtypes

assert dtypes['a'] == T, dtypes['a']

Issue Description

Extension dtypes (including categoricals) are down-cast to object if they are part of a multi-column data frame with more than one label on the non-stacked level(s).

The function _stack_multi_columns contains a branch for a homogenous extension dtype frame. (https://github.com/pandas-dev/pandas/blob/4f0e22c1c6d9cebed26f45a9a5b873ebbe67cd17/pandas/core/reshape/reshape.py#L744,L745)

The task of this branch is to ensure that extension dtypes are preserved by the stacking.

There’s a bug that causes this branch to not be executed.

Expected Behavior

Expectation: Extension Dtypes should be preserved under stacking.

Installed Versions

Pandas 1.4