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[ENH] Auto-convert categorical columns to string in attributes_arff_from_df #1490

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[ENH] Auto-convert categorical columns to string in attributes_arff_from_df #1490

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859c05e
[ENH] Auto-convert categorical columns to string in attributes_arff_f…
alphaleporus Nov 20, 2025
1ce5d02
Merge branch 'main' into enh/auto-convert-categories
alphaleporus Jan 7, 2026
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19 changes: 12 additions & 7 deletions openml/datasets/functions.py
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Original file line number Diff line number Diff line change
Expand Up @@ -564,13 +564,18 @@ def attributes_arff_from_df(df: pd.DataFrame) -> list[tuple[str, list[str] | str
categories = df[column_name].cat.categories
categories_dtype = pd.api.types.infer_dtype(categories)
if categories_dtype not in ("string", "unicode"):
raise ValueError(
f"The column '{column_name}' of the dataframe is of "
"'category' dtype. Therefore, all values in "
"this columns should be string. Please "
"convert the entries which are not string. "
f"Got {categories_dtype} dtype in this column.",
)
try:
# [ENH] Attempt to convert categories to string automatically
new_categories = categories.astype(str)
df[column_name] = df[column_name].cat.rename_categories(new_categories)
categories = df[column_name].cat.categories
except Exception as e:
raise ValueError(
f"The column '{column_name}' of the dataframe is of "
"'category' dtype. Therefore, all values in "
"this columns should be string. Automatic conversion failed. "
f"Got {categories_dtype} dtype in this column.",
) from e
attributes_arff.append((column_name, categories.tolist()))
elif column_dtype == "boolean":
# boolean are encoded as categorical.
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