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BUG: Raise clear error for duplicate id_vars in melt (GH61475) #61484

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Merged
merged 4 commits into from
May 30, 2025
Merged

BUG: Raise clear error for duplicate id_vars in melt (GH61475) #61484

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53e39f7
BUG: Raise clear error for duplicate id_vars in melt (GH61475)
May 23, 2025
8763b5e
Bug Fixed Issue GH61475
May 27, 2025
0cfc2ab
if statement changes
May 29, 2025
c1c53f1
Update pandas/core/reshape/melt.py
mroeschke May 30, 2025
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v3.0.0.rst
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Expand Up @@ -847,6 +847,7 @@ Reshaping
- Bug in :meth:`DataFrame.stack` with the new implementation where ``ValueError`` is raised when ``level=[]`` (:issue:`60740`)
- Bug in :meth:`DataFrame.unstack` producing incorrect results when manipulating empty :class:`DataFrame` with an :class:`ExtentionDtype` (:issue:`59123`)
- Bug in :meth:`concat` where concatenating DataFrame and Series with ``ignore_index = True`` drops the series name (:issue:`60723`, :issue:`56257`)
- Bug in :func:`melt` where calling with duplicate column names in ``id_vars`` raised a misleading ``AttributeError`` (:issue:`61475`)

Sparse
^^^^^^
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3 changes: 3 additions & 0 deletions pandas/core/reshape/melt.py
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Expand Up @@ -173,6 +173,9 @@ def melt(
1 b B E 3
2 c B E 5
"""
# GH61475 - prevent AttributeError when duplicate column in id_vars
if id_vars and any(frame.columns.tolist().count(col) > 1 for col in id_vars):
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  1. Can you move this after the id_vars = ensure_list_vars(id_vars, "id_vars", frame.columns) line
  2. Specify this as if id_vars and not frame.columns.is_unique and any(id_var in frame.column for id_var in id_vars):

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If I added not frame.columns.is_unique at the beginning of the function, wouldn't it cause test_melt_with_duplicate_columns to fail, since that test currently allows duplicate column names in value_vars?

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I'm not sure. You'll need to run the test to find out

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I added not frame.columns.is_unique at and it caused test_melt_with_duplicate_columns to fail since that test currently allows duplicate column names in value_vars, so we might need to handle duplicates differently since we allow certain duplicates.

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@mroeschke mroeschke May 29, 2025 •
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Ah ok you'll need to check that the id_var values is actually a duplicate in the columns.

You can use len(frame.columns.get_indexer_for(id_vars)) > len(id_vars)

raise ValueError("id_vars cannot contain duplicate columns.")
if value_name in frame.columns:
raise ValueError(
f"value_name ({value_name}) cannot match an element in "
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8 changes: 8 additions & 0 deletions pandas/tests/reshape/test_melt.py
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Expand Up @@ -555,6 +555,14 @@ def test_melt_multiindex_columns_var_name_too_many(self):
):
df.melt(var_name=["first", "second", "third"])

def test_melt_duplicate_column_header_raises(self):
# GH61475
df = DataFrame([[1, 2, 3], [3, 4, 5]], columns=["A", "A", "B"])
msg = "id_vars cannot contain duplicate columns."

with pytest.raises(ValueError, match=msg):
df.melt(id_vars=["A"], value_vars=["B"])


class TestLreshape:
def test_pairs(self):
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