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

* BUG: Raise clear error for duplicate id_vars in melt (GH61475) * Bug Fixed Issue GH61475 * if statement changes * Update pandas/core/reshape/melt.py --------- Co-authored-by: ZanirP <zanir@DESKTOP-JOHE35M> Co-authored-by: ZanirP <zanir@DESKTOP-JOHE35M.localdomain> Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

Author: 4 people

doc/source/whatsnew/v3.0.0.rst

@@ -851,6 +851,7 @@ 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
 ^^^^^^

pandas/core/reshape/melt.py

@@ -182,6 +182,10 @@ def melt(
  value_vars_was_not_none = value_vars is not None
  value_vars = ensure_list_vars(value_vars, "value_vars", frame.columns)
 
+ # GH61475 - prevent AttributeError when duplicate column in id_vars
+ if len(frame.columns.get_indexer_for(id_vars)) > len(id_vars):
+ raise ValueError("id_vars cannot contain duplicate columns.")
+
  if id_vars or value_vars:
  if col_level is not None:
  level = frame.columns.get_level_values(col_level)

pandas/tests/reshape/test_melt.py

@@ -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):