BUG: Raise ValueError when var_name collides with id_vars in melt()

[C1-BA-B1-F3]

Description

Fixes #65654

DataFrame.melt silently corrupts data when var_name matches an id_vars column. The original id_vars data gets overwritten with the variable labels.

Before (silent corruption):

df = pd.DataFrame({"id": [1, 2], "a": [10, 20], "b": [100, 200]})
out = df.melt(id_vars="id", var_name="id")
#   id id  value
# 0  a  a     10
# 1  a  a     20
# 2  b  b    100
# 3  b  b    200

After (ValueError raised):

ValueError: var_name ({'id'}) cannot match an element in id_vars. This would cause the id_vars data to be silently overwritten.

Changes

• pandas/core/reshape/melt.py: Added validation check after var_name is resolved to detect collisions with id_vars. Raises ValueError with a clear message explaining the issue.
• pandas/tests/reshape/test_melt.py: Added two test cases:
  • test_raise_of_var_name_in_id_vars: Tests scalar id_vars collision
  • test_raise_of_var_name_in_id_vars_list: Tests list-like id_vars collision

Testing

import pandas as pd

# Test 1: Collision detected
try:
    df = pd.DataFrame({"id": [1, 2], "a": [10, 20], "b": [100, 200]})
    pd.melt(df, id_vars="id", var_name="id")
except ValueError as e:
    print(f"PASS: {e}")

# Test 2: No collision works fine
df = pd.DataFrame({"id": [1, 2], "a": [10, 20], "b": [100, 200]})
result = pd.melt(df, id_vars="id", var_name="variable")
print(result)

This fix is consistent with the existing value_name collision check (line 179-183 in the original code).

[jbrockmendel]

closed by #65655