BUG: Index.union() drops Index component for duplicate Index elements not sorted · Issue #36289 · pandas-dev/pandas (original) (raw)

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• I have confirmed this bug exists on the latest version of pandas.
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Code Sample, a copy-pastable example

import pandas as pd

idx1 = pd.Index([0, 0, 1]) idx2 = pd.Index([0, 1])

print(idx1.union(idx2))

Int64Index([0, 0, 1], dtype='int64')

print(idx2.union(idx1))

Int64Index([0, 0, 1], dtype='int64')

idx1 = pd.Index([1, 0, 0])

print(idx1.union(idx2))

Int64Index([0, 0, 1], dtype='int64')

print(idx2.union(idx1))

Int64Index([0, 1], dtype='int64')

Problem description

In the fourth call idx2.union(idx1) one of the 0 is dropped. Meaning union() depends on the call order and on the index order, which seems buggy.

Expected Output

Int64Index([0, 0, 1], dtype='int64') for every print Statement

Output of pd.show_versions()

master