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BUG: Allow assignment by indexing with duplicate column names #12498

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BUG: Allow assignment by indexing with duplicate column names #12498

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1 change: 1 addition & 0 deletions doc/source/whatsnew/v0.18.0.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -1201,3 +1201,4 @@ Bug Fixes
- Bug when initializing categorical series with a scalar value. (:issue:`12336`)
- Bug when specifying a UTC ``DatetimeIndex`` by setting ``utc=True`` in ``.to_datetime`` (:issue:`11934`)
- Bug when increasing the buffer size of CSV reader in ``read_csv`` (:issue:`12494`)
- Bug when setting columns of a ``DataFrame`` with duplicate column names (:issue:`12344`)
2 changes: 1 addition & 1 deletion pandas/core/indexing.py
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Expand Up @@ -541,7 +541,7 @@ def can_do_equal_len():
if (len(indexer) > info_axis and
is_integer(indexer[info_axis]) and
all(is_null_slice(idx) for i, idx in enumerate(indexer)
if i != info_axis)):
if i != info_axis) and item_labels.is_unique):
self.obj[item_labels[indexer[info_axis]]] = value
return

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16 changes: 16 additions & 0 deletions pandas/tests/frame/test_nonunique_indexes.py
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Expand Up @@ -452,3 +452,19 @@ def test_as_matrix_duplicates(self):
dtype=object)

self.assertTrue(np.array_equal(result, expected))

def test_set_value_by_index(self):
# See gh-12344
df = DataFrame(np.arange(9).reshape(3, 3).T)
df.columns = list('AAA')
expected = df.iloc[:, 2]

df.iloc[:, 0] = 3
assert_series_equal(df.iloc[:, 2], expected)

df = DataFrame(np.arange(9).reshape(3, 3).T)
df.columns = [2, float(2), str(2)]
expected = df.iloc[:, 1]

df.iloc[:, 0] = 3
assert_series_equal(df.iloc[:, 1], expected)