BUG: index name lost with timedelta ops

[sinhrks]

import pandas as pd 

dtidx = pd.DatetimeIndex(['2011-01-01'], freq='D', name='dtidx')
(dtidx + 1).name
# dtidx

# NG
(dtidx + pd.Timedelta('1 day')).name
# None

tdidx = pd.TimedeltaIndex(['1 day'], freq='D', name='tdidx')
(tdidx + 1).name
# tdidx

# NG
(tdidx + pd.Timedelta('1 day')).name
# None

ref: #9862

[sinhrks]

Would like to define expected bahavior. Based on normal index, I understand first (left-side) index name should be prioritized.

idx1 = pd.Index([1], name='idx1')
idx2 = pd.Index([1], name='idx2')
(idx1 + 1).name
# idx1
(idx1 + idx1).name
# idx1
(idx1 + idx2).name
# idx1

But in case of datetime-likes, I feel it is natural that prioritize the name of DatetimeIndex. I'd like to ask whether I can prepare a fix based on following behavior.

left side	right side	prioritized name
DatetimeIndex	DatetimeIndex	name of left side
TimedeltaIndex	TimedeltaIndex	name of left side
DatetimeIndex	TimedeltaIndex	name of left side (DatetimeIndex)
TimedeltaIndex	DatetimeIndex	name of right side (DatetimeIndex)

dtidx = pd.DatetimeIndex(['2011-01-01'], freq='D', name='dtidx')
tdidx = pd.TimedeltaIndex(['1 day'], freq='D', name='tdidx')

dtidx + tdidx
# <class 'pandas.tseries.index.DatetimeIndex'>
# [2011-01-02]
# Length: 1, Freq: None, Timezone: None

(dtidx + tdidx).name
# dtidx

tdidx + dtidx
# <class 'pandas.tseries.index.DatetimeIndex'>
# [2011-01-02]
# Length: 1, Freq: None, Timezone: None

(tdidx + dtidx).name
# dtidx

[jreback]

no priority
append / op on an index has to have the same name (or None)
otherwise will be set to None

see Index.append

it may be a bug if add ops don't follow this pattern

[sinhrks]

Thanks, could you check following understanding is correct for both set (intersection, etc) and arithmetic ops (addition, etc)?

• In case of Index + Index, name is preserved if left and right index have the same name. Otherwise, name is reset to None.
• In case of Index + scalar or scalar + Index, name of the index is preserved.

Based on above understanding, normal index behaves incorrectly. All the below ops should reset the name to None.

idx1 = pd.Index([1, 2, 3], name='idx1')
idx2 = pd.Index([1, 2, 3], name='idx2')

result = idx1 + idx2
result, result.name
# (Int64Index([2, 4, 6], dtype='int64'), 'idx1')

result = idx1.__add__(idx2)
result, result.name
# (Int64Index([2, 4, 6], dtype='int64'), 'idx1')

result = idx1.intersection(idx2)
result, result.name
# (Int64Index([1, 2, 3], dtype='int64'), 'idx1')

[shoyer]

@sinhrks Yes, I think you correctly understand this now.

Here are some notes on this from @cpcloud: blaze/blaze#458 (comment)