Implement Series.keys

[itholic]

An alias for index.

>>> midx = pd.MultiIndex([['lama', 'cow', 'falcon'],
...                       ['speed', 'weight', 'length']],
...                      [[0, 0, 0, 1, 1, 1, 2, 2, 2],
...                       [0, 1, 2, 0, 1, 2, 0, 1, 2]])
>>> kser = ks.Series([45, 200, 1.2, 30, 250, 1.5, 320, 1, 0.3], index=midx)
>>> pser = kser.to_pandas()

>>> pser.keys()
MultiIndex([(  'lama',  'speed'),
            (  'lama', 'weight'),
            (  'lama', 'length'),
            (   'cow',  'speed'),
            (   'cow', 'weight'),
            (   'cow', 'length'),
            ('falcon',  'speed'),
            ('falcon', 'weight'),
            ('falcon', 'length')],
           )

>>> kser.keys()
MultiIndex([(  'lama',  'speed'),
            (  'lama', 'weight'),
            (  'lama', 'length'),
            (   'cow',  'speed'),
            (   'cow', 'weight'),
            (   'cow', 'length'),
            ('falcon',  'speed'),
            ('falcon', 'weight'),
            ('falcon', 'length')],
           )

[codecov-io]

Codecov Report

Merging #935 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #935      +/-   ##
==========================================
+ Coverage   94.46%   94.46%   +<.01%     
==========================================
  Files          34       34              
  Lines        6382     6383       +1     
==========================================
+ Hits         6029     6030       +1     
  Misses        353      353

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/series.py	95.6% <100%> (+0.01%)	⬆️

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[ueshin]

Otherwise, LGTM.

[HyukjinKwon]

LGTM too except https://github.com/databricks/koalas/pull/935/files#r336508255

[itholic]

@ueshin @HyukjinKwon i think they have different behavior about showing index as print out,

in python3.7

Expected:
    MultiIndex([(  'lama',  'speed'),
                (  'lama', 'weight'),
                (  'lama', 'length'),
                (   'cow',  'speed'),
                (   'cow', 'weight'),
                (   'cow', 'length'),
                ('falcon',  'speed'),
                ('falcon', 'weight'),
                ('falcon', 'length')],
               )

but python3.5 & python3.6

Got:
    MultiIndex(levels=[['cow', 'falcon', 'lama'], ['length', 'speed', 'weight']],
               labels=[[2, 2, 2, 0, 0, 0, 1, 1, 1], [1, 2, 0, 1, 2, 0, 1, 2, 0]])

so the doctest raises error,

any idea how to fix this?

i think maybe we can just skip doctest or remove examples from doctest

[ueshin]

@itholic Let's keep the 3.7 result and skip the doctest.

[itholic]

@ueshin i think it's better too. thanks for your comment!!

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