Implement Series.first_valid_index

[itholic]

Like pandas Series.first_valid_index (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.first_valid_index.html#pandas.Series.first_valid_index)

implemented function first_valid_index for Series.

>>> s = ks.Series([None, None, 3, 4, 5], index=[100, 200, 300, 400, 500])
>>> s
100    NaN
200    NaN
300    3.0
400    4.0
500    5.0
Name: 0, dtype: float64

>>> s.first_valid_index()
300
>>> s.to_pandas().first_valid_index()
300

And act like below when Series has MultiIndex

>>> 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]])
>>> s = ks.Series([None, None, None, None, 250, 1.5, 320, 1, 0.3], index=midx)
>>> s
lama    speed       NaN
        weight      NaN
        length      NaN
cow     speed       NaN
        weight    250.0
        length      1.5
falcon  speed     320.0
        weight      1.0
        length      0.3
Name: 0, dtype: float64

>>> s.first_valid_index()
('cow', 'weight')
>>> s.to_pandas().first_valid_index()
('cow', 'weight')

[codecov-io]

Codecov Report

Merging #936 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #936      +/-   ##
==========================================
+ Coverage   94.52%   94.53%   +<.01%     
==========================================
  Files          34       34              
  Lines        6465     6472       +7     
==========================================
+ Hits         6111     6118       +7     
  Misses        354      354

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/series.py	96.14% <100%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c8dcb64...bad5c3f. Read the comment docs.

[HyukjinKwon]

Looks fine otherwise.

[softagram-bot]

Softagram Impact Report for pull/936 (head commit: bad5c3f)

⭐ Change Overview

(Open in Softagram Desktop for full details)

📄 Full report

• Permalink: Full report for pull/936

Impact Report explained. Give feedback on this report to [email protected]