Implement Series.truncate

[itholic]

Like pandas Series.truncate (https://pandas.pydata.org/pandas-docs/version/0.21/generated/pandas.Series.truncate.html#pandas.Series.truncate)

implemented function truncate for Series.

>>> s = ks.Series([10, 20, 30, 40, 50, 60, 70],
...               index=[1, 2, 3, 4, 5, 6, 7])
>>> s
1    10
2    20
3    30
4    40
5    50
6    60
7    70
Name: 0, dtype: int64

>>> s.truncate(2, 5)
2    20
3    30
4    40
5    50
Name: 0, dtype: int64

>>> s = ks.Series([10, 20, 30, 40, 50, 60, 70],
...               index=['a', 'b', 'c', 'd', 'e', 'f', 'g'])
>>> s
a    10
b    20
c    30
d    40
e    50
f    60
g    70
Name: 0, dtype: int64

>>> s.truncate('b', 'e')
b    20
c    30
d    40
e    50
Name: 0, dtype: int64

[codecov-io]

Codecov Report

Merging #928 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #928      +/-   ##
=========================================
+ Coverage   94.49%   94.5%   +0.01%     
=========================================
  Files          34      34              
  Lines        6434    6447      +13     
=========================================
+ Hits         6080    6093      +13     
  Misses        354     354

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/series.py	96% <100%> (+0.08%)	⬆️

---

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 a924df6...558d978. Read the comment docs.

[HyukjinKwon]

@itholic, this will collect everything in index into driver's memory. We should avoid this.

[HyukjinKwon]

Koalas index implements is_monotonic_increasing and is_monotonic_decreasing. The problem is that this is an expensive operation as warned in their documentations as it leads all data into single node.

Can you add a note that this API will be expensive (see is_monotonic_increasing as an example). We even might have to explicitly don't implement. cc @ueshin

[itholic]

@HyukjinKwon Thanks for commenting!

yeah you're right. i used is_monotonic_increasing of pandas' because there was a bug for koalas'. (so i fixed in #930 )

now we can use koalas', but there still have an issue about cost as you said.

and first, i wrote note about warning in this API docstring as you said

[HyukjinKwon]

Looks fine to me. I'll take it back.

[softagram-bot]

Softagram Impact Report for pull/928 (head commit: 558d978)

⭐ Change Overview

(Open in Softagram Desktop for full details)

📄 Full report

• Permalink: Full report for pull/928

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