Implemented to_list to Index/MultiIndex

[itholic]

Added to_list - tolist as an alias - to the Index and MultiIndex.

This implementation idea is basically inspired from Series.to_list.

>>> idx = ks.Index([1, 2, 3, 4, 5])
>>> idx.to_list()
[1, 2, 3, 4, 5]

>>> tuples = [(1, 'red'), (1, 'blue'), (2, 'red'), (2, 'green')]
>>> midx = ks.MultiIndex.from_tuples(tuples)
>>> midx.to_list()
[(1, 'red'), (1, 'blue'), (2, 'red'), (2, 'green')]

[itholic]

FYI: This implementation idea is basically inspired from Series.to_list.

koalas/databricks/koalas/series.py

Lines 1497 to 1515 in 138c7b8

	def to_list(self) -> List:
	"""
	Return a list of the values.
	These are each a scalar type, which is a Python scalar
	(for str, int, float) or a pandas scalar
	(for Timestamp/Timedelta/Interval/Period)
	.. note:: This method should only be used if the resulting list is expected
	to be small, as all the data is loaded into the driver's memory.
	"""
	# pandas<0.24 doesn't support `to_list()` as an alias for `tolist()`
	if LooseVersion(pd.__version__) < LooseVersion("0.24"):
	return self._to_internal_pandas().tolist()
	else:
	return self._to_internal_pandas().to_list()
	tolist = to_list

[codecov-io]

Codecov Report

Merging #1948 (d2df72f) into master (e598f44) will decrease coverage by 3.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1948      +/-   ##
==========================================
- Coverage   94.50%   91.19%   -3.31%     
==========================================
  Files          45       49       +4     
  Lines       10682    10712      +30     
==========================================
- Hits        10095     9769     -326     
- Misses        587      943     +356     

Impacted Files	Coverage Δ
databricks/koalas/missing/indexes.py	100.00% <ø> (ø)
databricks/koalas/indexes.py	96.94% <100.00%> (+0.09%)	⬆️
databricks/koalas/usage_logging/__init__.py	25.66% <0.00%> (-66.65%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	47.82% <0.00%> (-52.18%)	⬇️
databricks/koalas/__init__.py	68.75% <0.00%> (-21.88%)	⬇️
databricks/conftest.py	89.83% <0.00%> (-10.17%)	⬇️
databricks/koalas/spark/functions.py	88.88% <0.00%> (-7.41%)	⬇️
databricks/koalas/accessors.py	86.43% <0.00%> (-7.04%)	⬇️
databricks/koalas/namespace.py	78.17% <0.00%> (-6.07%)	⬇️
databricks/koalas/generic.py	87.89% <0.00%> (-5.03%)	⬇️
... and 24 more

---

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[xinrong-meng]

Do we exclude "tolist" on purpose?

[itholic]

Yes, because pandas also exclude tolist in their docs.

[xinrong-meng]

Sounds good, thanks!

[itholic]

I'll merge this for now. Let's address #1948 (comment)
cc @ueshin

[xinrong-meng]

ref #1929