Repository: databricks/koalas

Issue:
Implement missing DataFrame/Series/Index APIs
I am opening this as a reminder. Probably I might keep opening and closing for each release - not sure yet about it. I don't intend to open this so long time. 

To get to the point, eventually we should fix all of the missing APIs here:
https://github.com/databricks/koalas/blob/master/databricks/koalas/missing/series.py
https://github.com/databricks/koalas/blob/master/databricks/koalas/missing/frame.py
https://github.com/databricks/koalas/blob/master/databricks/koalas/missing/indexes.py
https://github.com/databricks/koalas/blob/master/databricks/koalas/missing/groupby.py

**Please open a separate issue to start to work on implementing them.**


Discussion:
@HyukjinKwon any particular functions that need to be done first? 

I don't have in my mind yet. We should implement all of them (unless it violates our design principles) and I wanted to pin the reminder for now :).

Resolving patch:
diff --git a/databricks/koalas/missing/series.py b/databricks/koalas/missing/series.py
--- a/databricks/koalas/missing/series.py
+++ b/databricks/koalas/missing/series.py
@@ -182,7 +182,6 @@ class _MissingPandasLikeSeries(object):
     to_dense = unsupported_function('to_dense')
     to_frame = unsupported_function('to_frame')
     to_hdf = unsupported_function('to_hdf')
-    to_list = unsupported_function('to_list')
     to_msgpack = unsupported_function('to_msgpack')
     to_period = unsupported_function('to_period')
     to_pickle = unsupported_function('to_pickle')
@@ -190,7 +189,6 @@ class _MissingPandasLikeSeries(object):
     to_sql = unsupported_function('to_sql')
     to_timestamp = unsupported_function('to_timestamp')
     to_xarray = unsupported_function('to_xarray')
-    tolist = unsupported_function('tolist')
     transform = unsupported_function('transform')
     transpose = unsupported_function('transpose')
     truediv = unsupported_function('truediv')
diff --git a/databricks/koalas/series.py b/databricks/koalas/series.py
--- a/databricks/koalas/series.py
+++ b/databricks/koalas/series.py
@@ -24,7 +24,6 @@
 
 import numpy as np
 import pandas as pd
-from pandas.api.types import is_list_like
 
 from pyspark import sql as spark
 from pyspark.sql import functions as F
@@ -557,6 +556,22 @@ def to_pandas(self):
     # Alias to maintain backward compatibility with Spark
     toPandas = to_pandas
 
+    def to_list(self):
+        """
+        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.
+
+        """
+        return self.to_pandas().to_list()
+
+    tolist = to_list
+
     def fillna(self, value=None, axis=None, inplace=False):
         """Fill NA/NaN values.
 


Test patch:
diff --git a/databricks/koalas/tests/test_series.py b/databricks/koalas/tests/test_series.py
--- a/databricks/koalas/tests/test_series.py
+++ b/databricks/koalas/tests/test_series.py
@@ -20,8 +20,8 @@
 import pandas as pd
 
 from databricks import koalas
+from distutils.version import LooseVersion
 from databricks.koalas import Series
-from databricks.koalas.generic import max_display_count
 from databricks.koalas.testing.utils import ReusedSQLTestCase, SQLTestUtils
 from databricks.koalas.exceptions import PandasNotImplementedError
 from databricks.koalas.missing.series import _MissingPandasLikeSeries
@@ -323,3 +323,7 @@ def test_is_unique(self):
         pser = pd.Series([1, 1, 1])
         kser = koalas.from_pandas(pser)
         self.assertEqual(pser.is_unique, kser.is_unique)
+
+    def test_to_list(self):
+        if LooseVersion(pd.__version__) >= LooseVersion("0.24.0"):
+            self.assertEqual(self.ks.to_list(), self.ps.to_list())