Fix Series.astype to work properly

databricks/koalas/series.py

@@ -1027,6 +1027,10 @@ def astype(self, dtype) -> "Series":
                 scol = F.when(self.spark.column.isNull(), F.lit(False)).otherwise(
                     self.spark.column.cast(spark_type)
                 )
+        elif isinstance(spark_type, StringType):
+            scol = F.when(self.spark.column.isNull(), str(None)).otherwise(
+                self.spark.column.cast(spark_type)
+            )
         else:
             scol = self.spark.column.cast(spark_type)
         return self._with_new_scol(scol)

databricks/koalas/tests/test_series.py

@@ -1123,6 +1123,9 @@ def test_astype(self):
         kser = ks.Series(pser)
 
         self.assert_eq(kser.astype(bool), pser.astype(bool))
+        # TODO: restore after pandas 1.1.4 is released.
+        # self.assert_eq(kser.astype(str).tolist(), pser.astype(str).tolist())
+        self.assert_eq(kser.astype(str).tolist(), ["hi", "hi ", " ", " \t", "", "None"])
         self.assert_eq(kser.str.strip().astype(bool), pser.str.strip().astype(bool))
 
         pser = pd.Series([True, False, None], name="x")