Rename get_column_names to column_names and make property (#254)

* get_column_names -> column_names property

* get_column_names -> column_names

Author: MarcoGorelli

Diff for: spec/API_specification/dataframe_api/dataframe_object.py

@@ -199,7 +199,7 @@ def insert_column(self, column: Column[Any]) -> DataFrame:
         .. code-block:: python
 
             new_column = df.get_column_by_name('a') + 1
-            new_columns_names = ['a_plus_1'] + df.get_column_names()
+            new_columns_names = ['a_plus_1'] + df.column_names
             df = df.insert_column(new_column.rename('a_plus_1'))
             df = df.select(new_column_names)
 
@@ -268,7 +268,8 @@ def rename_columns(self, mapping: Mapping[str, str]) -> DataFrame:
         """
         ...
 
-    def get_column_names(self) -> list[str]:
+    @property
+    def column_names(self) -> list[str]:
         """
         Get column names.
 

Diff for: spec/purpose_and_scope.md

@@ -291,7 +291,7 @@ def my_dataframe_agnostic_function(df):
     mask = df.get_column_by_name('species') != 'setosa'
     df = df.filter(mask)
 
-    for column_name in df.get_column_names():
+    for column_name in df.column_names
         if column_name == 'species':
             continue
         new_column = df.get_column_by_name(column_name)