add __len__ and __getitem__ to Column

spec/API_specification/dataframe_api/column_object.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import Sequence
+from typing import NoReturn, Sequence
 
 from ._types import dtype
 
@@ -35,3 +35,37 @@ def from_sequence(cls, sequence: Sequence[object], dtype: dtype) -> Column:
         -------
         Column
         """
+        ...
+
+    def __len__(self) -> int:
+        """
+        Return the number of rows.
+        """
+
+    def __getitem__(self, row: int) -> object:
+        """
+        Get the element at row index `key`.
+        """
+
+    def __iter__(self) -> NoReturn:
+        """
+        Iterate over elements.
+
+        This is intentionally "poisoned" to discourage inefficient code patterns.
+
+        Raises
+        ------
+        NotImplementedError
+        """
+        raise NotImplementedError("'__iter__' is intentionally not implemented.")
+
+    def get_rows(self, indices: Column[int]) -> Column:
+        """
+        Select a subset of rows, similar to `ndarray.take`.
+
+        Parameters
+        ----------
+        indices : Column[int]
+            Positions of rows to select.
+        """
+        ...