Add note about parent dataframes and cross-dataframe column compariso…

…ns (#310)

* wip

* add notes about parent dataframes

* add note about free-standing columns

* post merge fixup

* 🚚 Column.dataframe -> Column.parent_dataframe, note unsupportedness rather than impossibility

spec/API_specification/dataframe_api/column_object.py

@@ -5,6 +5,8 @@
 if TYPE_CHECKING:
     from typing_extensions import Self
 
+    from dataframe_api.dataframe_object import DataFrame
+
     from .typing import DType, Namespace, NullType, Scalar
 
 
@@ -17,9 +19,45 @@ class Column(Protocol):
     Note that this column object is not meant to be instantiated directly by
     users of the library implementing the dataframe API standard. Rather, use
     constructor functions or an already-created dataframe object retrieved via
-
+    :meth:`DataFrame.col`.
+
+    The parent dataframe (which can be retrieved via the :meth:`parent_dataframe`
+    property) plays a key role here:
+
+    - If two columns were retrieved from the same dataframe,
+      then they can be combined and compared at will.
+    - If two columns were retrieved from different dataframes,
+      then there is no guarantee about how or whether they can be combined and
+      compared, this may vary across implementations.
+    - If two columns are both "free-standing" (i.e. not retrieved from a dataframe
+      but constructed directly from a 1D array or sequence), then they can be
+      combined and compared with each other. Note, however, that there's no guarantee
+      about whether they can be compared or combined with columns retrieved from a
+      different dataframe, this may vary across implementations.
     """
 
+    @property
+    def parent_dataframe(self) -> DataFrame | None:
+        """Return parent DataFrame, if present.
+
+        For example, if we have the following
+
+        .. code-block:: python
+
+            df: DataFrame
+            column = df.col('a')
+
+        then `column.parent_dataframe` should return `df`.
+
+        On the other hand, if we had:
+
+        .. code-block:: python
+
+            column = column_from_1d_array(...)
+
+        then `column.parent_dataframe` should return `None`.
+        """
+
     def __column_namespace__(self) -> Namespace:
         """Return an object that has all the Dataframe Standard API functions on it.
 
@@ -201,6 +239,11 @@ def __eq__(self, other: Self | Scalar) -> Self:  # type: ignore[override]
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -219,6 +262,11 @@ def __ne__(self, other: Self | Scalar) -> Self:  # type: ignore[override]
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -235,6 +283,11 @@ def __ge__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -251,6 +304,11 @@ def __gt__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -267,6 +325,11 @@ def __le__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -283,6 +346,11 @@ def __lt__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -300,6 +368,11 @@ def __and__(self, other: Self | bool) -> Self:
         -------
         Column
 
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
+
         Raises
         ------
         ValueError
@@ -321,6 +394,11 @@ def __or__(self, other: Self | bool) -> Self:
         -------
         Column
 
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
+
         Raises
         ------
         ValueError
@@ -338,6 +416,11 @@ def __add__(self, other: Self | Scalar) -> Self:
             "Scalar" here is defined implicitly by what scalar types are allowed
             for the operation by the underling dtypes.
 
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
+
         Returns
         -------
         Column
@@ -357,6 +440,11 @@ def __sub__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -373,6 +461,11 @@ def __mul__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -389,6 +482,11 @@ def __truediv__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -405,6 +503,11 @@ def __floordiv__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -425,6 +528,11 @@ def __pow__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -441,6 +549,11 @@ def __mod__(self, other: Self | Scalar) -> Self:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -457,6 +570,11 @@ def __divmod__(self, other: Self | Scalar) -> tuple[Column, Column]:
         Returns
         -------
         Column
+
+        Notes
+        -----
+        `other`'s parent DataFrame must be the same as `self`'s - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 

spec/API_specification/dataframe_api/dataframe_object.py

@@ -136,6 +136,11 @@ def get_rows(self, indices: Column) -> Self:
         Returns
         -------
         DataFrame
+
+        Notes
+        -----
+        `indices`'s parent DataFrame must be `self` - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -172,8 +177,8 @@ def filter(self, mask: Column) -> Self:
 
         Notes
         -----
-        Some participants preferred a weaker type Arraylike[bool] for mask,
-        where 'Arraylike' denotes an object adhering to the Array API standard.
+        `mask`'s parent DataFrame must be `self` - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...
 
@@ -201,6 +206,11 @@ def assign(self, *columns: Column) -> Self:
         Returns
         -------
         DataFrame
+
+        Notes
+        -----
+        All of `columns`'s parent DataFrame must be `self` - else,
+        the operation is unsupported and may vary across implementations.
         """
         ...