feat: add FunctionalTableTransformer (#901)

Closes #858 

### Summary of Changes

This implements a Transformer that wraps a Callable[[Table], Table] so
operations on Tables can be inserted into the operation order of a
SequentialTableTransformer.
Note that this transformer inherently cannot be invertible.
Since there is no type checking at runtime, callables with wrong
argument or return types will only throw an exception upon calling
transform but not during init.

---------

Co-authored-by: Simon <[email protected]>
Co-authored-by: srose <[email protected]>
Co-authored-by: megalinter-bot <[email protected]>
Co-authored-by: Lars Reimann <[email protected]>

src/safeds/data/tabular/transformation/__init__.py

@@ -6,6 +6,7 @@
 
 if TYPE_CHECKING:
     from ._discretizer import Discretizer
+    from ._functional_table_transformer import FunctionalTableTransformer
     from ._invertible_table_transformer import InvertibleTableTransformer
     from ._k_nearest_neighbors_imputer import KNearestNeighborsImputer
     from ._label_encoder import LabelEncoder
@@ -16,10 +17,12 @@
     from ._standard_scaler import StandardScaler
     from ._table_transformer import TableTransformer
 
+
 apipkg.initpkg(
     __name__,
     {
         "Discretizer": "._discretizer:Discretizer",
+        "FunctionalTableTransformer": "._functional_table_transformer:FunctionalTableTransformer",
         "InvertibleTableTransformer": "._invertible_table_transformer:InvertibleTableTransformer",
         "LabelEncoder": "._label_encoder:LabelEncoder",
         "OneHotEncoder": "._one_hot_encoder:OneHotEncoder",
@@ -34,6 +37,7 @@
 
 __all__ = [
     "Discretizer",
+    "FunctionalTableTransformer",
     "InvertibleTableTransformer",
     "LabelEncoder",
     "OneHotEncoder",

src/safeds/data/tabular/transformation/_functional_table_transformer.py

@@ -0,0 +1,112 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from safeds._utils import _structural_hash
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
+
+    from safeds.data.tabular.containers import Table
+
+from ._table_transformer import TableTransformer
+
+
+class FunctionalTableTransformer(TableTransformer):
+    """
+    Wraps a callable so that it conforms to the TableTransformer interface.
+
+    Parameters
+    ----------
+    transformer:
+        A callable that receives a table and returns a table.
+    """
+
+    # ------------------------------------------------------------------------------------------------------------------
+    # Dunder methods
+    # ------------------------------------------------------------------------------------------------------------------
+
+    def __init__(
+        self,
+        transformer: Callable[[Table], Table],
+    ) -> None:
+        super().__init__(None)
+        self._transformer = transformer
+
+    def __hash__(self) -> int:
+        return _structural_hash(
+            super().__hash__(),
+            self._transformer,
+        )
+
+    # ------------------------------------------------------------------------------------------------------------------
+    # Properties
+    # ------------------------------------------------------------------------------------------------------------------
+
+    @property
+    def is_fitted(self) -> bool:
+        """FunctionalTableTransformer is always considered to be fitted."""
+        return True
+
+    # ------------------------------------------------------------------------------------------------------------------
+    # Learning and transformation
+    # ------------------------------------------------------------------------------------------------------------------
+
+    def fit(self, table: Table) -> FunctionalTableTransformer:  # noqa: ARG002
+        """
+        **Note:** For FunctionalTableTransformer this is a no-OP.
+
+        Parameters
+        ----------
+        table:
+            Required only to be consistent with other transformers.
+
+        Returns
+        -------
+        fitted_transformer:
+            Returns self, because this transformer is always fitted.
+        """
+        return self
+
+    def transform(self, table: Table) -> Table:
+        """
+        Apply the callable to a table.
+
+        **Note:** The given table is not modified.
+
+        Parameters
+        ----------
+        table:
+            The table on which on which the callable is executed.
+
+        Returns
+        -------
+        transformed_table:
+            The transformed table.
+
+        Raises
+        ------
+        Exception:
+            Raised when the wrapped callable encounters an error.
+        """
+        return self._transformer(table)
+
+    def fit_and_transform(self, table: Table) -> tuple[FunctionalTableTransformer, Table]:
+        """
+        **Note:** For the FunctionalTableTransformer this is the same as transform().
+
+        Parameters
+        ----------
+        table:
+            The table on which the callable is to be executed.
+
+        Returns
+        -------
+        fitted_transformer:
+            Return self because the transformer is always fitted.
+        transformed_table:
+            The transformed table.
+        """
+        fitted_transformer = self
+        transformed_table = self.transform(table)
+        return fitted_transformer, transformed_table

tests/safeds/data/tabular/transformation/test_functional_table_transformer.py

@@ -0,0 +1,102 @@
+import pytest
+from safeds.data.tabular.containers import Table
+from safeds.data.tabular.transformation import FunctionalTableTransformer
+from safeds.exceptions import ColumnNotFoundError
+
+
+def valid_callable(table: Table) -> Table:
+    return table.remove_columns(["col1"])
+
+
+class TestInit:
+    def test_should_not_raise_type_error(self) -> None:
+        FunctionalTableTransformer(valid_callable)
+
+
+class TestFit:
+    def test_should_return_self(self) -> None:
+        table = Table(
+            {
+                "col1": [1, 2, 3],
+                "col2": [1, 2, 3],
+            },
+        )
+        transformer = FunctionalTableTransformer(valid_callable)
+        assert transformer.fit(table) is transformer
+
+
+class TestIsFitted:
+    def test_should_always_be_fitted(self) -> None:
+        transformer = FunctionalTableTransformer(valid_callable)
+        assert transformer.is_fitted
+
+
+class TestTransform:
+    def test_should_raise_specific_error_when_error_in_method(self) -> None:
+        table = Table(
+            {
+                "col2": [1, 2, 3],
+            },
+        )
+        transformer = FunctionalTableTransformer(valid_callable)
+        with pytest.raises(ColumnNotFoundError):
+            transformer.transform(table)
+
+    def test_should_not_modify_original_table(self) -> None:
+        table = Table(
+            {
+                "col1": [1, 2, 3],
+                "col2": [1, 2, 3],
+            },
+        )
+        transformer = FunctionalTableTransformer(valid_callable)
+        transformer.transform(table)
+        assert table == Table(
+            {
+                "col1": [1, 2, 3],
+                "col2": [1, 2, 3],
+            },
+        )
+
+    def test_should_return_modified_table(self) -> None:
+        table = Table(
+            {
+                "col1": [1, 2, 3],
+                "col2": [1, 2, 3],
+            },
+        )
+        transformer = FunctionalTableTransformer(valid_callable)
+        transformed_table = transformer.transform(table)
+        assert transformed_table == Table(
+            {
+                "col2": [1, 2, 3],
+            },
+        )
+
+
+class TestFitAndTransform:
+    def test_should_return_self(self) -> None:
+        table = Table(
+            {
+                "col1": [1, 2, 3],
+                "col2": [1, 2, 3],
+            },
+        )
+        transformer = FunctionalTableTransformer(valid_callable)
+        assert transformer.fit_and_transform(table)[0] is transformer
+
+    def test_should_not_modify_original_table(self) -> None:
+        table = Table(
+            {
+                "col1": [1, 2, 3],
+                "col2": [1, 2, 3],
+            },
+        )
+        transformer = FunctionalTableTransformer(valid_callable)
+        transformer.fit_and_transform(table)
+        assert table == Table(
+            {
+                "col1": [1, 2, 3],
+                "col2": [1, 2, 3],
+            },
+        )

tests/safeds/data/tabular/transformation/test_table_transformer.py

@@ -4,6 +4,7 @@
 from safeds.data.tabular.containers import Table
 from safeds.data.tabular.transformation import (
     Discretizer,
+    FunctionalTableTransformer,
     KNearestNeighborsImputer,
     LabelEncoder,
     OneHotEncoder,
@@ -51,6 +52,10 @@ def transformers_non_numeric() -> list[TableTransformer]:
     ]
 
 
+def valid_callable_for_functional_table_transformer(table: Table) -> Table:
+    return table.remove_columns(["col1"])
+
+
 def transformers() -> list[TableTransformer]:
     """
     Return the list of all transformers to test.
@@ -69,6 +74,7 @@ def transformers() -> list[TableTransformer]:
         + [
             SimpleImputer(strategy=SimpleImputer.Strategy.mode()),
             KNearestNeighborsImputer(neighbor_count=3, value_to_replace=None),
+            FunctionalTableTransformer(valid_callable_for_functional_table_transformer),
         ]
     )