Feature generator

README.rst

@@ -240,6 +240,7 @@ in a list::
            [ 0.        ],
            [ 1.22474487]])
 
+
 Columns that don't need any transformation
 ******************************************
 
@@ -282,6 +283,59 @@ passing it as the ``default`` argument to the mapper:
 Using ``default=False`` (the default) drops unselected columns. Using
 ``default=None`` pass the unselected columns unchanged.
 
+
+Same transformer for the multiple columns
+*****************************************
+
+Sometimes it is required to apply the same transformation to several dataframe columns.
+To simplify this process, the package provides ``gen_features`` function which accepts a list
+of columns and feature transformer class (or list of classes), and generates a feature definition,
+acceptable by ``DataFrameMapper``.
+
+For example, consider a dataset with three categorical columns, 'col1', 'col2', and 'col3',
+To binarize each of them, one could pass column names and ``LabelBinarizer`` transformer class
+into generator, and then use returned definition as ``features`` argument for ``DataFrameMapper``:
+
+    >>> from sklearn_pandas import gen_features
+    >>> feature_def = gen_features(
+    ...     columns=['col1', 'col2', 'col3'],
+    ...     classes=[sklearn.preprocessing.LabelEncoder]
+    ... )
+    >>> feature_def
+    [('col1', [LabelEncoder()]), ('col2', [LabelEncoder()]), ('col3', [LabelEncoder()])]
+    >>> mapper5 = DataFrameMapper(feature_def)
+    >>> data5 = pd.DataFrame({
+    ...     'col1': ['yes', 'no', 'yes'],
+    ...     'col2': [True, False, False],
+    ...     'col3': ['one', 'two', 'three']
+    ... })
+    >>> mapper5.fit_transform(data5)
+    array([[1, 1, 0],
+           [0, 0, 2],
+           [1, 0, 1]])
+
+If it is required to override some of transformer parameters, then a dict with 'class' key and
+transformer parameters should be provided. For example, consider a dataset with missing values.
+Then the following code could be used to override default imputing strategy:
+
+    >>> feature_def = gen_features(
+    ...     columns=[['col1'], ['col2'], ['col3']],
+    ...     classes=[{'class': sklearn.preprocessing.Imputer, 'strategy': 'most_frequent'}]
+    ... )
+    >>> mapper6 = DataFrameMapper(feature_def)
+    >>> data6 = pd.DataFrame({
+    ...     'col1': [None, 1, 1, 2, 3],
+    ...     'col2': [True, False, None, None, True],
+    ...     'col3': [0, 0, 0, None, None]
+    ... })
+    >>> mapper6.fit_transform(data6)
+    array([[ 1.,  1.,  0.],
+           [ 1.,  0.,  0.],
+           [ 1.,  1.,  0.],
+           [ 2.,  1.,  0.],
+           [ 3.,  1.,  0.]])
+
+
 Feature selection and other supervised transformations
 ******************************************************
 

sklearn_pandas/__init__.py

@@ -3,3 +3,4 @@
 from .dataframe_mapper import DataFrameMapper  # NOQA
 from .cross_validation import cross_val_score, GridSearchCV, RandomizedSearchCV  # NOQA
 from .categorical_imputer import CategoricalImputer  # NOQA
+from .features_generator import gen_features  # NOQA

sklearn_pandas/features_generator.py

@@ -0,0 +1,55 @@
+def gen_features(columns, classes=None):
+    """Generates a feature definition list which can be passed
+    into DataFrameMapper
+
+    Params:
+
+    columns     a list of column names to generate features for.
+
+    classes     a list of classes for each feature, a list of dictionaries with
+                transformer class and init parameters, or None.
+
+                If list of classes is provided, then each of them is
+                instantiated with default arguments. Example:
+
+                    classes = [StandardScaler, LabelBinarizer]
+
+                If list of dictionaries is provided, then each of them should
+                have a 'class' key with transformer class. All other keys are
+                passed into 'class' value constructor. Example:
+
+                    classes = [
+                        {'class': StandardScaler, 'with_mean': False},
+                        {'class': LabelBinarizer}
+                    }]
+
+                If None value selected, then each feature left as is.
+
+    """
+    if classes is None:
+        return [(column, None) for column in columns]
+
+    feature_defs = []
+
+    for column in columns:
+        feature_transformers = []
+
+        classes = [cls for cls in classes if cls is not None]
+        if not classes:
+            feature_defs.append((column, None))
+
+        else:
+            for definition in classes:
+                if isinstance(definition, dict):
+                    params = definition.copy()
+                    klass = params.pop('class')
+                    feature_transformers.append(klass(**params))
+                else:
+                    feature_transformers.append(definition())
+
+            if not feature_transformers:
+                feature_transformers = None
+
+            feature_defs.append((column, feature_transformers))
+
+    return feature_defs

tests/test_features_generator.py

@@ -0,0 +1,118 @@
+from collections import Counter
+
+import pytest
+import numpy as np
+from pandas import DataFrame
+from numpy.testing import assert_array_equal
+
+from sklearn_pandas import DataFrameMapper
+from sklearn_pandas.features_generator import gen_features
+
+
+class MockClass(object):
+
+    def __init__(self, value=1, name='class'):
+        self.value = value
+        self.name = name
+
+
+class MockTransformer(object):
+
+    def __init__(self):
+        self.most_common_ = None
+
+    def fit(self, X, y=None):
+        [(value, _)] = Counter(X).most_common(1)
+        self.most_common_ = value
+        return self
+
+    def transform(self, X, y=None):
+        return np.asarray([self.most_common_] * len(X))
+
+
+@pytest.fixture
+def simple_dataset():
+    return DataFrame({
+        'feat1': [1, 2, 1, 3, 1],
+        'feat2': [1, 2, 2, 2, 3],
+        'feat3': [1, 2, 3, 4, 5],
+    })
+
+
+def test_generate_features_with_default_parameters():
+    """
+    Tests generating features from classes with default init arguments.
+    """
+    columns = ['colA', 'colB', 'colC']
+    feature_defs = gen_features(columns=columns, classes=[MockClass])
+    assert len(feature_defs) == len(columns)
+
+    feature_dict = dict(feature_defs)
+    assert columns == sorted(feature_dict.keys())
+
+    # default init arguments for MockClass for clarification.
+    expected = {'value': 1, 'name': 'class'}
+    for column, transformers in feature_dict.items():
+        for obj in transformers:
+            assert_attributes(obj, **expected)
+
+
+def test_generate_features_with_several_classes():
+    """
+    Tests generating features pipeline with different transformers parameters.
+    """
+    feature_defs = gen_features(
+        columns=['colA', 'colB', 'colC'],
+        classes=[
+            {'class': MockClass},
+            {'class': MockClass, 'name': 'mockA'},
+            {'class': MockClass, 'name': 'mockB', 'value': None}
+        ]
+    )
+
+    for transformers in dict(feature_defs).values():
+        assert_attributes(transformers[0], name='class', value=1)
+        assert_attributes(transformers[1], name='mockA', value=1)
+        assert_attributes(transformers[2], name='mockB', value=None)
+
+
+def test_generate_features_with_none_only_transformers():
+    """
+    Tests generating "dummy" feature definition which doesn't apply any
+    transformation.
+    """
+    feature_defs = gen_features(
+        columns=['colA', 'colB', 'colC'], classes=[None])
+
+    expected = [('colA', None),
+                ('colB', None),
+                ('colC', None)]
+
+    assert feature_defs == expected
+
+
+def test_compatibility_with_data_frame_mapper(simple_dataset):
+    """
+    Tests compatibility of generated feature definition with DataFrameMapper.
+    """
+    features_defs = gen_features(
+        columns=['feat1', 'feat2'],
+        classes=[MockTransformer])
+    features_defs.append(('feat3', None))
+
+    mapper = DataFrameMapper(features_defs)
+    X = mapper.fit_transform(simple_dataset)
+    expected = np.asarray([
+        [1, 2, 1],
+        [1, 2, 2],
+        [1, 2, 3],
+        [1, 2, 4],
+        [1, 2, 5]
+    ])
+
+    assert_array_equal(X, expected)
+
+
+def assert_attributes(obj, **attrs):
+    for attr, value in attrs.items():
+        assert getattr(obj, attr) == value