ENH Sample weights for median_absolute_error

doc/whats_new/v0.24.rst

@@ -76,7 +76,13 @@ Changelog
   attribute name/path or a `callable` for extracting feature importance from
   the estimator.  :pr:`15361` by :user:`Venkatachalam N <venkyyuvy>`
 
-
+:mod:`sklearn.metrics`
+......................
+
+- |Enhancement| Add `sample_weight` parameter to
+  :class:`metrics.median_absolute_error`.
+  :pr:`17225` by :user:`Lucy Liu <lucyleeow>`.
+
 :mod:`sklearn.tree`
 ...................
 

sklearn/ensemble/tests/test_gradient_boosting_loss_functions.py

@@ -8,7 +8,6 @@
 import pytest
 
 from sklearn.utils import check_random_state
-from sklearn.utils.stats import _weighted_percentile
 from sklearn.ensemble._gb_losses import RegressionLossFunction
 from sklearn.ensemble._gb_losses import LeastSquaresError
 from sklearn.ensemble._gb_losses import LeastAbsoluteError
@@ -103,36 +102,6 @@ def test_sample_weight_init_estimators():
         assert_allclose(out, sw_out, rtol=1e-2)
 
 
-def test_weighted_percentile():
-    y = np.empty(102, dtype=np.float64)
-    y[:50] = 0
-    y[-51:] = 2
-    y[-1] = 100000
-    y[50] = 1
-    sw = np.ones(102, dtype=np.float64)
-    sw[-1] = 0.0
-    score = _weighted_percentile(y, sw, 50)
-    assert score == 1
-
-
-def test_weighted_percentile_equal():
-    y = np.empty(102, dtype=np.float64)
-    y.fill(0.0)
-    sw = np.ones(102, dtype=np.float64)
-    sw[-1] = 0.0
-    score = _weighted_percentile(y, sw, 50)
-    assert score == 0
-
-
-def test_weighted_percentile_zero_weight():
-    y = np.empty(102, dtype=np.float64)
-    y.fill(1.0)
-    sw = np.ones(102, dtype=np.float64)
-    sw.fill(0.0)
-    score = _weighted_percentile(y, sw, 50)
-    assert score == 1.0
-
-
 def test_quantile_loss_function():
     # Non regression test for the QuantileLossFunction object
     # There was a sign problem when evaluating the function

sklearn/metrics/_regression.py

@@ -30,6 +30,8 @@
                                 _num_samples)
 from ..utils.validation import column_or_1d
 from ..utils.validation import _deprecate_positional_args
+from ..utils.validation import _check_sample_weight
+from ..utils.stats import _weighted_percentile
 from ..exceptions import UndefinedMetricWarning
 
 
@@ -335,7 +337,8 @@ def mean_squared_log_error(y_true, y_pred, *,
 
 
 @_deprecate_positional_args
-def median_absolute_error(y_true, y_pred, *, multioutput='uniform_average'):
+def median_absolute_error(y_true, y_pred, *, multioutput='uniform_average',
+                          sample_weight=None):
     """Median absolute error regression loss
 
     Median absolute error output is non-negative floating point. The best value
@@ -360,6 +363,11 @@ def median_absolute_error(y_true, y_pred, *, multioutput='uniform_average'):
         'uniform_average' :
             Errors of all outputs are averaged with uniform weight.
 
+    sample_weight : array-like of shape (n_samples,), default=None
+        Sample weights.
+
+        .. versionadded:: 0.24
+
     Returns
     -------
     loss : float or ndarray of floats
@@ -387,7 +395,12 @@ def median_absolute_error(y_true, y_pred, *, multioutput='uniform_average'):
     """
     y_type, y_true, y_pred, multioutput = _check_reg_targets(
         y_true, y_pred, multioutput)
-    output_errors = np.median(np.abs(y_pred - y_true), axis=0)
+    if sample_weight is None:
+        output_errors = np.median(np.abs(y_pred - y_true), axis=0)
+    else:
+        sample_weight = _check_sample_weight(sample_weight, y_pred)
+        output_errors = _weighted_percentile(np.abs(y_pred - y_true),
+                                             sample_weight=sample_weight)
     if isinstance(multioutput, str):
         if multioutput == 'raw_values':
             return output_errors

sklearn/metrics/tests/test_score_objects.py

@@ -33,7 +33,7 @@
 from sklearn.linear_model import Ridge, LogisticRegression, Perceptron
 from sklearn.tree import DecisionTreeClassifier, DecisionTreeRegressor
 from sklearn.datasets import make_blobs
-from sklearn.datasets import make_classification
+from sklearn.datasets import make_classification, make_regression
 from sklearn.datasets import make_multilabel_classification
 from sklearn.datasets import load_diabetes
 from sklearn.model_selection import train_test_split, cross_val_score
@@ -89,7 +89,7 @@ def _make_estimators(X_train, y_train, y_ml_train):
     # Make estimators that make sense to test various scoring methods
     sensible_regr = DecisionTreeRegressor(random_state=0)
     # some of the regressions scorers require strictly positive input.
-    sensible_regr.fit(X_train, y_train + 1)
+    sensible_regr.fit(X_train, _require_positive_y(y_train))
     sensible_clf = DecisionTreeClassifier(random_state=0)
     sensible_clf.fit(X_train, y_train)
     sensible_ml_clf = DecisionTreeClassifier(random_state=0)
@@ -474,8 +474,9 @@ def test_raises_on_score_list():
 
 
 @ignore_warnings
-def test_scorer_sample_weight():
-    # Test that scorers support sample_weight or raise sensible errors
+def test_classification_scorer_sample_weight():
+    # Test that classification scorers support sample_weight or raise sensible
+    # errors
 
     # Unlike the metrics invariance test, in the scorer case it's harder
     # to ensure that, on the classifier output, weighted and unweighted
@@ -493,31 +494,70 @@ def test_scorer_sample_weight():
     estimator = _make_estimators(X_train, y_train, y_ml_train)
 
     for name, scorer in SCORERS.items():
+        if name in REGRESSION_SCORERS:
+            # skip the regression scores
+            continue
         if name in MULTILABEL_ONLY_SCORERS:
             target = y_ml_test
         else:
             target = y_test
-        if name in REQUIRE_POSITIVE_Y_SCORERS:
-            target = _require_positive_y(target)
         try:
             weighted = scorer(estimator[name], X_test, target,
                               sample_weight=sample_weight)
             ignored = scorer(estimator[name], X_test[10:], target[10:])
             unweighted = scorer(estimator[name], X_test, target)
             assert weighted != unweighted, (
-                "scorer {0} behaves identically when "
-                "called with sample weights: {1} vs "
-                "{2}".format(name, weighted, unweighted))
+                f"scorer {name} behaves identically when called with "
+                f"sample weights: {weighted} vs {unweighted}")
             assert_almost_equal(weighted, ignored,
-                                err_msg="scorer {0} behaves differently when "
-                                "ignoring samples and setting sample_weight to"
-                                " 0: {1} vs {2}".format(name, weighted,
-                                                        ignored))
+                                err_msg=f"scorer {name} behaves differently "
+                                "when ignoring samples and setting "
+                                "sample_weight to 0: {weighted} vs {ignored}")
 
         except TypeError as e:
             assert "sample_weight" in str(e), (
-                "scorer {0} raises unhelpful exception when called "
-                "with sample weights: {1}".format(name, str(e)))
+                   f"scorer {name} raises unhelpful exception when called "
+                   f"with sample weights: {str(e)}")
+
+
+@ignore_warnings
+def test_regression_scorer_sample_weight():
+    # Test that regression scorers support sample_weight or raise sensible
+    # errors
+
+    # Odd number of test samples req for neg_median_absolute_error
+    X, y = make_regression(n_samples=101, n_features=20, random_state=0)
+    y = _require_positive_y(y)
+    X_train, X_test, y_train, y_test = train_test_split(X, y, random_state=0)
+
+    sample_weight = np.ones_like(y_test)
+    # Odd number req for neg_median_absolute_error
+    sample_weight[:11] = 0
+
+    reg = DecisionTreeRegressor(random_state=0)
+    reg.fit(X_train, y_train)
+
+    for name, scorer in SCORERS.items():
+        if name not in REGRESSION_SCORERS:
+            # skip classification scorers
+            continue
+        try:
+            weighted = scorer(reg, X_test, y_test,
+                              sample_weight=sample_weight)
+            ignored = scorer(reg, X_test[11:], y_test[11:])
+            unweighted = scorer(reg, X_test, y_test)
+            assert weighted != unweighted, (
+                f"scorer {name} behaves identically when called with "
+                f"sample weights: {weighted} vs {unweighted}")
+            assert_almost_equal(weighted, ignored,
+                                err_msg=f"scorer {name} behaves differently "
+                                "when ignoring samples and setting "
+                                "sample_weight to 0: {weighted} vs {ignored}")
+
+        except TypeError as e:
+            assert "sample_weight" in str(e), (
+                   f"scorer {name} raises unhelpful exception when called "
+                   f"with sample weights: {str(e)}")
 
 
 @pytest.mark.parametrize('name', SCORERS)

sklearn/utils/fixes.py

@@ -165,3 +165,39 @@ class loguniform(scipy.stats.reciprocal):
 )
 class MaskedArray(_MaskedArray):
     pass  # TODO: remove in 0.25
+
+
+def _take_along_axis(arr, indices, axis):
+    """Implements a simplified version of np.take_along_axis if numpy
+    version < 1.15"""
+    if np_version > (1, 14):
+        return np.take_along_axis(arr=arr, indices=indices, axis=axis)
+    else:
+        if axis is None:
+            arr = arr.flatten()
+
+        if not np.issubdtype(indices.dtype, np.intp):
+            raise IndexError('`indices` must be an integer array')
+        if arr.ndim != indices.ndim:
+            raise ValueError(
+                "`indices` and `arr` must have the same number of dimensions")
+
+        shape_ones = (1,) * indices.ndim
+        dest_dims = (
+            list(range(axis)) +
+            [None] +
+            list(range(axis+1, indices.ndim))
+        )
+
+        # build a fancy index, consisting of orthogonal aranges, with the
+        # requested index inserted at the right location
+        fancy_index = []
+        for dim, n in zip(dest_dims, arr.shape):
+            if dim is None:
+                fancy_index.append(indices)
+            else:
+                ind_shape = shape_ones[:dim] + (-1,) + shape_ones[dim+1:]
+                fancy_index.append(np.arange(n).reshape(ind_shape))
+
+        fancy_index = tuple(fancy_index)
+        return arr[fancy_index]

sklearn/utils/stats.py

@@ -1,18 +1,61 @@
 import numpy as np
 
 from .extmath import stable_cumsum
+from .fixes import _take_along_axis
 
 
 def _weighted_percentile(array, sample_weight, percentile=50):
+    """Compute weighted percentile
+
+    Computes lower weighted percentile. If `array` is a 2D array, the
+    `percentile` is computed along the axis 0.
+
+        .. versionchanged:: 0.24
+            Accepts 2D `array`.
+
+    Parameters
+    ----------
+    array : 1D or 2D array
+        Values to take the weighted percentile of.
+
+    sample_weight: 1D or 2D array
+        Weights for each value in `array`. Must be same shape as `array` or
+        of shape `(array.shape[0],)`.
+
+    percentile: int, default=50
+        Percentile to compute. Must be value between 0 and 100.
+
+    Returns
+    -------
+    percentile : int if `array` 1D, ndarray if `array` 2D
+        Weighted percentile.
     """
-    Compute the weighted ``percentile`` of ``array`` with ``sample_weight``.
-    """
-    sorted_idx = np.argsort(array)
+    n_dim = array.ndim
+    if n_dim == 0:
+        return array[()]
+    if array.ndim == 1:
+        array = array.reshape((-1, 1))
+    # When sample_weight 1D, repeat for each array.shape[1]
+    if (array.shape != sample_weight.shape and
+            array.shape[0] == sample_weight.shape[0]):
+        sample_weight = np.tile(sample_weight, (array.shape[1], 1)).T
+    sorted_idx = np.argsort(array, axis=0)
+    sorted_weights = _take_along_axis(sample_weight, sorted_idx, axis=0)
 
     # Find index of median prediction for each sample
-    weight_cdf = stable_cumsum(sample_weight[sorted_idx])
-    percentile_idx = np.searchsorted(
-        weight_cdf, (percentile / 100.) * weight_cdf[-1])
-    # in rare cases, percentile_idx equals to len(sorted_idx)
-    percentile_idx = np.clip(percentile_idx, 0, len(sorted_idx)-1)
-    return array[sorted_idx[percentile_idx]]
+    weight_cdf = stable_cumsum(sorted_weights, axis=0)
+    adjusted_percentile = percentile / 100 * weight_cdf[-1]
+    percentile_idx = np.array([
+        np.searchsorted(weight_cdf[:, i], adjusted_percentile[i])
+        for i in range(weight_cdf.shape[1])
+    ])
+    percentile_idx = np.array(percentile_idx)
+    # In rare cases, percentile_idx equals to sorted_idx.shape[0]
+    max_idx = sorted_idx.shape[0] - 1
+    percentile_idx = np.apply_along_axis(lambda x: np.clip(x, 0, max_idx),
+                                         axis=0, arr=percentile_idx)
+
+    col_index = np.arange(array.shape[1])
+    percentile_in_sorted = sorted_idx[percentile_idx, col_index]
+    percentile = array[percentile_in_sorted, col_index]
+    return percentile[0] if n_dim == 1 else percentile