scikit-learn · ogrisel · Mar 11, 2024 · Aug 18, 2023 · Aug 18, 2023 · Aug 19, 2023
diff --git a/doc/modules/array_api.rst b/doc/modules/array_api.rst
@@ -105,6 +105,7 @@ Metrics
 -------
 
 - :func:`sklearn.metrics.accuracy_score`
+- :func:`sklearn.metrics.r2_score`
 - :func:`sklearn.metrics.zero_one_loss`
 
 Tools
@@ -115,6 +116,22 @@ Tools
 Coverage is expected to grow over time. Please follow the dedicated `meta-issue on GitHub
 <https://github.com/scikit-learn/scikit-learn/issues/22352>`_ to track progress.
 
+Type of return values and fitted attributes
+-------------------------------------------
+
+When calling functions or methods with Array API compatible inputs, the
+convention is to return array values of the same array container type and
+device as the input data.
+
+Similarly, when an estimator is fitted with Array API compatible inputs, the
+fitted attributes will be arrays from the same library as the input and stored
+on the same device. The `predict` and `transform` method subsequently expect
+inputs from the same array library and device as the data passed to the `fit`
+method.
+
+Note however that scoring functions that return scalar values return Python
+scalars (typically a `float` instance) instead of an array scalar value.
+
 Common estimator checks
 =======================
 

diff --git a/doc/whats_new/v1.5.rst b/doc/whats_new/v1.5.rst
@@ -22,6 +22,22 @@ Version 1.5.0
 
 **In Development**
 
+Support for Array API
+---------------------
+
+Additional estimators and functions have been updated to include support for all
+`Array API <https://data-apis.org/array-api/latest/>`_ compliant inputs.
+
+See :ref:`array_api` for more details.
+
+**Functions:**
+
+- :func:`sklearn.metrics.r2_score` now supports Array API compliant inputs.
+  :pr:`27904` by :user:`Eric Lindgren <elindgren>`, `Franck Charras <fcharras>`,
+  `Olivier Grisel <ogrisel>` and `Tim Head <betatim>`.
+
+**Classes:**
+
 Support for building with Meson
 -------------------------------
 

diff --git a/sklearn/metrics/_classification.py b/sklearn/metrics/_classification.py
@@ -38,7 +38,11 @@
     check_consistent_length,
     column_or_1d,
 )
-from ..utils._array_api import _union1d, _weighted_sum, get_namespace
+from ..utils._array_api import (
+    _average,
+    _union1d,
+    get_namespace,
+)
 from ..utils._param_validation import (
     Hidden,
     Interval,
@@ -224,7 +228,7 @@ def accuracy_score(y_true, y_pred, *, normalize=True, sample_weight=None):
     else:
         score = y_true == y_pred
 
-    return _weighted_sum(score, sample_weight, normalize)
+    return float(_average(score, weights=sample_weight, normalize=normalize))
 
 
 @validate_params(
@@ -2809,7 +2813,7 @@ def hamming_loss(y_true, y_pred, *, sample_weight=None):
         return n_differences / (y_true.shape[0] * y_true.shape[1] * weight_average)
 
     elif y_type in ["binary", "multiclass"]:
-        return _weighted_sum(y_true != y_pred, sample_weight, normalize=True)
+        return float(_average(y_true != y_pred, weights=sample_weight, normalize=True))
     else:
         raise ValueError("{0} is not supported".format(y_type))
 
@@ -2994,7 +2998,7 @@ def log_loss(
     y_pred = y_pred / y_pred_sum[:, np.newaxis]
     loss = -xlogy(transformed_labels, y_pred).sum(axis=1)
 
-    return _weighted_sum(loss, sample_weight, normalize)
+    return float(_average(loss, weights=sample_weight, normalize=normalize))
 
 
 @validate_params(

diff --git a/sklearn/metrics/_regression.py b/sklearn/metrics/_regression.py
@@ -34,6 +34,12 @@
 from scipy.special import xlogy
 
 from ..exceptions import UndefinedMetricWarning
+from ..utils._array_api import (
+    _average,
+    _find_matching_floating_dtype,
+    device,
+    get_namespace,
+)
 from ..utils._param_validation import Hidden, Interval, StrOptions, validate_params
 from ..utils.stats import _weighted_percentile
 from ..utils.validation import (
@@ -65,7 +71,7 @@
 ]
 
 
-def _check_reg_targets(y_true, y_pred, multioutput, dtype="numeric"):
+def _check_reg_targets(y_true, y_pred, multioutput, dtype="numeric", xp=None):
     """Check that y_true and y_pred belong to the same regression task.
 
     Parameters
@@ -99,15 +105,17 @@ def _check_reg_targets(y_true, y_pred, multioutput, dtype="numeric"):
         just the corresponding argument if ``multioutput`` is a
         correct keyword.
     """
+    xp, _ = get_namespace(y_true, y_pred, multioutput, xp=xp)
+
     check_consistent_length(y_true, y_pred)
     y_true = check_array(y_true, ensure_2d=False, dtype=dtype)
     y_pred = check_array(y_pred, ensure_2d=False, dtype=dtype)
 
     if y_true.ndim == 1:
-        y_true = y_true.reshape((-1, 1))
+        y_true = xp.reshape(y_true, (-1, 1))
 
     if y_pred.ndim == 1:
-        y_pred = y_pred.reshape((-1, 1))
+        y_pred = xp.reshape(y_pred, (-1, 1))
 
     if y_true.shape[1] != y_pred.shape[1]:
         raise ValueError(
@@ -855,9 +863,10 @@ def median_absolute_error(
 
 
 def _assemble_r2_explained_variance(
-    numerator, denominator, n_outputs, multioutput, force_finite
+    numerator, denominator, n_outputs, multioutput, force_finite, xp, device
 ):
     """Common part used by explained variance score and :math:`R^2` score."""
+    dtype = numerator.dtype
 
     nonzero_denominator = denominator != 0
 
@@ -868,12 +877,14 @@ def _assemble_r2_explained_variance(
         nonzero_numerator = numerator != 0
         # Default = Zero Numerator = perfect predictions. Set to 1.0
         # (note: even if denominator is zero, thus avoiding NaN scores)
-        output_scores = np.ones([n_outputs])
+        output_scores = xp.ones([n_outputs], device=device, dtype=dtype)
         # Non-zero Numerator and Non-zero Denominator: use the formula
         valid_score = nonzero_denominator & nonzero_numerator
+
         output_scores[valid_score] = 1 - (
             numerator[valid_score] / denominator[valid_score]
         )
+
         # Non-zero Numerator and Zero Denominator:
         # arbitrary set to 0.0 to avoid -inf scores
         output_scores[nonzero_numerator & ~nonzero_denominator] = 0.0
@@ -887,15 +898,18 @@ def _assemble_r2_explained_variance(
             avg_weights = None
         elif multioutput == "variance_weighted":
             avg_weights = denominator
-            if not np.any(nonzero_denominator):
+            if not xp.any(nonzero_denominator):
                 # All weights are zero, np.average would raise a ZeroDiv error.
                 # This only happens when all y are constant (or 1-element long)
                 # Since weights are all equal, fall back to uniform weights.
                 avg_weights = None
     else:
         avg_weights = multioutput
 
-    return np.average(output_scores, weights=avg_weights)
+    result = _average(output_scores, weights=avg_weights)
+    if result.size == 1:
+        return float(result)
+    return result
 
 
 @validate_params(
@@ -1033,6 +1047,9 @@ def explained_variance_score(
         n_outputs=y_true.shape[1],
         multioutput=multioutput,
         force_finite=force_finite,
+        xp=get_namespace(y_true)[0],
+        # TODO: update once Array API support is added to explained_variance_score.
+        device=None,
     )
 
 
@@ -1177,8 +1194,14 @@ def r2_score(
     >>> r2_score(y_true, y_pred, force_finite=False)
     -inf
     """
-    y_type, y_true, y_pred, multioutput = _check_reg_targets(
-        y_true, y_pred, multioutput
+    input_arrays = [y_true, y_pred, sample_weight, multioutput]
+    xp, _ = get_namespace(*input_arrays)
+    device_ = device(*input_arrays)
+
+    dtype = _find_matching_floating_dtype(y_true, y_pred, sample_weight, xp=xp)
+
+    _, y_true, y_pred, multioutput = _check_reg_targets(
+        y_true, y_pred, multioutput, dtype=dtype, xp=xp
     )
     check_consistent_length(y_true, y_pred, sample_weight)
 
@@ -1188,22 +1211,25 @@ def r2_score(
         return float("nan")
 
     if sample_weight is not None:
-        sample_weight = column_or_1d(sample_weight)
-        weight = sample_weight[:, np.newaxis]
+        sample_weight = column_or_1d(sample_weight, dtype=dtype)
+        weight = sample_weight[:, None]
     else:
         weight = 1.0
 
-    numerator = (weight * (y_true - y_pred) ** 2).sum(axis=0, dtype=np.float64)
-    denominator = (
-        weight * (y_true - np.average(y_true, axis=0, weights=sample_weight)) ** 2
-    ).sum(axis=0, dtype=np.float64)
+    numerator = xp.sum(weight * (y_true - y_pred) ** 2, axis=0)
+    denominator = xp.sum(
+        weight * (y_true - _average(y_true, axis=0, weights=sample_weight, xp=xp)) ** 2,
+        axis=0,
+    )
 
     return _assemble_r2_explained_variance(
         numerator=numerator,
         denominator=denominator,
         n_outputs=y_true.shape[1],
         multioutput=multioutput,
         force_finite=force_finite,
+        xp=xp,
+        device=device_,
     )
 
 

diff --git a/sklearn/metrics/tests/test_common.py b/sklearn/metrics/tests/test_common.py
@@ -55,6 +55,7 @@
 from sklearn.utils import shuffle
 from sklearn.utils._array_api import (
     _atol_for_type,
+    _convert_to_numpy,
     yield_namespace_device_dtype_combinations,
 )
 from sklearn.utils._testing import (
@@ -1749,7 +1750,7 @@ def check_array_api_metric(
         metric_xp = metric(y_true_xp, y_pred_xp, sample_weight=sample_weight)
 
         assert_allclose(
-            metric_xp,
+            _convert_to_numpy(xp.asarray(metric_xp), xp),
             metric_np,
             atol=_atol_for_type(dtype_name),
         )
@@ -1813,6 +1814,33 @@ def check_array_api_multiclass_classification_metric(
     )
 
 
+def check_array_api_regression_metric(metric, array_namespace, device, dtype_name):
+    y_true_np = np.array([[1, 3], [1, 2]], dtype=dtype_name)
+    y_pred_np = np.array([[1, 4], [1, 1]], dtype=dtype_name)
+
+    check_array_api_metric(
+        metric,
+        array_namespace,
+        device,
+        dtype_name,
+        y_true_np=y_true_np,
+        y_pred_np=y_pred_np,
+        sample_weight=None,
+    )
+
+    sample_weight = np.array([0.1, 2.0], dtype=dtype_name)
+
+    check_array_api_metric(
+        metric,
+        array_namespace,
+        device,
+        dtype_name,
+        y_true_np=y_true_np,
+        y_pred_np=y_pred_np,
+        sample_weight=sample_weight,
+    )
+
+
 array_api_metric_checkers = {
     accuracy_score: [
         check_array_api_binary_classification_metric,
@@ -1822,6 +1850,7 @@ def check_array_api_multiclass_classification_metric(
         check_array_api_binary_classification_metric,
         check_array_api_multiclass_classification_metric,
     ],
+    r2_score: [check_array_api_regression_metric],
 }