MNT Add _check_sample_weights to classification metrics (#31701)

lucyleeow · web-flow · commit 27e52567278a · 2025-07-27T12:12:38.000+05:00
diff --git a/doc/whats_new/upcoming_changes/sklearn.metrics/31701.fix.rst b/doc/whats_new/upcoming_changes/sklearn.metrics/31701.fix.rst
@@ -0,0 +1,22 @@
+
+- Additional `sample_weight` checking has been added to
+  :func:`metrics.accuracy_score`,
+  :func:`metrics.balanced_accuracy_score`,
+  :func:`metrics.brier_score_loss`,
+  :func:`metrics.class_likelihood_ratios`,
+  :func:`metrics.classification_report`,
+  :func:`metrics.cohen_kappa_score`,
+  :func:`metrics.confusion_matrix`,
+  :func:`metrics.f1_score`,
+  :func:`metrics.fbeta_score`,
+  :func:`metrics.hamming_loss`,
+  :func:`metrics.jaccard_score`,
+  :func:`metrics.matthews_corrcoef`,
+  :func:`metrics.multilabel_confusion_matrix`,
+  :func:`metrics.precision_recall_fscore_support`,
+  :func:`metrics.precision_score`,
+  :func:`metrics.recall_score` and
+  :func:`metrics.zero_one_loss`.
+  `sample_weight` can only be 1D, consistent to `y_true` and `y_pred` in length,and
+  all values must be finite and not complex.
+  By :user:`Lucy Liu <lucyleeow>`.
diff --git a/sklearn/metrics/_classification.py b/sklearn/metrics/_classification.py
@@ -66,7 +66,7 @@ def _check_zero_division(zero_division):
         return np.nan
 
 
-def _check_targets(y_true, y_pred):
+def _check_targets(y_true, y_pred, sample_weight=None):
     """Check that y_true and y_pred belong to the same classification task.
 
     This converts multiclass or binary types to a common shape, and raises a
@@ -83,6 +83,8 @@ def _check_targets(y_true, y_pred):
 
     y_pred : array-like
 
+    sample_weight : array-like, default=None
+
     Returns
     -------
     type_true : one of {'multilabel-indicator', 'multiclass', 'binary'}
@@ -92,11 +94,17 @@ def _check_targets(y_true, y_pred):
     y_true : array or indicator matrix
 
     y_pred : array or indicator matrix
+
+    sample_weight : array or None
     """
-    xp, _ = get_namespace(y_true, y_pred)
-    check_consistent_length(y_true, y_pred)
+    xp, _ = get_namespace(y_true, y_pred, sample_weight)
+    check_consistent_length(y_true, y_pred, sample_weight)
     type_true = type_of_target(y_true, input_name="y_true")
     type_pred = type_of_target(y_pred, input_name="y_pred")
+    if sample_weight is not None:
+        sample_weight = _check_sample_weight(
+            sample_weight, y_true, force_float_dtype=False
+        )
 
     y_type = {type_true, type_pred}
     if y_type == {"binary", "multiclass"}:
@@ -148,7 +156,7 @@ def _check_targets(y_true, y_pred):
             y_pred = csr_matrix(y_pred)
         y_type = "multilabel-indicator"
 
-    return y_type, y_true, y_pred
+    return y_type, y_true, y_pred, sample_weight
 
 
 def _validate_multiclass_probabilistic_prediction(
@@ -200,6 +208,9 @@ def _validate_multiclass_probabilistic_prediction(
         raise ValueError(f"y_prob contains values lower than 0: {y_prob.min()}")
 
     check_consistent_length(y_prob, y_true, sample_weight)
+    if sample_weight is not None:
+        _check_sample_weight(sample_weight, y_true, force_float_dtype=False)
+
     lb = LabelBinarizer()
 
     if labels is not None:
@@ -356,8 +367,9 @@ def accuracy_score(y_true, y_pred, *, normalize=True, sample_weight=None):
     xp, _, device = get_namespace_and_device(y_true, y_pred, sample_weight)
     # Compute accuracy for each possible representation
     y_true, y_pred = attach_unique(y_true, y_pred)
-    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
-    check_consistent_length(y_true, y_pred, sample_weight)
+    y_type, y_true, y_pred, sample_weight = _check_targets(
+        y_true, y_pred, sample_weight
+    )
 
     if y_type.startswith("multilabel"):
         differing_labels = _count_nonzero(y_true - y_pred, xp=xp, device=device, axis=1)
@@ -464,7 +476,9 @@ def confusion_matrix(
     (0, 2, 1, 1)
     """
     y_true, y_pred = attach_unique(y_true, y_pred)
-    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
+    y_type, y_true, y_pred, sample_weight = _check_targets(
+        y_true, y_pred, sample_weight
+    )
     if y_type not in ("binary", "multiclass"):
         raise ValueError("%s is not supported" % y_type)
 
@@ -482,10 +496,6 @@ def confusion_matrix(
 
     if sample_weight is None:
         sample_weight = np.ones(y_true.shape[0], dtype=np.int64)
-    else:
-        sample_weight = np.asarray(sample_weight)
-
-    check_consistent_length(y_true, y_pred, sample_weight)
 
     n_labels = labels.size
     # If labels are not consecutive integers starting from zero, then
@@ -654,11 +664,10 @@ def multilabel_confusion_matrix(
             [1, 2]]])
     """
     y_true, y_pred = attach_unique(y_true, y_pred)
-    xp, _, device_ = get_namespace_and_device(y_true, y_pred)
-    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
-    if sample_weight is not None:
-        sample_weight = column_or_1d(sample_weight, device=device_)
-    check_consistent_length(y_true, y_pred, sample_weight)
+    xp, _, device_ = get_namespace_and_device(y_true, y_pred, sample_weight)
+    y_type, y_true, y_pred, sample_weight = _check_targets(
+        y_true, y_pred, sample_weight
+    )
 
     if y_type not in ("binary", "multiclass", "multilabel-indicator"):
         raise ValueError("%s is not supported" % y_type)
@@ -1171,8 +1180,9 @@ def matthews_corrcoef(y_true, y_pred, *, sample_weight=None):
     -0.33
     """
     y_true, y_pred = attach_unique(y_true, y_pred)
-    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
-    check_consistent_length(y_true, y_pred, sample_weight)
+    y_type, y_true, y_pred, sample_weight = _check_targets(
+        y_true, y_pred, sample_weight
+    )
     if y_type not in {"binary", "multiclass"}:
         raise ValueError("%s is not supported" % y_type)
 
@@ -1759,7 +1769,7 @@ def _check_set_wise_labels(y_true, y_pred, average, labels, pos_label):
         raise ValueError("average has to be one of " + str(average_options))
 
     y_true, y_pred = attach_unique(y_true, y_pred)
-    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
+    y_type, y_true, y_pred, _ = _check_targets(y_true, y_pred)
     # Convert to Python primitive type to avoid NumPy type / Python str
     # comparison. See https://github.com/numpy/numpy/issues/6784
     present_labels = _tolist(unique_labels(y_true, y_pred))
@@ -2227,7 +2237,9 @@ class are present in `y_true`): both likelihood ratios are undefined.
     # remove `FutureWarning`, and the Warns section in the docstring should not mention
     # `raise_warning` anymore.
     y_true, y_pred = attach_unique(y_true, y_pred)
-    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
+    y_type, y_true, y_pred, sample_weight = _check_targets(
+        y_true, y_pred, sample_weight
+    )
     if y_type != "binary":
         raise ValueError(
             "class_likelihood_ratios only supports binary classification "
@@ -2945,7 +2957,9 @@ class 2       1.00      0.67      0.80         3
     """
 
     y_true, y_pred = attach_unique(y_true, y_pred)
-    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
+    y_type, y_true, y_pred, sample_weight = _check_targets(
+        y_true, y_pred, sample_weight
+    )
 
     if labels is None:
         labels = unique_labels(y_true, y_pred)
@@ -3134,15 +3148,15 @@ def hamming_loss(y_true, y_pred, *, sample_weight=None):
     0.75
     """
     y_true, y_pred = attach_unique(y_true, y_pred)
-    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
-    check_consistent_length(y_true, y_pred, sample_weight)
+    y_type, y_true, y_pred, sample_weight = _check_targets(
+        y_true, y_pred, sample_weight
+    )
 
     xp, _, device = get_namespace_and_device(y_true, y_pred, sample_weight)
 
     if sample_weight is None:
         weight_average = 1.0
     else:
-        sample_weight = xp.asarray(sample_weight, device=device)
         weight_average = _average(sample_weight, xp=xp)
 
     if y_type.startswith("multilabel"):
@@ -3440,6 +3454,8 @@ def _validate_binary_probabilistic_prediction(y_true, y_prob, sample_weight, pos
     assert_all_finite(y_prob)
 
     check_consistent_length(y_prob, y_true, sample_weight)
+    if sample_weight is not None:
+        _check_sample_weight(sample_weight, y_true, force_float_dtype=False)
 
     y_type = type_of_target(y_true, input_name="y_true")
     if y_type != "binary":
diff --git a/sklearn/metrics/tests/test_classification.py b/sklearn/metrics/tests/test_classification.py
@@ -596,7 +596,7 @@ def test_multilabel_confusion_matrix_errors():
     # Bad sample_weight
     with pytest.raises(ValueError, match="inconsistent numbers of samples"):
         multilabel_confusion_matrix(y_true, y_pred, sample_weight=[1, 2])
-    with pytest.raises(ValueError, match="should be a 1d array"):
+    with pytest.raises(ValueError, match="Sample weights must be 1D array or scalar"):
         multilabel_confusion_matrix(
             y_true, y_pred, sample_weight=[[1, 2, 3], [2, 3, 4], [3, 4, 5]]
         )
@@ -2541,7 +2541,7 @@ def test__check_targets():
                         _check_targets(y1, y2)
 
         else:
-            merged_type, y1out, y2out = _check_targets(y1, y2)
+            merged_type, y1out, y2out, _ = _check_targets(y1, y2)
             assert merged_type == expected
             if merged_type.startswith("multilabel"):
                 assert y1out.format == "csr"
diff --git a/sklearn/metrics/tests/test_common.py b/sklearn/metrics/tests/test_common.py
@@ -881,6 +881,38 @@ def test_format_invariance_with_1d_vectors(name):
                     metric(y1_row, y2_row)
 
 
+@pytest.mark.parametrize("metric", CLASSIFICATION_METRICS.values())
+def test_classification_with_invalid_sample_weight(metric):
+    # Check invalid `sample_weight` raises correct error
+    random_state = check_random_state(0)
+    n_samples = 20
+    y1 = random_state.randint(0, 2, size=(n_samples,))
+    y2 = random_state.randint(0, 2, size=(n_samples,))
+
+    sample_weight = random_state.random_sample(size=(n_samples - 1,))
+    with pytest.raises(ValueError, match="Found input variables with inconsistent"):
+        metric(y1, y2, sample_weight=sample_weight)
+
+    sample_weight = random_state.random_sample(size=(n_samples,))
+    sample_weight[0] = np.inf
+    with pytest.raises(ValueError, match="Input sample_weight contains infinity"):
+        metric(y1, y2, sample_weight=sample_weight)
+
+    sample_weight[0] = np.nan
+    with pytest.raises(ValueError, match="Input sample_weight contains NaN"):
+        metric(y1, y2, sample_weight=sample_weight)
+
+    sample_weight = np.array([1 + 2j, 3 + 4j, 5 + 7j])
+    with pytest.raises(ValueError, match="Complex data not supported"):
+        metric(y1[:3], y2[:3], sample_weight=sample_weight)
+
+    sample_weight = random_state.random_sample(size=(n_samples * 2,)).reshape(
+        (n_samples, 2)
+    )
+    with pytest.raises(ValueError, match="Sample weights must be 1D array or scalar"):
+        metric(y1, y2, sample_weight=sample_weight)
+
+
 @pytest.mark.parametrize(
     "name", sorted(set(CLASSIFICATION_METRICS) - METRIC_UNDEFINED_BINARY_MULTICLASS)
 )
diff --git a/sklearn/utils/validation.py b/sklearn/utils/validation.py
@@ -2134,7 +2134,13 @@ def _check_psd_eigenvalues(lambdas, enable_warnings=False):
 
 
 def _check_sample_weight(
-    sample_weight, X, *, dtype=None, ensure_non_negative=False, copy=False
+    sample_weight,
+    X,
+    *,
+    dtype=None,
+    force_float_dtype=True,
+    ensure_non_negative=False,
+    copy=False,
 ):
     """Validate sample weights.
 
@@ -2162,6 +2168,10 @@ def _check_sample_weight(
         If `dtype` is not `{np.float32, np.float64, None}`, then output will
         be `np.float64`.
 
+    force_float_dtype : bool, default=True
+        Whether `X` should be forced to be float dtype, when `dtype` is a non-float
+        dtype or None.
+
     ensure_non_negative : bool, default=False,
         Whether or not the weights are expected to be non-negative.
 
@@ -2185,15 +2195,15 @@ def _check_sample_weight(
     float_dtypes = (
         [xp.float32] if max_float_type == xp.float32 else [xp.float64, xp.float32]
     )
-    if dtype is not None and dtype not in float_dtypes:
+    if force_float_dtype and dtype is not None and dtype not in float_dtypes:
         dtype = max_float_type
 
     if sample_weight is None:
         sample_weight = xp.ones(n_samples, dtype=dtype, device=device)
     elif isinstance(sample_weight, numbers.Number):
         sample_weight = xp.full(n_samples, sample_weight, dtype=dtype, device=device)
     else:
-        if dtype is None:
+        if force_float_dtype and dtype is None:
             dtype = float_dtypes
         sample_weight = check_array(
             sample_weight,
