FIX Raise informative error when validation set is too small in MLPRegressor (#24788)

mohitthakur13 · Mohit Singh Thakur · Mohit Singh Thakur · web-flow · commit eec4449e2b36 · 2025-04-13T17:23:13.000+02:00
Co-authored-by: Mohit Singh Thakur &lt;mohit.thakur@tum.de&gt;
Co-authored-by: Mohit Singh Thakur &lt;thakur.mohit@gmail.com&gt;
Co-authored-by: Jérémie du Boisberranger &lt;jeremie@probabl.ai&gt;
diff --git a/doc/whats_new/upcoming_changes/sklearn.neural_network/24788.fix.rst b/doc/whats_new/upcoming_changes/sklearn.neural_network/24788.fix.rst
@@ -0,0 +1,3 @@
+:class:`neural_network.MLPRegressor` now raises an informative error when
+`early_stopping` is set and the computed validation set is too small.
+By :user:`David Shumway <davidshumway>`.
diff --git a/sklearn/neural_network/_multilayer_perceptron.py b/sklearn/neural_network/_multilayer_perceptron.py
@@ -668,6 +668,12 @@ def _fit_stochastic(
                     test_size=self.validation_fraction,
                     stratify=stratify,
                 )
+            if X_val.shape[0] < 2:
+                raise ValueError(
+                    "The validation set is too small. Increase 'validation_fraction' "
+                    "or the size of your dataset."
+                )
+
             if is_classifier(self):
                 y_val = self._label_binarizer.inverse_transform(y_val)
         else:
diff --git a/sklearn/neural_network/tests/test_mlp.py b/sklearn/neural_network/tests/test_mlp.py
@@ -1084,3 +1084,11 @@ def test_mlp_vs_poisson_glm_equivalent(global_random_seed):
         random_state=np.random.RandomState(global_random_seed + 1),
     ).fit(X, y)
     assert not np.allclose(mlp.predict(X), glm.predict(X), rtol=1e-4)
+
+
+def test_minimum_input_sample_size():
+    """Check error message when the validation set is too small."""
+    X, y = make_regression(n_samples=2, n_features=5, random_state=0)
+    model = MLPRegressor(early_stopping=True, random_state=0)
+    with pytest.raises(ValueError, match="The validation set is too small"):
+        model.fit(X, y)

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+:class:`neural_network.MLPRegressor` now raises an informative error when
	`2`	+`early_stopping` is set and the computed validation set is too small.
	`3`	+By :user:`David Shumway <davidshumway>`.