scikit-learn · jeremiedbb · Feb 20, 2023 · Jan 20, 2023 · Jan 20, 2023 · Jan 24, 2023
diff --git a/doc/whats_new/v1.3.rst b/doc/whats_new/v1.3.rst
@@ -179,6 +179,15 @@ Changelog
   dissimilarity is not a metric and cannot be supported by the BallTree.
   :pr:`25417` by :user:`Guillaume Lemaitre <glemaitre>`.
 
+:mod:`sklearn.neural_network`
+.............................
+
+- |Fix| :class:`neural_network.MLPRegressor` and :class:`neural_network.MLPClassifier`
+  reports the right `n_iter_` when `warm_start=True`. It corresponds to the number
+  of iterations performed on the current call to `fit` instead of the total number
+  of iterations performed since the initialization of the estimator.
+  :pr:`25443` by :user:`Marvin Krawutschke <Marvvxi>`.
+
 :mod:`sklearn.pipeline`
 .......................
 

diff --git a/sklearn/neural_network/_multilayer_perceptron.py b/sklearn/neural_network/_multilayer_perceptron.py
@@ -607,6 +607,7 @@ def _fit_stochastic(
             batch_size = np.clip(self.batch_size, 1, n_samples)
 
         try:
+            self.n_iter_ = 0
             for it in range(self.max_iter):
                 if self.shuffle:
                     # Only shuffle the sample indices instead of X and y to

diff --git a/sklearn/neural_network/tests/test_mlp.py b/sklearn/neural_network/tests/test_mlp.py
@@ -752,7 +752,7 @@ def test_warm_start_full_iteration(MLPEstimator):
     clf.fit(X, y)
     assert max_iter == clf.n_iter_
     clf.fit(X, y)
-    assert 2 * max_iter == clf.n_iter_
+    assert max_iter == clf.n_iter_
 
 
 def test_n_iter_no_change():
@@ -926,3 +926,25 @@ def test_mlp_warm_start_with_early_stopping(MLPEstimator):
     mlp.set_params(max_iter=20)
     mlp.fit(X_iris, y_iris)
     assert len(mlp.validation_scores_) > n_validation_scores
+
+
+@pytest.mark.parametrize("MLPEstimator", [MLPClassifier, MLPRegressor])
+@pytest.mark.parametrize("solver", ["sgd", "adam", "lbfgs"])
+def test_mlp_warm_start_no_convergence(MLPEstimator, solver):
+    """Check that we stop the number of iteration at `max_iter` when warm starting.
+
+    Non-regression test for:
+    https://github.com/scikit-learn/scikit-learn/issues/24764
+    """
+    model = MLPEstimator(
+        solver=solver, warm_start=True, early_stopping=False, max_iter=10
+    )
+
+    with pytest.warns(ConvergenceWarning):
+        model.fit(X_iris, y_iris)
+    assert model.n_iter_ == 10
+
+    model.set_params(max_iter=20)
+    with pytest.warns(ConvergenceWarning):
+        model.fit(X_iris, y_iris)
+    assert model.n_iter_ == 20