[MRG+1] Catch cases for different class size in MLPClassifier with wa…

…rm start (scikit-learn#7976)  (scikit-learn#8035)

* added test that fails

* generate standard value error for different class size

* moved num_classes one class down

* fixed over-indented lines

* standard error occurs a layer up.

* created a different label comparison for warm_start

* spaces around multiplication sign.

* reworded error and added another edge case.

* fixed pep8 violation

* make test shorter

* updated ignore warning

sklearn/neural_network/multilayer_perceptron.py

@@ -908,6 +908,13 @@ def _validate_input(self, X, y, incremental):
             self._label_binarizer = LabelBinarizer()
             self._label_binarizer.fit(y)
             self.classes_ = self._label_binarizer.classes_
+        elif self.warm_start:
+            classes = unique_labels(y)
+            if set(classes) != set(self.classes_):
+                raise ValueError("warm_start can only be used where `y` has "
+                                 "the same classes as in the previous "
+                                 "call to fit. Previously got %s, `y` has %s" %
+                                 (self.classes_, classes))
         else:
             classes = unique_labels(y)
             if np.setdiff1d(classes, self.classes_, assume_unique=True):
@@ -939,6 +946,25 @@ def predict(self, X):
 
         return self._label_binarizer.inverse_transform(y_pred)
 
+    def fit(self, X, y):
+        """Fit the model to data matrix X and target(s) y.
+
+        Parameters
+        ----------
+        X : array-like or sparse matrix, shape (n_samples, n_features)
+            The input data.
+
+        y : array-like, shape (n_samples,) or (n_samples, n_outputs)
+            The target values (class labels in classification, real numbers in
+            regression).
+
+        Returns
+        -------
+        self : returns a trained MLP model.
+        """
+        return self._fit(X, y, incremental=(self.warm_start and
+                                            hasattr(self, "classes_")))
+
     @property
     def partial_fit(self):
         """Fit the model to data matrix X and target y.

sklearn/neural_network/tests/test_mlp.py

@@ -12,7 +12,7 @@
 
 from numpy.testing import assert_almost_equal, assert_array_equal
 
-from sklearn.datasets import load_digits, load_boston
+from sklearn.datasets import load_digits, load_boston, load_iris
 from sklearn.datasets import make_regression, make_multilabel_classification
 from sklearn.exceptions import ConvergenceWarning
 from sklearn.externals.six.moves import cStringIO as StringIO
@@ -24,6 +24,7 @@
 from scipy.sparse import csr_matrix
 from sklearn.utils.testing import (assert_raises, assert_greater, assert_equal,
                                    assert_false, ignore_warnings)
+from sklearn.utils.testing import assert_raise_message
 
 
 np.seterr(all='warn')
@@ -49,6 +50,11 @@
 Xboston = StandardScaler().fit_transform(boston.data)[: 200]
 yboston = boston.target[:200]
 
+iris = load_iris()
+
+X_iris = iris.data
+y_iris = iris.target
+
 
 def test_alpha():
     # Test that larger alpha yields weights closer to zero
@@ -556,3 +562,29 @@ def test_adaptive_learning_rate():
     clf.fit(X, y)
     assert_greater(clf.max_iter, clf.n_iter_)
     assert_greater(1e-6, clf._optimizer.learning_rate)
+
+
+@ignore_warnings(RuntimeError)
+def test_warm_start():
+    X = X_iris
+    y = y_iris
+
+    y_2classes = np.array([0] * 75 + [1] * 75)
+    y_3classes = np.array([0] * 40 + [1] * 40 + [2] * 70)
+    y_3classes_alt = np.array([0] * 50 + [1] * 50 + [3] * 50)
+    y_4classes = np.array([0] * 37 + [1] * 37 + [2] * 38 + [3] * 38)
+    y_5classes = np.array([0] * 30 + [1] * 30 + [2] * 30 + [3] * 30 + [4] * 30)
+
+    # No error raised
+    clf = MLPClassifier(hidden_layer_sizes=2, solver='lbfgs',
+                        warm_start=True).fit(X, y)
+    clf.fit(X, y)
+    clf.fit(X, y_3classes)
+
+    for y_i in (y_2classes, y_3classes_alt, y_4classes, y_5classes):
+        clf = MLPClassifier(hidden_layer_sizes=2, solver='lbfgs',
+                            warm_start=True).fit(X, y)
+        message = ('warm_start can only be used where `y` has the same '
+                   'classes as in the previous call to fit.'
+                   ' Previously got [0 1 2], `y` has %s' % np.unique(y_i))
+        assert_raise_message(ValueError, message, clf.fit, X, y_i)