scikit-learn · OmarManzoor · Jul 24, 2023 · Jun 18, 2023 · Jun 18, 2023 · Jul 21, 2023
diff --git a/doc/whats_new/v1.4.rst b/doc/whats_new/v1.4.rst
@@ -66,6 +66,13 @@ Changelog
 - |Enhancement| :func:`base.clone` now supports `dict` as input and creates a
   copy. :pr:`26786` by `Adrin Jalali`_.
 
+:mod:`sklearn.cross_decomposition`
+..................................
+
+- |Fix| :class:`cross_decomposition.PLSRegression` now automatically ravels the output
+  of `predict` if fitted with one dimensional `y`.
+  :pr:`26602` by :user:`Yao Xiao <Charlie-XIAO>`.
+
 :mod:`sklearn.decomposition`
 ............................
 

diff --git a/sklearn/cross_decomposition/_pls.py b/sklearn/cross_decomposition/_pls.py
@@ -238,7 +238,10 @@ def fit(self, X, Y):
             Y, input_name="Y", dtype=np.float64, copy=self.copy, ensure_2d=False
         )
         if Y.ndim == 1:
+            self._predict_1d = True
             Y = Y.reshape(-1, 1)
+        else:
+            self._predict_1d = False
 
         n = X.shape[0]
         p = X.shape[1]
@@ -469,8 +472,8 @@ def predict(self, X, copy=True):
         # Normalize
         X -= self._x_mean
         X /= self._x_std
-        Ypred = X @ self.coef_.T
-        return Ypred + self.intercept_
+        Ypred = X @ self.coef_.T + self.intercept_
+        return Ypred.ravel() if self._predict_1d else Ypred
 
     def fit_transform(self, X, y=None):
         """Learn and apply the dimension reduction on the train data.

diff --git a/sklearn/cross_decomposition/tests/test_pls.py b/sklearn/cross_decomposition/tests/test_pls.py
@@ -12,7 +12,9 @@
     _svd_flip_1d,
 )
 from sklearn.datasets import load_linnerud, make_regression
+from sklearn.ensemble import VotingRegressor
 from sklearn.exceptions import ConvergenceWarning
+from sklearn.linear_model import LinearRegression
 from sklearn.utils import check_random_state
 from sklearn.utils.extmath import svd_flip
 
@@ -621,3 +623,24 @@ def test_pls_set_output(Klass):
     assert isinstance(y_trans, np.ndarray)
     assert isinstance(X_trans, pd.DataFrame)
     assert_array_equal(X_trans.columns, est.get_feature_names_out())
+
+
+def test_pls_regression_fit_1d_y():
+    """Check that when fitting with 1d `y`, prediction should also be 1d.
+
+    Non-regression test for Issue #26549.
+    """
+    X = np.array([[1, 1], [2, 4], [3, 9], [4, 16], [5, 25], [6, 36]])
+    y = np.array([2, 6, 12, 20, 30, 42])
+    expected = y.copy()
+
+    plsr = PLSRegression().fit(X, y)
+    y_pred = plsr.predict(X)
+    assert y_pred.shape == expected.shape
+
+    # Check that it works in VotingRegressor
+    lr = LinearRegression().fit(X, y)
+    vr = VotingRegressor([("lr", lr), ("plsr", plsr)])
+    y_pred = vr.fit(X, y).predict(X)
+    assert y_pred.shape == expected.shape
+    assert_allclose(y_pred, expected)