FIX Preserve parameters when slicing a pipeline (#18429)

Co-authored-by: Paweł Biernat <pawel.biernat@ardigen.com>

doc/whats_new/v0.24.rst

@@ -530,6 +530,11 @@ Changelog
   will raise a ``ValueError``.
   :pr:`17876` by :user:`Cary Goltermann <Ultramann>`.
 
+- |Fix| A slice of a :class:`pipeline.Pipeline` now inherits the parameters of
+  the original pipeline (`memory` and `verbose`).
+  :pr:`18429` by :user:`Albert Villanova del Moral <albertvillanova>` and
+  :user:`Paweł Biernat <pwl>`.
+
 :mod:`sklearn.preprocessing`
 ............................
 

sklearn/pipeline.py

@@ -207,8 +207,10 @@ def __getitem__(self, ind):
         """
         if isinstance(ind, slice):
             if ind.step not in (1, None):
-                raise ValueError('Pipeline slicing only supports a step of 1')
-            return self.__class__(self.steps[ind])
+                raise ValueError("Pipeline slicing only supports a step of 1")
+            return self.__class__(
+                self.steps[ind], memory=self.memory, verbose=self.verbose
+            )
         try:
             name, est = self.steps[ind]
         except TypeError:

sklearn/tests/test_pipeline.py

@@ -558,15 +558,32 @@ def test_pipeline_fit_transform():
     assert_array_almost_equal(X_trans, X_trans2)
 
 
-def test_pipeline_slice():
-    pipe = Pipeline([('transf1', Transf()),
-                     ('transf2', Transf()),
-                     ('clf', FitParamT())])
-    pipe2 = pipe[:-1]
-    assert isinstance(pipe2, Pipeline)
-    assert pipe2.steps == pipe.steps[:-1]
-    assert 2 == len(pipe2.named_steps)
-    assert_raises(ValueError, lambda: pipe[::-1])
+@pytest.mark.parametrize("start, end", [(0, 1), (0, 2), (1, 2), (1, 3),
+                                        (None, 1), (1, None), (None, None)])
+def test_pipeline_slice(start, end):
+    pipe = Pipeline(
+        [("transf1", Transf()), ("transf2", Transf()), ("clf", FitParamT())],
+        memory="123",
+        verbose=True,
+    )
+    pipe_slice = pipe[start:end]
+    # Test class
+    assert isinstance(pipe_slice, Pipeline)
+    # Test steps
+    assert pipe_slice.steps == pipe.steps[start:end]
+    # Test named_steps attribute
+    assert list(pipe_slice.named_steps.items()) == list(
+        pipe.named_steps.items())[start:end]
+    # Test the rest of the parameters
+    pipe_params = pipe.get_params(deep=False)
+    pipe_slice_params = pipe_slice.get_params(deep=False)
+    del pipe_params["steps"]
+    del pipe_slice_params["steps"]
+    assert pipe_params == pipe_slice_params
+    # Test exception
+    msg = "Pipeline slicing only supports a step of 1"
+    with pytest.raises(ValueError, match=msg):
+        pipe[start:end:-1]
 
 
 def test_pipeline_index():