scikit-learn · jnothman · Feb 22, 2017
diff --git a/sklearn/pipeline.py b/sklearn/pipeline.py
@@ -7,6 +7,8 @@
 #         Virgile Fritsch
 #         Alexandre Gramfort
 #         Lars Buitinck
+#         Joel Nothman
+#         Guillaume Lemaitre
 # License: BSD
 
 from collections import defaultdict
@@ -233,6 +235,41 @@ def named_steps(self):
     def _final_estimator(self):
         return self.steps[-1][1]
 
+    def get_subsequence(self, start=None, stop=None):
+        """Extract a Pipeline consisting of a subsequence of steps
+
+        Parameters
+        ----------
+        start : int or str, optional
+            The index (0-based) or name of the step where the extracted
+            subsequence begins (inclusive bound).  By default, get from the
+            beginning.  Negative integers are intpreted as subtracted from the
+            number of steps in the Pipeline.
+        stop : int or str, optional
+            The index (0-based) or name of the step before which the extracted
+            subsequence ends (exclusive bound).  By default, get until the end.
+            Negative integers are intpreted as subtracted from the number of
+            steps in the Pipeline.
+
+        Returns
+        -------
+        sub_pipeline : Pipeline instance
+            The steps of this pipeline range from the start step to the stop
+            step specified.  The constituent estimators are not copied: if the
+            Pipeline had been fit, so will be the returned Pipeline.
+
+            The return type will be of the same type as self, if a subclass
+            is used and if its constructor is compatible.
+        """
+        if isinstance(start, six.string_types):
+            start = [name for name, _ in self.steps].index(start)
+        if isinstance(stop, six.string_types):
+            stop = [name for name, _ in self.steps].index(stop)
+
+        kwargs = self.get_params(deep=False)
+        kwargs['steps'] = self.steps[start:stop]
+        return type(self)(**kwargs)
+
     # Estimator interface
 
     def _fit(self, X, y=None, **fit_params):

diff --git a/sklearn/tests/test_pipeline.py b/sklearn/tests/test_pipeline.py
@@ -894,3 +894,55 @@ def test_pipeline_memory():
         assert_equal(ts, cached_pipe_2.named_steps['transf_2'].timestamp_)
     finally:
         shutil.rmtree(cachedir)
+
+
+class MyPipeline(Pipeline):
+    pass
+
+
+class MyPipelineNoMemory(Pipeline):
+    def __init__(self, steps, other_param):
+        super(MyPipelineNoMemory, self).__init__(steps)
+        self.other_param = other_param
+
+
+def test_pipeline_get_subsequence():
+    pipe = Pipeline([('transf1', Transf()),
+                     ('transf2', Transf()),
+                     ('predict', Mult())])
+    pipe.fit(np.arange(5)[:, None], np.arange(5))
+
+    for start, stop, expected_slice in [
+        (None, None, slice(None, None)),
+        ('transf2', None, slice(1, None)),
+        (None, 'predict', slice(None, 2)),
+        (1, 'predict', slice(1, 2)),
+        (1, -1, slice(1, -1)),
+        (-1, None, slice(-1, None)),
+    ]:
+        new_pipe = pipe.get_subsequence(start, stop)
+        expected_steps = pipe.steps[expected_slice]
+        assert_equal(new_pipe.steps, expected_steps)
+        assert_dict_equal(new_pipe.named_steps, dict(expected_steps))
+        for name in new_pipe.named_steps:
+            assert_true(new_pipe.named_steps[name] is pipe.named_steps[name])
+
+    # invalid step name
+    assert_raise_message(ValueError, "'foo' is not in list",
+                         pipe.get_subsequence, 'foo')
+
+    # test subtype is maintained by get_subsequence
+    for memory in [None, '/path/to/somewhere']:
+        pipe = MyPipeline([('transf1', Transf()),
+                           ('predict', Mult())],
+                          memory=memory)
+        new_pipe = pipe.get_subsequence(1)
+        assert_equal(new_pipe.steps, pipe.steps[1:])
+        assert_equal(pipe.memory, new_pipe.memory)
+
+    pipe = MyPipelineNoMemory([('transf1', Transf()),
+                               ('predict', Mult())],
+                              other_param='blah')
+    new_pipe = pipe.get_subsequence(1)
+    assert_equal(new_pipe.steps, pipe.steps[1:])
+    assert_equal(pipe.other_param, new_pipe.other_param)