Steps in pipelines are named; where a name equals that of an existing attribute on Pipeline (e.g. transform, predict, steps), calling set_params and setting that name to some value causes the attribute to be overwritten.

Example:

Consider the following:

import sklearn.pipeline, sklearn.dummy
class DummyTransformer(sklearn.dummy.DummyClassifier):
    def transform(self, X):
        return X

clf = sklearn.pipeline.Pipeline([('transform', DummyTransformer()), ('predict', sklearn.dummy.DummyClassifier())])
print(clf.transform) # (1)
clf.set_params(transform=DummyTransformer())
print(clf.transform) # (2)

This prints <bound method Pipeline.transform of Pipeline(steps=[('transform', DummyTransformer(random_state=None, strategy='stratified')), ('est', DummyClassifier(random_state=None, strategy='stratified'))])> at (1) and DummyTransformer(random_state=None, strategy=stratified) at (2).

Mechanism:

The general mechanism of BaseEstimator.set_params is to set the attribute corresponding to the parameter name. In general, this is restricted to a small set of possible names (usually corresponding to constructor arguments) that would not conflict with non-parameter attributes (though I don't think there's a test to assure this). Where double-underscore notation is used to set sub-estimators' parameters, the sub-estimator's name (e.g. est in est__some_param) needs to be returned by the estimator's get_params. Hence set_params allows est to be set, in turn calling setattr, without regard to existing attributes.

Resolution:

(a) Don't allow step names to be used as parameter names in set_params

or

(b) Allow step names to be used as parameter names in set_params meaningfully while:

1. prohibiting constructor arguments and existing attributes as names; or
2. prohibiting constructor arguments as names, but special-casing the setting of steps, so that it doesn't involve setattr (rather, the modification of the steps attribute, which needs to happen somehow anyway). This approach is taken by [MRG + 2] ENH enable setting pipeline components as parameters #1769.

Also, a test should be added for this case, and perhaps more generally for all estimators to ensure set_params does not overwrite class attributes (methods, etc.).