RFC Allow passthrough as pipeline step instead of None

[amueller]

As I mentioned here it might make sense to deprecate setting pipeline steps to None and requiring "passthrough" instead to avoid ambiguities with FeatureUnion and ColumnTransformer.

This might also give us an opportunity to easily specify "blank" pipelines for which you want to search the steps. Right now it's not easy to say "I want a pipeline with three steps and then let the GridSearch figure out the steps" because you need to provide actual steps at instantiation.
Something like

pipe = Pipeline([("imputer", "passthrough"), ("scaler", "passthrough"), ("classifier", "passthrough")])

or something.

[jnothman]

sounds good to me, or at least providing this as an alternative to None

[jnothman]

we should change None in FeatureUnion to 'drop'. These together would clear up a lot

…

On 27 May 2018 9:22 am, "Joel Nothman" ***@***.***> wrote: sounds good to me, or at least providing this as an alternative to None