• General meta-estimator issues
• Duck-typing and methods (#1805, #2019)
• Accessing fitted attributes (cf. #2561, #2568, #2630 wrt Pipeline)
• Pipeline / FeatureUnion issues
• Passing parameters such as sample_weight to methods (cf. #2630)
• Pipeline.get_feature_names() (#2007)
• Inconsistency between get_params and set_params treatment of sub-estimators (#1769, #1800)
• Efficiently reusing partial models/transformations during grid search (#2086)
• Doesn't clone
• Minor functionality and syntax issues: