To use _gettags you need to extend BaseEstimator

bob/pipelines/mixins.py

@@ -7,7 +7,7 @@ import cloudpickle
 import functools
 import bob.io.base
 from sklearn.preprocessing import FunctionTransformer
-from sklearn.base import TransformerMixin
+from sklearn.base import TransformerMixin, BaseEstimator
 from sklearn.pipeline import Pipeline
 from dask import delayed
 import dask.bag
@@ -191,7 +191,7 @@ def _make_kwargs_from_samples(samples, arg_attr_list):
     return kwargs
 
 
-class SampleMixin:
+class SampleMixin(BaseEstimator):
     """Mixin class to make scikit-learn estimators work in :any:`Sample`-based
     pipelines.
     Do not use this class except for scikit-learn estimators.