'Learn a list of feature name -> indices mappings and transform X.
Like fit(X) followed by transform(X), but does not require
materializing X in memory.
Parameters
X : Mapping or iterable over Mappings
Dict(s) or Mapping(s) from feature names (arbitrary Python
objects) to feature values (strings or convertible to dtype).
y : (ignored)
Returns
Xa : {array, sparse matrix}
Feature vectors; always 2-d.'

def fit_transform(self, X, y=None):

    return self._transform(X, fitting=True)
