code: def featurizer(topfile):
      r
      from pyemma.coordinates.data.featurization.featurizer import MDFeaturizer
      return MDFeaturizer(topfile)
docs: r""" Featurizer to select features from MD data.
  
      Parameters
      ----------
      topfile : str or mdtraj.Topology instance
          path to topology file (e.g pdb file) or a mdtraj.Topology object
  
      Returns
      -------
      feat : :class:`Featurizer <pyemma.coordinates.data.featurization.featurizer.MDFeaturizer>`
  
      Examples
      --------
  
      Create a featurizer and add backbone torsion angles to active features.
      Then use it in :func:`source`
  
      >>> import pyemma.coordinates # doctest: +SKIP
      >>> feat = pyemma.coordinates.featurizer('my_protein.pdb') # doctest: +SKIP
      >>> feat.add_backbone_torsions() # doctest: +SKIP
      >>> reader = pyemma.coordinates.source(["my_traj01.xtc", "my_traj02.xtc"], features=feat) # doctest: +SKIP
  
      or
  
      >>> traj = mdtraj.load('my_protein.pdb') # # doctest: +SKIP
      >>> feat = pyemma.coordinates.featurizer(traj.topology) # doctest: +SKIP
  
      .. autoclass:: pyemma.coordinates.data.featurization.featurizer.MDFeaturizer
          :members:
          :undoc-members:
  
          .. rubric:: Methods
  
          .. autoautosummary:: pyemma.coordinates.data.featurization.featurizer.MDFeaturizer
             :methods:
  
          .. rubric:: Attributes
  
          .. autoautosummary:: pyemma.coordinates.data.featurization.featurizer.MDFeaturizer
              :attributes: