old_contents: #! /usr/bin/env python
  # -*- coding: utf-8 -*-
  
  from __future__ import absolute_import
  
  # Local module
  from .structure import Chain, Atom
  from .PDB import PDB
  
  # Conditional import
  try:
      import MDAnalysis
  except ImportError:
      IS_MDANALYSIS = False
  else:
      IS_MDANALYSIS = True
  
  
  # Create the __all__ keyword according to the conditional import
  __all__ = ['chains_from_files']
  if IS_MDANALYSIS:
      __all__ += ['chains_from_trajectory']
  
  
  def chains_from_files(path_list):
      for pdb_name in path_list:
          pdb = PDB(pdb_name)
          for chain in pdb.get_chains():
              # build comment
              comment = pdb_name
              if chain.model:
                  comment += " | model %s" % (chain.model)
              if chain.name:
                  comment += " | chain %s" % (chain.name)
              yield comment, chain
  
  
  def chains_from_trajectory(trajectory, topology):
      comment = ""
      universe = MDAnalysis.Universe(topology, trajectory)
      for ts in universe.trajectory:
          structure = Chain()
          selection = universe.select_atoms("backbone")
          for atm in selection:
              atom = Atom()
              atom.read_from_xtc(atm)
              # append structure with atom
              structure.add_atom(atom)
              # define structure comment
              # when the structure contains 1 atom
              if structure.size() == 1:
                  comment = "%s | frame %s" % (trajectory, ts.frame)
          yield comment, structure
  
new_contents: #! /usr/bin/env python
  # -*- coding: utf-8 -*-
  
  from __future__ import absolute_import
  
  # Local module
  from .structure import Chain, Atom
  from .PDB import PDB
  
  # Conditional import
  try:
      import MDAnalysis
  except ImportError:
      IS_MDANALYSIS = False
  else:
      IS_MDANALYSIS = True
  
  
  # Create the __all__ keyword according to the conditional import
  __all__ = ['chains_from_files']
  if IS_MDANALYSIS:
      __all__ += ['chains_from_trajectory']
  
  
  def chains_from_files(path_list):
      for pdb_name in path_list:
          pdb = PDB(pdb_name)
          for chain in pdb.get_chains():
              # build comment
              comment = pdb_name
              if chain.model:
                  comment += " | model %s" % (chain.model)
              if chain.name:
                  comment += " | chain %s" % (chain.name)
              yield comment, chain
  
  
  def chains_from_trajectory(trajectory, topology):
      comment = ""
      universe = MDAnalysis.Universe(topology, trajectory)
      selection = universe.select_atoms("backbone")
      for ts in universe.trajectory:
          structure = Chain()
          for atm in selection:
              atom = Atom()
              atom.read_from_xtc(atm)
              # append structure with atom
              structure.add_atom(atom)
              # define structure comment
              # when the structure contains 1 atom
              if structure.size() == 1:
                  comment = "%s | frame %s" % (trajectory, ts.frame)
          yield comment, structure
  
subject: Create only one MDAnalysis selection