code: def _get_doc_from_filename(filename):
      
      if isinstance(filename, string_types):
          if ("win" in platform) and (len(filename) < 260) \
                  and os.path.exists(filename):
              # path (win)
              doc = libsbml.readSBMLFromFile(filename)  # noqa: E501 type: libsbml.SBMLDocument
          elif ("win" not in platform) and os.path.exists(filename):
              # path other
              doc = libsbml.readSBMLFromFile(filename)  # noqa: E501 type: libsbml.SBMLDocument
          else:
              # string representation
              if "<sbml" not in filename:
                  raise IOError("The file with 'filename' does not exist, "
                                "or is not an SBML string. Provide the path to "
                                "an existing SBML file or a valid SBML string "
                                "representation: \n%s", filename)
  
              doc = libsbml.readSBMLFromString(filename)  # noqa: E501 type: libsbml.SBMLDocument
  
      elif hasattr(filename, "read"):
          # file handle
          doc = libsbml.readSBMLFromString(filename.read())  # noqa: E501 type: libsbml.SBMLDocument
      else:
          raise CobraSBMLError("Input type '%s' for 'filename' is not supported."
                               " Provide a path, SBML str, "
                               "or file handle.", type(filename))
  
      return doc
docs: Get SBMLDocument from given filename.
  
      Parameters
      ----------
      filename : path to SBML, or SBML string, or filehandle
  
      Returns
      -------
      libsbml.SBMLDocument