code: def exists_by_source(self):
          
          exists_list = []
          for source in self.gtfs_sources:
              if isinstance(source, dict):
                  # source can now be either a dict or a zipfile
                  if self.fname in source:
                      if source[self.fname]:
                          exists_list.append(True)
                          continue
              # Handle zipfiles specially
              if "zipfile" in source:
                  try:
                      Z = zipfile.ZipFile(source['zipfile'], mode='r')
                      Z.getinfo(os.path.join(source['zip_commonprefix'], self.fname))
                      exists_list.append(True)
                      continue
                  # File does not exist in the zip archive
                  except KeyError:
                      print(self.fname, ' missing in ', source)
                      exists_list.append(False)
                      continue
              # Normal filename
              elif isinstance(source, string_types):
                  if os.path.exists(os.path.join(source, self.fname)):
                      exists_list.append(True)
                      continue
              exists_list.append(False)
          # the "file" was not found in any of the sources, return false
          return exists_list
docs: Does this GTFS contain this file? (file specified by the class)