old_contents: 
  from console import Console
  from rom import ROM
  from functools import reduce
  
  
  class ROMFinder(object):
  
    def __init__(self, filesystem):
      self.filesystem = filesystem
  
    def roms_for_console(self, console):
      """
      @param console - A console object
      @returns A list of ROM objects representing all of the valid ROMs for a
               given console.
  
      Valid ROMs are defined as ROMs for which `console`'s `is_valid_rom` method
      returns True.
  
      Returns an empty list if `console` is not enabled
      """
      if not console.is_enabled():
        return []
  
      paths = self.filesystem.files_in_directory(console.roms_directory())
      valid_rom_paths = filter(console.is_valid_rom, paths)
      return map(lambda path: ROM(path, console), valid_rom_paths)
  
    def roms_for_consoles(self, consoles):
      """
      @param consoles - An iterable list of consoles
      @returns A list of all of the ROMs for all of the consoles in `consoles`
  
      Equivalent to calling `roms_for_console` on every element of `consoles`
      and combining the results
      """
      assert hasattr(
          consoles, '__iter__'), "Expecting an iterable list of consoles"
  
      def rom_collector(roms, console):
        roms.extend(self.roms_for_console(console))
        return roms
      return reduce(rom_collector, consoles, [])
  
new_contents: 
  from console import Console
  from rom import ROM
  from functools import reduce
  
  
  class ROMFinder(object):
  
    def __init__(self, filesystem):
      self.filesystem = filesystem
  
    def roms_for_console(self, console):
      """
      @param console - A console object
      @returns A list of ROM objects representing all of the valid ROMs for a
               given console.
  
      Valid ROMs are defined as ROMs for which `console`'s `is_valid_rom` method
      returns True.
  
      Returns an empty list if `console` is not enabled
      """
      if not console.is_enabled():
        return []
  
      paths = self.filesystem.files_in_directory(console.roms_directory())
      valid_rom_paths = filter(console.is_valid_rom, paths)
      return map(lambda path: ROM(path, console), valid_rom_paths)
  
    def roms_for_consoles(self, consoles):
      """
      @param consoles - An iterable list of consoles
      @returns A list of all of the ROMs for all of the consoles in `consoles`
  
      Equivalent to calling `roms_for_console` on every element of `consoles`
      and combining the results
      """
      return reduce(lambda roms, console: roms + self.roms_for_console(console), consoles, [])
  
subject: Replace 'list.extend' call with '+' operator