The point of using a function is to allow the function special hanling of the encoding name, which goes beyond a simple map lookup, i.e. you could do the following:

   import codecs

   def search_function(encoding):
      if not encoding.startswith("append-"):
         return None

      suffix = encoding[7:]

      def encode(s, errors="strict"):
         s = (s + suffix).encode("utf-8", errors)
         return (s, len(s))

      def decode(s, errors="strict"):
         s = bytes(s).decode("utf-8", errors)
         if s.endswith(suffix):
            s = s[:-len(suffix)]
         return (s, len(s))

      return codecs.CodecInfo(encode, decode, name=encoding)

   codecs.register(search_function)

   $ python
   Python 3.3.1 (default, Apr 29 2013, 15:35:47)
   [GCC 4.2.1 Compatible Apple LLVM 4.2 (clang-425.0.24)] on darwin
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import appendcodec
   >>> 'foo'.encode('append-bar')
   b'foobar'
   >>> b'foobar'.decode('append-bar')
   'foo'

The search function can't return a list of codec names in this case, as the list is infinite.