I don't see exactly what James is proposing.

> For my needs, I would like the decoding parts of the utf_8 module
> to treat an initial BOM as an optional signature and skip it if
> there is one (just like the utf_8_sig decoder). In fact I have
> a working patch that replaces the utf_8_sig  decode,
> IncrementalDecoder and StreamReader components by direct
> transplants from utf_8_sig (as recently repaired -- there was a
> SteamReader error).

I've you want a decoder that behave like the utf-8-sig decoder, use the
utf-8-sig decoder. I don't see how changing the utf-8 decoder helps here.

> I can imagine there might be utf_8 client code out there which
> expects to see a leading U+feff as (perhaps) a clue that the
> output should be returned with a BOM-signature (say) to
> accomodate the guessed input requirements of the remote
> correspondant.

In this case use UTF-8: The leading BOM will be passed to the application.

> I can just live with code like
>  if input[0] == u"\ufeff": 
>    input=input[1:}
> spread around, and of course slightly different for incremental
> and stream inputs.

Can you post an example that requires this code?