Guessing the encoding from a BOM

[Ben Finney]

Steven D'Aprano <steve+comp.lang.python at pearwood.info> writes:

> enc = guess_encoding_from_bom("filename")
> if enc == something:
>      # Can't guess, fall back on an alternative strategy
>      ...
> else:
>      f = open("filename", encoding=enc)
>
>
> If I forget to check the returned result, I should get an explicit
> failure as soon as I try to use it, rather than silently returning the
> wrong results.

Yes, agreed.

> What should I return as the default default? I have four possibilities:
>
>     (1) 'undefined', which is an standard encoding guaranteed to 
>         raise an exception when used;

+0.5. This describes the outcome of the guess.

>     (2) 'unknown', which best describes the result, and currently 
>         there is no encoding with that name;

+0. This *better* describes the outcome, but I don't think adding a new
name is needed nor very helpful.

>     (3) None, which is not the name of an encoding; or

−1. This is too much like a real result and doesn't adequately indicate
the failure.

>     (4) Don't return anything, but raise an exception. (But 
>         which exception?)

+1. I'd like a custom exception class, sub-classed from ValueError.

-- 
 \       “I love to go down to the schoolyard and watch all the little |
  `\   children jump up and down and run around yelling and screaming. |
_o__)             They don't know I'm only using blanks.” —Emo Philips |
Ben Finney