[Python-Dev] Quick sum up about open() + BOM

MRAB python at mrabarnett.plus.com
Sat Jan 9 02:12:28 CET 2010

Glenn Linderman wrote:
> On approximately 1/8/2010 3:59 PM, came the following characters from 
> the keyboard of Victor Stinner:
>> Hi,
>> Thanks for all the answers! I will try to sum up all ideas here.
> One concern I have with this implementation encoding="BOM" is that if 
> there is no BOM it assumes UTF-8.  That is probably a good assumption in 
> some circumstances, but not in others.
> * It is not required that UTF-16LE, UTF-16BE, UTF-32LE, or UTF-32BE 
> encoded files include a BOM.  It is only required that UTF-16 and UTF-32 
> (cases where the endianness is unspecified) contain a BOM.  Hence, it 
> might be that someone would expect a UTF-16LE (or any of the formats 
> that don't require a BOM, rather than UTF-8), but be willing to accept 
> any BOM-discriminated format.
> * Potentially, this could be expanded beyond the various Unicode 
> encodings... one could envision that a program whose data files 
> historically were in any particular national language locale, could want 
> to be enhance to accept Unicode, and could declare that they will accept 
> any BOM-discriminated format, but want to default, in the absence of a 
> BOM, to the original national language locale that they historically 
> accepted.  That would provide a migration path for their old data files.
> So the point is, that it might be nice to have 
> "BOM-otherEncodingForDefault" for each other encoding that Python 
> supports.  Not sure that is the right API, but I think it is expressive 
> enough to handle the cases above.  Whether the cases solve actual 
> problems or not, I couldn't say, but they seem like reasonable cases.
> It would, of course, be nicest if OS metadata had been invented way back 
> when, for all OSes, such that all text files were flagged with their 
> encoding... then languages could just read the encoding and do the right 
> thing! But we live in the real world, instead.
What about listing the possible encodings? It would try each in turn
until it found one where the BOM matched or had no BOM:

     my_file = open(filename, 'r', encoding='UTF-8-sig|UTF-16|UTF-8')

or is that taking it too far?

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