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

Glenn Linderman v+python at g.nevcal.com
Sat Jan 9 01:47:38 CET 2010

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.

Glenn -- http://nevcal.com/
A protocol is complete when there is nothing left to remove.
-- Stuart Cheshire, Apple Computer, regarding Zero Configuration Networking

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