[Python-Dev] Improve open() to support reading file starting with an unicode BOM (original) (raw)

Guido van Rossum guido at python.org
Fri Jan 8 05:21:04 CET 2010

On Thu, Jan 7, 2010 at 7:34 PM, Glyph Lefkowitz <glyph at twistedmatrix.com> wrote:

On Jan 7, 2010, at 7:52 PM, Guido van Rossum wrote: On Thu, Jan 7, 2010 at 4:10 PM, Victor Stinner <victor.stinner at haypocalc.com> wrote: Hi, Builtin open() function is unable to open an UTF-16/32 file starting with a BOM if the encoding is not specified (raise an unicode error). For an UTF-8 file starting with a BOM, read()/readline() returns also the BOM whereas the BOM should be "ignored". I'm a little hesitant about this. First of all, UTF-8 + BOM is crazy talk. And for the other two, perhaps it would make more sense to have a separate encoding-guessing function that takes a binary stream and returns a text stream wrapping it with the proper encoding? It is crazy, but unfortunately rather common.  Wikipedia has a good description of the issues: <http://en.wikipedia.org/wiki/UTF-8#Byte-ordermark>.  Basically, some Windows text APIs will emit a UTF-8 "BOM" in order to identify the file as being UTF-8, so it's become a convention to do that.  That's not good enough, so you need to guess the encoding as well to make sure, but if there is a BOM and you can otherwise verify that the file is probably UTF-8 encoded, you should discard it.

That doesn't make sense. If the file isn't UTF-8 you can't see the BOM, because the BOM itself is UTF-8-encoded.

(And yes, I know this happens. Doesn't mean we need to auto-guess by default; there are lots of issues e.g. what should happen after seeking to offset 0?)

-- --Guido van Rossum (python.org/~guido)

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