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

Victor Stinner victor.stinner at haypocalc.com
Fri Jan 8 01:10:35 CET 2010

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".

See recent issues related to reading an UTF-8 text file including a BOM: #7185 (csv) and #7519 (ConfigParser). Such file can be opened in unicode mode with the UTF-8-SIG encoding, but it's possible to do better.

I propose to improve open() (TextIOWrapper) by using the BOM to choose the right encoding. I think that only files opened in read only mode should support this new feature. Read the BOM in a write only file would cause unexpected behaviours.

Since my proposition changes the result TextIOWrapper.read()/readline() for files starting with a BOM, we might introduce an option to open() to enable the new behaviour. But is it really needed to keep the backward compatibility?

I wrote a proof of concept attached to the issue #7651. My patch only changes the behaviour of TextIOWrapper for reading files starting with a BOM. It doesn't work yet if a seek() is used before the first read.

-- Victor Stinner http://www.haypocalc.com/

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