[Zope-CMF] Charsets

Charlie Clark charlie at begeistert.org
Sun Jan 18 09:49:05 EST 2009

Am 29.12.2008 um 15:01 schrieb Charlie Clark:

>> The site should deliver all pages containing forms (if possible even
>> all pages) with a single charset, let's call it the "site charset".
>> Then it uses this same charset to interpret form data.
> While I understand this, I'm a bit at a loss as to why this is
> happening. I'm using forms based on CMFDefault's formlib
> implementation. Charsets are set for the site and zpublisher but
> something else is probably missing.

Delving deeper into this I think I understand things a little better.

The accept-charset attribute on a form tag requires the browser to  
encode any form data in the specific encoding. Ideally this would make  
additional negotiation unnecessary but this value isn't passed to the  
server as the HTTP_ACCEPT_CHARSET which is where the fun starts. As  
has been noted previously, http://mail.zope.org/pipermail/zope3-dev/2004-June/011483.html 
  , browsers don't all behave themselves when setting this header: IE  
6 + 7 and Safari set an empty header whereas Opera and Firefox usually  
set something like "iso-8859-1, utf-8, utf-16, *;q=0.1"

getPreferredCharsets() will return 'iso-8859-1' where  
HTTP_ACCEPT_CHARSET is empty. But this will cause problems if the  
browser is actually using UTF-8. But the way the CMF uses  
getPreferredCharsets() is right either:


def getBrowserCharset(request):
     """ Get charset preferred by the browser.
     envadapter = IUserPreferredCharsets(request)
     charsets = envadapter.getPreferredCharsets() or ['utf-8']
     return charsets[0]

This will always be iso-8859-1 for Opera and Firefox because all  
charsets have the same quality, again even if UTF-8 encoding is  
specified. I haven't been able to track where the decoding of form  
data occurs for Zope 2 stuff but I can identify the problem in  

     def _decode(self, text):
         """Try to decode the text using one of the available  
         if self.charsets is None:
             envadapter = IUserPreferredCharsets(self)
             self.charsets = envadapter.getPreferredCharsets() or  
         for charset in self.charsets:
                 text = unicode(text, charset)
             except UnicodeError:
         return text

Here the naive assumption is that we decode from a charset without an  
error then we have the correct charset. Sometimes this goes unnoticed  
but with characters like u2013 and u2014 (en-dash and em-dash) it will  
raise errors as those codepoints are not in the Latin-1 charset but it  
has it's own equivalents.

I would suggest that we work towards enforcing UTF-8 in where possible  
but at the very least add the accept-charset attribute to forms and  
use the portal's default_charset for this.

I'd very much appreciate your comments on this.

Charlie Clark
Helmholtzstr. 20
D- 40215
Tel: +49-211-938-5360
GSM: +49-178-782-6226

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