import urllib
from django import template
from django.template.defaultfilters import stringfilter
from django.utils.html import conditional_escape
from django.utils.safestring import mark_safe

register = template.Library()

@register.filter
@stringfilter
def qrcode(value, alt=None):
    """
    Generate QR Code image from a string with the Google charts API
    
    http://code.google.com/intl/fr-FR/apis/chart/types.html#qrcodes
    
    Exemple usage --
    {{ my_string|qrcode:"my alt" }}
    
    <img src="http://chart.apis.google.com/chart?chs=150x150&amp;cht=qr&amp;chl=my_string&amp;choe=UTF-8" alt="my alt" />
    """
    
    url = conditional_escape("http://chart.apis.google.com/chart?%s" % \
            urllib.urlencode({'chs':'150x150', 'cht':'qr', 'chl':value, 'choe':'UTF-8'}))
    alt = conditional_escape(alt or value)
    
    return mark_safe(u"""<img class="qrcode" src="%s" width="150" height="150" alt="%s" />""" % (url, alt))