from django import template

register = template.Library()

@register.simple_tag(takes_context=True)
def query_transform(context, **kwargs):
    '''
    Returns the URL-encoded querystring for the current page,
    updating the params with the key/value pairs passed to the tag.
    
    E.g: given the querystring ?foo=1&bar=2
    {% query_transform bar=3 %} outputs ?foo=1&bar=3
    {% query_transform foo='baz' %} outputs ?foo=baz&bar=2
    {% query_transform foo='one' bar='two' baz=99 %} outputs ?foo=one&bar=two&baz=99
    
    A RequestContext is required for access to the current querystring.
    '''
    query = context['request'].GET.copy()
    for k, v in kwargs.items():
        query[k] = v
    return query.urlencode()