Django templatetag to output the current page's querystring updated with the specified values including context variables.

gistfile1.py

from django import template

register = template.Library()


class UpdateQuerystringNode(template.Node):
    def __init__(self, **kwargs):
        self.kwargs = kwargs

    def render(self, context):
        query_dict = context['request'].GET.copy()

        for k, v in self.kwargs.items():

            try:
                variable = template.Variable(v).resolve(context)

            except template.VariableDoesNotExist:
                variable = v

            finally:
                query_dict[k] = variable

        return query_dict.urlencode()


@register.tag
def update_querystring(parser, token):
    """
    From /?foo=bar, {% update_querystring foo=baz %}
    will output foo=baz.

    or use a context variable
    {% update_querystring foo=request.user %}
    foo=whatever

    where whatever is the current user's username.
    """
    bits = token.split_contents()
    return UpdateQuerystringNode(**dict([bit.split('=') for bit in bits[1:]]))