Query String alteration template tag for Django.

gistfile1.py

"""
Query String manipulation filters
"""

import logging

from django import template
from django.http import QueryDict
from django.utils.translation import ugettext as _

register = template.Library()


class QueryStringAlterer(template.Node):
    """
    Query String alteration template tag

    Receives a query string (either text or a QueryDict such as request.GET)
    and a list of changes to apply. The result will be returned as text query
    string, allowing use like this::

        <a href="?{% qs_alter request.GET type=object.type %}">{{ label }}</a>

    There are four available alterations:

        Assignment:

            name=var

        Deletion - removes the named parameter:

            -name

        Appending (treating the qstring as a list):

            name+=var

        Removing (treating the qstring as a list):

            name-=var

    Examples:

    Query string provided as QueryDict::

        {% qs_alter request.GET foo=bar %}
        {% qs_alter request.GET foo=bar baaz=quux %}
        {% qs_alter request.GET foo+=bar %}
        {% qs_alter request.GET foo-=bar %}
        {% qs_alter request.GET foo=bar baaz+=quux -corge %}

    Query string provided as string::

        {% qs_alter "foo=baaz" foo=bar %}">
    """

    def __init__(self, base_qs, *args):
        self.base_qs = template.Variable(base_qs)
        self.args = args

    def render(self, context):
        base_qs = self.base_qs.resolve(context)

        if isinstance(base_qs, QueryDict):
            qs = base_qs.copy()
        else:
            qs = QueryDict(base_qs, mutable=True)

        for arg in self.args:
            # look for += (add to assumed list or concat to string)
            if arg.find('+=') > -1:
                k,v = arg.split("+=", 2)
                value = template.Variable(v).resolve(context)
                if k in qs:
                    qs[k] = '%s,%s' % (qs[k],str(value))
                else:
                    qs[k] = str(value)
            # look for -= (remove from assumed list or string)
            elif arg.find('-=') > -1:
                k,v = arg.split("-=", 2)
                value = template.Variable(v).resolve(context)
                if k in qs:
                    ql = qs[k].split(',')
                    if str(value) in ql:
                        ql.remove(str(value))
                    if len(ql) > 0:
                        qs[k] = ','.join(ql)
                    else:
                        del qs[k]
            # look for just = (set, overriding existing value if it exists)
            elif arg.find('=') > -1:
                k, v = arg.split("=", 2)
                if k in qs:
                    qs[k] = template.Variable(v).resolve(context)
                else:
                    qs[k] = template.Variable(v).resolve(context)
            # look for just - (remove from dict)
            elif arg.find('-') > -1:
                k, v = arg.split("-", 2)
                if k in qs:
                    del qs[k]


        return qs.urlencode(safe=',')

    @classmethod
    def qs_alter_tag(cls, parser, token):
        try:
            bits = token.split_contents()
        except ValueError:
            raise template.TemplateSyntaxError(
                _('qs_alter requires at least two arguments: the initial query string and at least one alteration')
            )

        return QueryStringAlterer(bits[1], *bits[2:])

register.tag('qs_alter', QueryStringAlterer.qs_alter_tag)