Django's template filters for prams (QueryDict's instance). - query_string: Takes QueryDict's instance and argument. It returns a querystirng only including the values specified by argument. The argument should be constructed by query names separated by spaces.

params_extra.py

from urllib import urlencode

from django.template import Library

register = Library()

@register.filter(name='query_string'
def query_string(value, arg):
    """
    Takes QueryDict's instance and argument.
    It returns a querystirng only including the values
    specified by argument.
    The argument should be constructed by query names
    separated by spaces.
    
    Usage:
    
    {{ params|query_string:'page next' }}
    
    then, it will be
    
    'page=2&next=/'
    
    even thogh the params contains `'number': [123]` for example.
    """
    def dict_filter(dict_, included):
        d = dict_.copy()
        for k in dict_.keys():
            if k not in included:
                try:
                    d.pop(k)
                except KeyError:
                    pass
        return d

    if not isinstance(value, (dict, QueryDict)):
        return ""

    included = arg.split()
    filtered = dict_filter(value, included)

    if isinstance(filtered, QueryDict):
        items = filtered.iterlists()
    else:
        items = filtered.items()

    output = []
    for k, list_ in items:
        output.extend([urlencode({k: v}) for v in list_])

    return '&'.join(output)

query_string.is_safe = True