Capture all incoming bound data which passed validation into a new querydict, such that a form can return something which may be output as a querystring where appropriate. Obviously it's not always appropriate (eg: POST forms, forms with files etc). Also I can't entirely remember how well it works for `&multiple=of&multiple=the&multiple=samething`

forms.py

class F(forms.Form):
    def to_querydict(self):
        if not self.is_bound or not hasattr(self, 'cleaned_data'):
            return QueryDict('')
        qd = QueryDict('', mutable=True)
        get_or_default = self.data.get
        if isinstance(self.data, QueryDict):
            get_or_default = self.data.getlist
        for key, value in self.cleaned_data.items():
            prefixed_key = self.add_prefix(key)
            if value:
                # Sigh. If you pass in QueryDict(query_string="") to a Form
                # subclass, you'll actually have a data attr which is a plain
                # dictionary, and so won't have the getlist method.
                request_val = [x for x in get_or_default(prefixed_key, ()) if x]
                if request_val:
                    qd.setlist(prefixed_key, request_val)
        qd._mutable = False
        return qd