They are several approach to your issue. One approach is to play with Complex lookups with Q objects

from django.db.models import Q

user = User.objects.get(pk=1)
category = Category.objects.get(pk=1)

f1 = Q( user=user, date=now() )
f_cat_is_none = Q( category__isnull = True )
f_cat_is_not_none = Q( category=category )

todays_items = Item.objects.filter( f1 & ( f_cat_is_none | f_cat_is_not_none ) )

I don’t right understand in your answer if this is the query you are looking for, but, with this example you can compose easily your own query.

Edited due OP comment

category__isnull == True means that, in database, the item has not an associated category.
Perhaps the query you are looking for is:

from django.db.models import Q

user_pk = 1
category_pk = 1  #some times None

f = Q( user__pk = user_pk, date=now() )
if category_pk is not None:
  f &= Q( category__pk = category_pk )

todays_items = Item.objects.filter( f  )

This is only a code sample, fit it to your requirements. Be careful with single _ and double __.

Well, this is rather old question but for those who would like to do the conditional filtering on one line, here is my approach (Btw, the following code can probably be written in a more generic way):

from django.db.models import Q

def conditional_category_filter(category):
    if category != None:
        return Q(category=category)
    else:
        return Q() #Dummy filter

user = User.objects.get(pk=1)
category = Category.objects.get(pk=1)
todays_items = Item.objects.filter(conditional_category_filter(category), user=user, date=now())

The only thing you need to watch is to use the conditional_category_filter(category) call before the keyword arguments like user=user. For example the following code would throw an error:

todays_items = Item.objects.filter(user=user, date=now(), conditional_category_filter(category))

To continue on @iuysal answer:

To make it generic you need to pass the key too as parameter, to do that you need to pass a dictionary, here’s how I did it:

Create your dictionary like this:

filters = {'filter1': 'value1', 'filter2__startswith': 'valu', ...}

Then pass it to your Item filters like this:

Item.objects.filter(*[Q(**{k: v}) for k, v in filters.items() if v], filter3='value3')

The first version less cryptic version I had:

def _filter(filters):
    filters = []
    for k, v in n.items():
        if v:
            filters.append(Q(**{k: v}))
    return filters

filters = _filter({'name': name})
return Item.objects.filter(*filters)

Unpacking explanation: We want to give Q (queries) as args to objects.filter as args while we want to give kwargs to Q()

I have this on production now (I will just modify the filters names because it’s sensitive):

def get_queryset(self):
    filter1 = self.request.GET.get('filter1 ', '')
    filter2__startswith = self.request.GET.get('filter2_prefix ', '')

    def filters_to_Qs(filters):
        return [Q(**{k: v}) for k, v in filters.items() if v]

    filters = {'filter1': filter1 ,
               'filter2__startswith': filter2__startswith }

    return Order.objects.filter(*filters_to_Qs(filters))

from django.db.models import Q

qs = Users.objects.filter(
                    p_id=parent_id,
                    status=True
                ).all()

if user_id>0:
    qs = qs.filter( ~Q(id=user_id) )

in qs we will get the filtered results