Use variable for django ORM order_by

def customers(request):
    filtered = False
    sort_field = request.GET.get('sort', 'first_name')
    form = SearchForm(request.POST or None, request.FILES or None)
    if sort_field == 'address__city' and form.city == '':# or None, depending on the field type for city
        sort_field = 'address__county'
    our_customers = Customer.objects.order_by(sort_field)

    if request.method == 'POST' and \
           form.is_valid() and \
           form.cleaned_data['search'] and \
           'clear-searchform' not in request.POST:
        search = form.cleaned_data['search']
        search_list = search.split()
        duplicated_customers = []
        for item in search_list:
        the_customers = Customer.objects.filter(
            Q(slug__icontains=item) |
            Q(first_name__icontains=item) |
            Q(surname__icontains=item) |
            Q(dob__icontains=item) |
            Q(phone__number__icontains=item) |
            Q(email__email__icontains=item) |
            Q(address__address__icontains=item) |
            Q(address__city__icontains=item) |
            Q(address__county__icontains=item) |
            Q(address__postcode__icontains=item) |
            Q(address__country__icontains=item) |
            Q(medical__icontains=item) |
            Q(notes__icontains=item)
            # need to amend so can search by organization
            )
            duplicated_customers = chain(duplicated_customers, the_customers)
            filtered = True