Deeply nested ifs are hard to follow: by the innermost line you're holding four conditions in your head.

def ship(order):
    if order is not None:
        if order.paid:
            if order.items:
                if order.address:
                    send(order)
                    return "shipped"
    return "not shipped"

Guard clauses handle the special cases first and return early, leaving the main path flat and obvious:

def ship(order):
    if order is None:
        return "not shipped"
    if not order.paid:
        return "not shipped"
    if not order.items or not order.address:
        return "not shipped"
    send(order)
    return "shipped"