Example

client = trello.Client()
boards = client.get_boards()
gtd = boards['Getting Things Done']
labels = gtd.get_labels()
needs_due = labels['needs-due-date']
for card in gtd.cards:
    print(f"Processing card {card.name}")
    if not card.due and not card.has_label(needs_due)
        card.add_label(needs_due)
    if card.due and card.has_label(needs_due)
        card.remove_label(needs_due)