def customers_mapper(data):
    """Return a dict containing customer_id as key and
    customer_unique_id as values."""
    customers = data['olist_customers_dataset'].copy()
    customers = customers[['customer_unique_id', 'customer_id']]
    customers.set_index('customer_id', inplace=True, drop=True)
    return customers.to_dict()['customer_unique_id']