def mean(li):
    if li:
        return sum(li) / len(li)
    return None
Returns all customers. Sometimes they are too much and cause internal 
dataset/ETHPy150Open artirix/logcabin/logcabin/statistics.py/mean--crossover with--dataset/ETHPy150Open SeanOC/sharpy/sharpy/product.py/CheddarProduct.get_customers