Python函数将循环通过，但在最后一次迭代时出错

Question

在以下 Python 函数中，我使用 Python 2.7.6 和 SQLAlchemy 作为 ORM。我很困惑为什么即使函数循环通过，我仍然会收到以下错误。

def vacantShelter():
#Used to stay in bounds of the all the entries in the DB
shelterCount = session.query(Shelter).count
shelter_id = 1
print "Here are the Shelter results:"
while(shelter_id<=shelterCount):
    shelter = session.query(Shelter).filter(Shelter.id == shelter_id).first()
    if(shelter.max_capacity >= getShelterOccupancy(shelter_id)):
        print shelter.name + " has available space"
    else:
        print shelter.name + " is full! :("
    shelter_id = shelter_id + 1

让我感到困惑的是，考虑到有结果，它一开始运行正常，我不明白为什么在最后一次迭代中它会失败或如何处理它。

`Here are the Shelter results:
Oakland Animal Services has available space
San Francisco SPCA Mission Adoption Center is full! :(
Wonder Dog Rescue has available space
Humane Society of Alameda is full! :(
Palo Alto Humane Society is full! :(
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pup_test.py", line 82, in vacantShelter
    if(shelter.max_capacity >= getShelterOccupancy(shelter_id)):
AttributeError: 'NoneType' object has no attribute 'max_capacity'`

Answer 1

此错误最可能的原因是Shelter.id 值中存在孔。例如，该表可能包含id 值为[1, 2, 3, 4, 5, 7] 的行。当您到达最后一个元素时，您将尝试查找 id 为 6 的 Shelter，但它不存在。

原则上，您的代码现在的工作方式是非常大胆地假设数据库中的所有行都具有从1 开始的连续id 值。但一般情况并非如此。相反，您可以直接迭代实例，而不是一个接一个地加载它们：

def vacantShelter():
    shelters = session.query(Shelter).all()
    for shelter in shelters:
        if(shelter.max_capacity >= getShelterOccupancy(shelter.id)):
            print shelter.name + " has available space"
        else:
            print shelter.name + " is full! :("

而且，它更干净。