python - 谷歌应用引擎 : Trouble with Datastore Query

Question

此查询有效:

item = db.GqlQuery("SELECT * FROM Item WHERE CSIN = 13")[0]

虽然如果没有返回任何结果，它会在我面前爆炸。 (我怎样才能解决这个问题？当我想要最多一次迭代时，for 循环似乎很可疑。)

这个查询不起作用:

item = db.GqlQuery("SELECT * FROM Item WHERE CSIN = :1", CSIN)[0]

CSIN 是一个代表数字的字符串。我收到此错误:

Traceback (most recent call last):
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\webapp\__init__.py", line 507, in __call__
    handler.get(*groups)
  File "path\to\src\Main.py", line 42, in get
    item = db.GqlQuery("SELECT * FROM Item WHERE CSIN = :1", CSIN)[0]
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1717, in __getitem__
    raise IndexError('The query returned fewer than %d results' % (arg+1))
IndexError: The query returned fewer than 1 results

我在这里做错了什么？

Answer 1

您正试图从一个空列表(或类似列表的对象)中获取一个项目。您正在做的事情类似于以下内容:

>>> results = [] # an empty list
>>> item = results[0] # Raises an IndexError, because there is nothing in the list

你需要做的是:

item = db.GqlQuery("SELECT * FROM Item WHERE CSIN = :1", CSIN).get()

然后，item 将是 None 或查询的第一个结果。