Google App Engine: Trouble with Datastore Query

Question

This query works:

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

although if there are no results returned, it blows up in my face. (How can I get around this? A for loop seems dubious when I want at max one iteration.)

This query does not work:

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

CSIN is a string representing a number. I get this error:

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

What am I doing wrong here?

Answer 1

You're trying to get an item from a list (or a list-like object) that is empty. What you're doing is sort of comparable to the following:

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

What you need to do instead is:

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

Then, item will be either None or the first result from the query.

Answer 2

It seems that that your query returns an array or list of some sort, which is empty. Trying to access its non-existing first item raises an exception.

Try to figure out the length of the returned result before accessing it.