Python 更新数据库列有覆盖问题吗？

Question

I have a simple function which should update a certain column for certain users:

for key, group in groupby(list_i):
    print key, type(key), len(list(group)), type(len(list(group)))
    setattr(User.query.filter_by(id=key).first(), "number_of_rooms", len(list(group)))
db_session.commit()

I have a list ( list_i ) which has the data, simply look at the print:

The data is correct, the first number is the id of the user and the second is the value which should be assign to column number_of_rooms

The problem is it sets all values of the users to 0 . I assume it is somehow an override issue of the last user, I dont know. The thing is if I change it to:

setattr(User.query.filter_by(id=key).first(), "number_of_rooms", 2)

It will change the value to 2 for all users with id=key so the function works. I am here very confused, because len(list(group)) is a simple integer and it should work.

EDIT:

I double checked everything, I also made an other test with a variable ( t ) which increments by 1 and added an onther print to check everything, with the variable it also works, how is that possible?

t = 0
for key, group in groupby(list_i):
    print key, type(key), len(list(group)), type(len(list(group)))
    my_user = User.query.filter_by(id=key).first()
    setattr(my_user, "number_of_rooms", t)
    print my_user.number_of_rooms
    t+=1
db_session.commit()

The value of number_of_rooms changes:

Answer 1

As Roman Mindlin and glibdud have specified, list(group) can only be used once. Since group is of type <type 'itertools._grouper'> , after you iterate over group once (eg by calling list() on it), it's exhausted. The key is to save the list(group) inside a variable.

Here is the code that fixes the issue:

for key, group in groupby(list_i):
    l_group = list(group)
    print(l_group)
    my_user = User.query.filter_by(id=key).first()
    setattr(my_user, "number_of_rooms", len(l_group))
db_session.commit()