嵌套for循环意外输出

Question

I've been trying to make a nested for loop, but for some reason the inside one only loops once and I can't figure out why. I've already reduced my code to the bare minimum just to figure out whats happening:

pupil = db.session.query(Pupil).all()
result = db.session.query(Pupil_OLD).all()
for row in pupil:
    for sublist in result:   
        print("sublist"+str(sublist.PUPIL_ID))
    print("pupil"+str(row.PUPIL_ID))

This produces:

pupil1
sublist1
sublist2
sublist3
pupil2
pupil3

While it should produce

pupil1
sublist1
sublist2
sublist3
pupil2
sublist1
sublist2
sublist3
pupil3
sublist1
sublist2
sublist3

Does anyone have an idea what I'm doing wrong?

Answer 1

I assume result is a generator which is reaching the end.

You can reset it like so:

pupil = db.session.query(Pupil).all()
for row in pupil:
    result = db.session.query(Pupil_OLD).all()
    for sublist in result:   
        print("sublist"+str(sublist.PUPIL_ID))
    print("pupil"+str(row.PUPIL_ID))

Converting it to a list is another option, but you'd be missing out on cool generators ;)

My love for generator's aside, if resetting it means re-querying a database, you might be better off with the list.

Answer 2

You should rather do something like

pupil = db.session.query(Pupil).all()
result = list(db.session.query(Pupil_OLD).all())

So the result of db.session.query(Pupil_OLD).all() which appears to be a generator, is converted to a list which won't be consumed like your generator is.