Python嵌套的Try / Except / Else控制流

Question

What I'd like to achieve in the most elegant Python way: Try to find an object in the database on a certain filter. Only if there is no result, then try to find the object using another filter. If that returns no result, then the object does not exist, so insert a new instance.

What I'm thinking:

try:
    obj = session.query(model).filter_by(field_a).one()
except NoObjFound:
    try:
        obj = session.query(model).filter_by(field_b).one()
    except NoObjFound:
        insert_into_db(brand_new_obj)
else:
    update_the_obj(obj)

I'm not sure if this block is correct or the best way to handle what I'm trying to accomplish.

Answer 1

I would suggest something like:

for filter in (field_a, field_b):
    try:
        obj = session.query(model).filter_by(filter).one()
    except NoObjFound:
        pass # or 'continue'
    else:
        update_the_obj(obj)
        break
else:
    insert_into_db(brand_new_obj)

This will ensure that the correct things happen in the correct order, is more readable than nested try blocks and can easily be extended to add more filter s.

Answer 2

This is incorrect. Assuming you want to update_the_obj(obj) after finding the object independent of which filter found it, the else clause at the end is hidden to the inner filter.

You can try:

obj = None
try:
    obj = session.query(model).filter_by(field_a).one()
except NoObjFound:
    try:
        obj = session.query(model).filter_by(field_b).one()
    except NoObjFound:
        insert_into_db(brand_new_obj)

if obj is not None:
    update_the_obj(obj)