How to use try, except, else correctly in Python

Question

So I want to know which is the right way to write try except statements. I'm new to error handling in Python.

Option 1

try:
    itemCode = items["itemCode"]
    dbObject=db.GqlQuery("SELECT * FROM %s WHERE code=:1" % dbName,itemCode).get()
    dbObject.delete() 
except AttributeError:
    print "There's no item with that code"
except KeyError:
    print "Bad parameter name"
except:
    print "Unknow error" 

Option 2

try:
    itemCode = items["itemCode"]
except KeyError:
    print "Bad parameter name"
else:    
    dbObject=db.GqlQuery("SELECT * FROM %s WHERE code=:1" % dbName,itemCode).get()
    try:    
        dbObject.delete() 
    except AttributeError:
        print "There's no item with that code"
    except:
        print "Unknow error" 

Option 3 Any other better option you can think of.

Option 1, we see that I wrap all the code in a try block. Option 2, it uses nested blocks. It raises an exception on specific line statements.

If there's an error somewhere I will be glad to know about it.

Answer 1

From the zen of python, "flat is better than nested." I'd go with Option #1 style in general, though I'm a bit confused as to whether dbObject=db.GqlQuery("SELECT.... or dbObject.delete() raises an AttributeError. In any case though, you shouldn't have to call the dbObject.delete() more than once.

Answer 2

Simple - it depends. If you are certain about what exceptions you're going to get, you can stick with 1) and frankly, it is the case 90% of the time. The second way is useful if you know that many code routines can raise the same exception type.

Answer 3

Instead of catching KeyError, I prefer using get method of dicts with validation.

itemCode = items.get("itemCode") # itemCode will be None, if no such key
if itemCode is not None:
    print "code missing"

This is not "general" solution (see comments to my answer), but in this case it'll help.

And in first case I don't understand, why you trying to delete two times.

try:
    itemCode = items.get("itemCode") # itemCode will be None, if no such key
    if itemCode is not None:
        print "code missing"
    dbObject=db.GqlQuery("SELECT * FROM %s WHERE code=:1" % dbName,itemCode).get()
    dbObject.delete() 
except AttributeError:
    print "There's no item with that code"
except StandardError as ex:    # good idea to be prepared to handle various fails
    print "Unexpected error deleting item {}".format(ex)

And also, don't forget that python have finally section. It's oftem comes in handy sometimes.