Sorting objects in Python

Question

I want to sort objects using by one of their attributes. As of now, I am doing it in the following way

USpeople.sort(key=lambda person: person.utility[chosenCar],reverse=True)

This works fine, but I have read that using operator.attrgetter() might be a faster way to achieve this sort. First, is this correct? Assuming that it is correct, how do I use operator.attrgetter() to achieve this sort?

I tried,

 keyFunc=operator.attrgetter('utility[chosenCar]')
 USpeople.sort(key=keyFunc,reverse=True)

However, I get an error saying that there is no attribute 'utility[chosenCar]'.

The problem is that the attribute by which I want to sort is in a dictionary. For example, the utility attribute is in the following form:

utility={chosenCar:25000,anotherCar:24000,yetAnotherCar:24500}

I want to sort by the utility of the chosenCar using operator.attrgetter(). How could I do this?

Thanks in advance.

Answer 1

No, attrgetter will not be any faster than the lambda - it's really just another way of doing the same thing.

You may have been confused by a recommendation to use key instead of cmp, which is indeed significantly faster, but you're already doing that.

Answer 2

to access chosenCar item you'd have to use:

>>> P.utility={'chosenCar':25000,'anotherCar':24000,'yetAnotherCar':24500}
>>> operator.itemgetter('chosenCar')(operator.attrgetter('utility')(P))
25000

for the key function you'll have to do the following:

>>> def keyfunc(P):
    util = operator.attrgetter('utility')(P)
    return operator.itemgetter('chosenCar')(util)

>>> USpeople.sort(key=keyfunc,reverse=True)

However, your main claim re the better performance of this approach seems poorly researched. I'd suggest to use timeit module to test performance of both approaches for your own data.