Question

I’m trying to emulate the following output (from Marakana.com’s excellent Python tutorial:

>>> for c in lot.cars_by_age():
...     print c
1981 VW Vanagon
1988 Buick Regal
2010 Audi R8

My code so far:

class ParkingLot(object):
    def __init__(self, spaces, cars=[]):
        self.spaces = spaces
        self.cars = cars
    def park(self, car):
        if self.spaces == 0:
            print "The lot is full."
        else:
            self.spaces -= 1
            self.cars.append(car)
    def __iter__(self):
        return (car for car in self.cars)

class Car(object):
    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year
    def __str__(self):
        return '%s %s %s' % (car.year, car.make, car.model)

I want to add a method (cars_by_age()) to the ParkingLot() class. However, this method somehow needs to be iterable, as per the sample code. I’m not sure how to do this – for a class, you define an iter function, but how do you do that for a method?

Answer 1

It’s not that the method is iterable; the value returned by the method is iterable. A cars_by_age implementation could simply return a list of Cars.

def cars_by_age(self):
    return sorted(self.cars, key=lambda car: car.year)

Use sorted to create a new sorted list, and use lambda to specify that you want to sort by the car’s year attribute.

http://wiki.python.org/moin/HowTo/Sorting