Python - How to sort a list, by an attribute from another class

Question

I have some elements in a list in one class. I want them sorted in a new list, and they have to be sorted by an attribute from another class.

Can anyone give an exa

Answer 1

You say that you have classes already (show them!) so you can make them sortable by defining __lt__:

class Car(object):
    def __init__(self, year, plate):
        self.year = year
        self.plate = plate

    # natural sort for cars by year:
    def __lt__(self, other):
        return self.year < other.year

    def __repr__(self):
        return "Car (%d) %r" % (self.year, self.plate)

class Plate(object):
    def __init__(self, val):
        self.val = val

    def __repr__(self):
        return repr(self.val)

    # natural sort by val:
    def __lt__(self, other):
        return self.val < other.val

cars = [ Car(2009, Plate('A')),
              Car(2007, Plate('B')),
              Car(2006, Plate('C'))
]

print cars
print sorted(cars) # sort cars by their year
print sorted(cars, key=lambda car: car.plate) # sort by plate

Answer 2

To sort a list of objects that have the attribute bar:

anewlist = sorted(list, key=lambda x: x.bar)