Object appended to a list instance appears in a different instance of that list

Question

I was writing this little piece of code as an exercise in object-oriented programming.

Here I'm trying to define a house as a list of rooms and each room as a list of devices (lamps, for example).

First I created all the objects and them appended the two rooms to the house and a different device to each room. Pretty basic.

The problem is that it seems that the device is being appended to both rooms. Why is that?

The code:

#! /usr/bin/python

class House:
        def __init__(self, rooms = list()):
                self.rooms = rooms
                print('house created')


class Room:
        def __init__(self, name = 'a room', devs = list()):
            self.name = name
            self.devs = devs
            print('room ' + self.name + ' created')


class Device:
        def __init__(self, name = 'a device'):
                self.name = name
                print('device ' + self.name + ' created')


def main():
        #1
        h = House()
        r1 = Room(name = 'R1')
        r2 = Room(name = 'R2')
        d1 = Device(name = 'lamp1')
        d2 = Device(name = 'lamp2')

        #2
        h.rooms.append(r1)
        h.rooms.append(r2)

        for room in  h.rooms:
                print room.name

        print h.rooms[0]
        print h.rooms[1]
        h.rooms[1].devs.append(d1)

        #3
        for room in h.rooms:
                print room.name
                for dev in room.devs:
                        print('room ' + room.name + ' > ' + dev.name)
                        print room
                        print dev


if __name__ == '__main__' : main()

And the output.

house created
room R1 created
room R2 created
device lamp1 created
device lamp2 created
R1
R2
<__main__.Room instance at 0xb7d8a58c>
<__main__.Room instance at 0xb7d8a5ac>
R1
room R1 > lamp1
<__main__.Room instance at 0xb7d8a58c>
<__main__.Device instance at 0xb7d8a5cc>
R2
room R2 > lamp1
<__main__.Room instance at 0xb7d8a5ac>
<__main__.Device instance at 0xb7d8a5cc>

Note that the same instance of d1 is in both rooms, r1 and r2.

Answer 1

Default parameter values for functions are evaluated only once. This means that all instances of House will use the same list instance for self.rooms (if rooms parameter wasn't given in construction). In the same manner, all instances of Room will share the same list for self.devs.

To solve this, write the code like this:

def __init__(self, rooms = None):
    if rooms is None:
        rooms = []
    self.rooms = rooms
    print('house created')

And the same thing for the other classes.