如何打印子类的属性？

Question

I am making a text based game and this is the file I am using to establishing the attributes of the rooms. Whenever I try to test this file out by printing the room description of room3 (print Room3.desc) I receive the error: AttributeError: type object 'Room3' has no attribute 'desc'

  class Room:
    def __init__(self, x, y, desc):
        self.x=x
        self.y=y
        self.desc=desc



class Room1(Room):
    def __init__(self, x, y, desc):
        super(Room1, self).__init__()
        self.x=0
        self.y=0
        self.desc="""

        A Metal Hallway
        This place is dimly lit and cool
        ----------------------------------------------------
        You stand in the middle of an intersection in a metal room.
        There are blue glowing lamps lighting this area up. There
        is a sign on the wall.
        ----------------------------------------------------
        Obvious exits:East, North"""

class Room2(Room):
    def __init__(self, x, y, desc):
        super(Room2, self).__init__()
        self.x=1
        self.y=0
        self.desc="""

        Thacker's Main Control Room
        This place is well lit and cool
        ----------------------------------------------------
        There are multiple panels throughout with a variety of levers and buttons.
        People stand in uniforms infront of computers, which are scattered throughout the room.
        There is a glass window here revealing space.
        Thacker is sitting here in the back of the room in a large chair.
        ----------------------------------------------------
        Obvious exits:West"""

class Room3(Room):
    def __init__(self, x, y, desc):
        super(Room3, self).__init__()
        self.x=0
        self.y=1
        self.desc== """


        Large Hanger Bay
        This place is well lit and cool
        ----------------------------------------------------
        There are a variety of mobile suits here
        ----------------------------------------------------
        Obvious exits:South"""


print("%s" % Room3.desc)

Answer 1

You're trying to access desc via reference to the class itself, but desc is defined in the constructor, which means it's only available in instances of the class.

This points to a general problem with your code: you're defining subclasses of Room, when what you really want is to create instances of the class Room:

room1 = Room(0, 0, """A Metal Hallway...")
room2 = Room(1, 0, """Thacker's main control room...")
etc...

Answer 2

Your __init__ methods define attributes of instances, not of classes. If you had something like this:

class Room3(Room):
  desc = 'foo'
  ...

Then there would be such a thing as Room3.desc . As it is, you would need something like this:

r = Room3(x, y, 'foo')
print(r.desc)

For more, study the tutorial, the section on Classes, at docs.python.org .

Answer 3

Room3.desc is an instance attribute rather than a class attribute. You can't access instance attributes on a class object - they don't exist there.

You aren't using an instance of Room3, but the class, so you can't access the attribute that doesn't exist. You can create a throwaway instance if you want to access the default instance attribute:

print Room3(None, None, None).desc

Also, your calls to your superclass init methods will fail in instance construction, because you aren't passing required arguments.

Answer 4

As cmd says, desc is not an attribute of the Room3 class object, it's an attribute of each Room3 instance .

I think the real problem here is that you don't want Room3 to be a subclass of Room ; you just want an instance of Room . There's no code or attributes specific to Room3 , and it's hard to imagine you're going to be creating a bunch of Room3 instances.

So what you probably want is this:

room3 = Room(1, 0, """blah blah""")

Now you can print room3.desc and it will work fine.

---

Meanwhile, if you do want Room3 to be a class, there are a lot of problems with your design:

def __init__(self, x, y, desc):
    super(Room3, self).__init__()
    self.x=0
    self.y=1
    self.desc== """…"""

First, that super call is going to raise an exception, because Room.__init__ takes x , y , and desc parameters, and you aren't passing them. You have to change it to super(Room3, self).__init__(x, y, desc) .

But once you fix that, the base class is going to set self.x , etc., just for you to immediately replace them with new values. Why do this? And why even take x , y , and desc parameters just to ignore them?

I think what you want is this:

def __init__(self):
    super(Room3, self).__init__(0, 1, """…""")

This is actually a pretty common pattern (although not as common as in languages like C++ and Java), where you have a base class that holds some variables that can take arbitrary values, and subclasses that have specific values for some of those variables.

But, as I said, I don't think it's the pattern you want here. I think you just want a room3 instance of Room , not a subclass.

---

Finally, to answer the question you directly asked, "How do you print an attribute of a subclass?", that's easy. Just print it. That's how duck typing works. For example:

class Base(object):
    def print_x(self):
        print self.x

class Derived(Base):
    def __init__(self, x):
        self.x = x

base = Derived(3)
base.print_x()

Answer 5

print("%s" % Room3.desc)

desc is an attribute of an instance of Room3, not of class Room3 itself. If you instanciate Room3, then you can access its attribute desc. Also be careful with your __init()__ as you have an extra =

class Room3(Room):
    def __init__(self, x, y, desc):
        super(Room3, self).__init__()
        self.x=0
        self.y=1
        self.desc== """

last line should be self.desc= """

Answer 6

Room3 class does not have the attribute desc . Instances of that class do. So something like this would work:

room3 = Room3()
print(room3.desc)

Answer 7

self.desc== """

There is an extra = . And you must instantiate Room3 to access the desc attribute.