How to copy instances of a class?

Question

So far I've done programming in C, C++, and Java. Python is totally new so this problem seems kind of awkward to me. I created two instances box1 (class Rectangle) and box2 (also class Rectangle). The member p which is an instance of the class Point inside the class Rectangle seems to be shared between both instances (box1 and box2). How to make the instances completely independent?

import copy

class Point:
    x=0
    y=0

class Rectangle:
    width=0
    height=0
    p=Point()

box1=Rectangle()

box1.width=5
box1.height=6
box1.p.x=2
box1.p.y=3

box2=Rectangle()

print(box1 is box2)

print(box1.p is box2.p)

Output:

False True

That's why I remembered to mention that on my first line. I'm actually reading "Think Python" by Allen. — Minhaj Ansari, Aug 04 '18 at 13:32

score 1 · Accepted Answer · answered Aug 04 '18 at 13:00

1

This is why in python you instantiate classes with the __init__(self) method.

If you change your code to

class Point:
    def __init__(self):
        self.x = 0
        self.y = 0
class Rectangle:
     def __init__(self):
        self.width = 0
        self.height = 0
        self.p = Point()
box1=Rectangle()

box1.width=5
box1.height=6
box1.p.x=2
box1.p.y=3

box2=Rectangle()

print(box1 is box2) # output is false

print(box1.p is box2.p) # output is false

answered Aug 04 '18 at 13:00

Jonathan R

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Seems to be working now! Could you please tell me why we use "def __init__(self)"? – Minhaj Ansari Aug 04 '18 at 13:38
Look at this link https://stackoverflow.com/questions/1680528/how-to-avoid-having-class-data-shared-among-instances – Jonathan R Aug 04 '18 at 14:04
Just looked into that, thanks for the help :) – Minhaj Ansari Aug 04 '18 at 14:12

How to copy instances of a class?

1 Answers1