difference between class and instance attributes

Question

what is the difference between the following codes?

1. class A(object):
    foo = 3   #set default value

2. class B(object):
    def __init__(self, foo=3):
        self.foo = foo

How will it affect the performance or space requirements while creating a lot of instances?

Answer 1

In Python, there is no need to compare to True or False. In the following code, the opposite of dest in smaller.keys() is (dest not in smaller.keys())

So it must be written as:

if(dest not in smaller.keys()):

Answer 2

Apart from the performance, there is a significant semantic difference between the two codes. only one object is referred to in case of class attribute whereas there can be multiple objects referred to in case of instance-attribute-set-at-instantiation.

For example :

1.   class A: foo = [] 

     a, b = A(), A() 

     a.foo.append(3) 

      b.foo

2.  class A: 

     def __init__(self): self.foo = [] 

     a, b = A(), A() a.foo.append(3) 

      b.foo