Differences between Class Attribute and Instance Attribute of Python

example6.py

class MyClass(object):
    data = []
 
    def __init__(self, value):
      self.instance_data = value
         
my_instance = MyClass(3)
my_instance.data
# []

my_instance.data.append(1)
my_instance.data
# [1]

MyClass.data
# [1]
# MyClass.data was also changed, because list is mutable.

my_instance.data = [1,2,3]
my_instance.data
# [1, 2, 3]
MyClass.data
# [1]
# MyClass.data didn't change at this time.
# Because the list was totally replaced by a new list 
# and my_instance.data became its instance attribute.