python class var will share between obj?

Question

class cls():
    A = []

a = cls()
b = cls()
a.A.append("a")
print(b.A)

I run this on python 3.4. but i see it print a stting "a".

I cant understand why this happen. a is different from b, but why they are share one var A?

Answer 1

You've assigned a variable to the class. It only exists once for all instances (as long as you use it by reference, which .append does).

What you really want is an instance variable:

class cls():
    def __init__(self):
        self.A = []

a = cls()
b = cls()
a.A.append("a")
print(b.A)

Also, cls should not be used as the name of a class - use CamelCase names, and additionally cls should only be used with metaclasses or classmethods (you are unlikely to need either of these).