Class variables, instance variables and inheritance

Class variables, instance variables and inheritance

class C2:
x = 2
z = 2
class C3:
w = 3
z = 3
def __init__(self):
self.w += 1
class C1(C2, C3):
x = 1
y = 1
I1 = C1()
I1.name = 'I1'
I2 = C1()
I2.name = 'I2'
print I1.w # 4
print I2.w # 4
print C3.w # 3
Can you explain me the results from last 3 prints? I can't find the logic
in this :)