[Python] Confused about self.var vs self.__class__.var [duplicate]

I played with some code below:

class MyClass:
s_occ = 42
def __init__(self):
print( self.s_occ ) # print out 1
self.s_occ += 1
print(self.__class__.s_occ) # print out 1

cls1 = MyClass()


Given that self.s_occ += 1 does not change self.__class__.s_occ, it seems they are not aliased.

However, I didn't define self.s_occ, shouldn't it hence be an undefined symbol? Why it seems to be a symbol that has same value as self.__class__.s_occ?

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