Consider the following code, why don't I need to pass x to Y?

class X:    def __init__(self):self.a = 1self.b = 2self.c = 3class Y:        def A(self):print(x.a,x.b,x.c)x = X()
y = Y()
y.A()

Thank you to the top answers, they really helped me see what was the problem, namely misunderstanding regarding variable scope. I wish I could choose both as correct answer as they are enlightening in their own way.

When python compiles a def into a function, it tries to figure out if the things you are referencing are locals - and if they're not, you must be referring to a global. For example:

def f():print(x)

Well, you haven't defined x within the scope of f, so you must be referring to a global.

This is all that's happening in your above code. You haven't defined x within the scope of A, so x must be a global. As it happens, you define the global:

x = X()

before you call it:

y = Y()
y.A()

so everything works out okay.

In case you are going "hm, I'm not sure if I believe you, roippi" just look at the bytecode:

dis.dis(Y.A)3           0 LOAD_GLOBAL              0 (print) 3 LOAD_GLOBAL              1 (x) # aha6 LOAD_ATTR                2 (a) 9 LOAD_GLOBAL              1 (x) # aha!12 LOAD_ATTR                3 (b) 15 LOAD_GLOBAL              1 (x) # aha!!18 LOAD_ATTR                4 (c) 21 CALL_FUNCTION            3 (3 positional, 0 keyword pair) 24 POP_TOP              25 LOAD_CONST               0 (None) 28 RETURN_VALUE     

aha.