def f():
    import sys
    locs = sys._getframe(1).f_locals
    setattr(locs['self'], 'xy', 4*locs['x'] + 7*locs['y'])

class A:
    def __init__(self, x, y):
        self.xy = 2*x + 3*y
        f()

a = A(2, 3)
A.xy = 99
print(a.xy)