real & imag part

[buran]

(and the imag) so what differs between one method and the other is the return value, which for what I need, must exclusively return a (or b depending on the method).

yes and I gave you example for that :-) here it is again

class Complex():
    def __init__(self, real, imag):
        self.real = real
        self.imag = imag
         
c = Complex(1, 2)
print(c.real)
print(c.imag)

Output:
1
2
>>>

for the second part
You understand that your method will always return None, right? There is no explicit return, so by default it will return None.
Then read this https://docs.python.org/3.7/library/stdtypes.html#truth