Define all class attributes inside the constructor

attribute-definition-placement-good.py

class X:
    def __init__(self):
        self.a = 'a'
        self.b = None
        self.c = None
    def run(self, b)
        self.b = b

x = X()
x.c = 'c'
x.run('b')
print(x.a, x.b, x.c)
#|a b c