#api.py
from classes.a import class_a
from classes.b import class_b

a = class_a()
a.test = '123'
b = class_b()


print(b.start())

#classes/a.py
class class_a:
	def do_sth(self, x):
		return self.test + 5

#classes/b.py
from classes.a import class_a as a
class class_b:
    def start(self):
    	return a().do_sth(5)