Object has no attribute. Why?

Question

I have this simple Python 3 code.
I define 3 classes: class Test2 inherited class Test1 and class Test3 inherited Test2.
When I create an object for class Test3 and try to print it with the attribute "doi" from Class Test3, it error's out: the object has no attribute "doi"
Why?

class Test1:
def metoda1(self):
self.unu='unu'
class Test2(Test1):
def metoda2(self):
self.unu='doi'
class Test3(Test2):
def metoda3(self):
self.doi='doi'
obj=Test3()
print(obj.doi)

Answer 1

you have to call the method otherwise you need to declare it out of the method

Answer 2

class Test1:
def metoda1(self):
self.unu='unu'
class Test2(Test1):
def metoda2(self):
self.unu='doi'
class Test3(Test2):
def metoda3(self):
self.doi='doi'

obj=Test3()
obj.metoda3()
print(obj.doi)

Answer 3

The Great Saga: thank you very much!