Multiple inheritance in Python - error when invoking init

Question

I have a class D, this class inherits the classes B and C, which both inherit A, but when I created an instance of class D this error occurred:

johni@johni-pc:~/Draft$ python3.4 main.py 
Traceback (most recent call last):
  File "main.py", line 35, in <module>
    d = D()
  File "main.py", line 31, in __init__
    super().__init__()
  File "main.py", line 19, in __init__
    super().__init__("pg")
TypeError: __init__() takes 1 positional argument but 2 were given

I don't understand this.

The classes B and C are initializing the parameter driver in class A. My file main.py:

class A:   
    def __init__(self, driver):
        self.driver = driver

    @property
    def driver(self):
        return self.__driver

    @driver.setter
    def driver(self, valor):
        self.__driver = valor

    def run_driver(self):
        print("I am executing with %s" % self.driver)

class B(A):

    def __init__(self):
        super().__init__("pg")


class C(A):

    def __init__(self):
        super().__init__("conn_pg")


class D(B, C):

    def __init__(self):
        super().__init__()


d = D()
print(d.run_driver())

Why did this error occur?

Community · Accepted Answer · 2017-05-23 10:10:10Z

5

Note that super ensures that all superclass methods get called, according to the MRO (method resolution order - see e.g. How does Python's super() work with multiple inheritance?). D inherits from both B and C, so the following happens:

D() calls D.__init__
D.__init__ calls B.__init__
B.__init__ calls C.__init__ - here is the problem!
C.__init__ calls A.__init__ - we never get here

It is not clear why D inherits from both - you can't have two drivers, surely? If you do need to handle this, note that all __init__ methods should be written to take the same parameters, e.g. add:

def __init__(self, *args):
               # ^ ^ ^ ^ this part

to B and C.

edited May 23, 2017 at 10:10

CommunityBot

11 silver badge

answered Apr 14, 2015 at 10:53

jonrsharpe

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