Python: "self" in inheritance

Question

A basic question on inheritance and "self" here.

Good code with no error: (Correction: Turns out this is NOT GOOD, either. Please refer to MisterMiyagi's answers below.)

class A:
  def __init__(self, a):
    self.a = 10
    self.b = { 'a': 10, 'b': 20 }
    self.c = [ 1, 2, 3 ]

class B(A):
  def __init__(self):
    super().__init__(self)
    print(self.a)
    print(self.b)
    print(self.c)

i = B()

Bad code with errors:

class A:
  def __init__(self, a, b, c):
    self.a = 10
    self.b = { 'a': 10, 'b': 20 }
    self.c = [ 1, 2, 3 ]

class B(A):
  def __init__(self):
    super().__init__(self)
    print(self.a)
    print(self.b)
    print(self.c)

i = B()

ERROR:

Traceback (most recent call last):
  File "./o", line 16, in <module>
    i = B()
  File "./o", line 11, in __init__
    super().__init__(self)
TypeError: __init__() missing 2 required positional arguments: 'b' and 'c'

Could you please explain why it has to be:

class A:
  def __init__(self, a):

but neither

class A:
  def __init__(self):

nor

class A:
  def __init__(self, a, b, c):

?

Thank you!

Note that super().__init__(self) is not correct, unless the intention is to bind a=self. super().__init__() should be used, and A.__init__ should not take any arguments aside from self. — MisterMiyagi
– MisterMiyagi, Commented May 29, 2020 at 17:48
I don't understand the question. You are not using any of the arguments in the A()'s constructor, so why not either make them optional, or not pass them at all? — cs95
– cs95, Commented May 29, 2020 at 17:48
Your super call is busted - it should be super().__init__(), and A.__init__ shouldn't take an a argument. — user2357112
– user2357112, Commented May 29, 2020 at 17:48
Since def __init__(self, a) versus def __init__(self) is a typo/error, can you clarify why you would expect def __init__(self, a, b, c) to be correct? You are not providing b or c anywhere. — MisterMiyagi
– MisterMiyagi, Commented May 29, 2020 at 17:51
I think MisterMiyagi and Monica gave the direct answer. When I used super().__init__() instead, everything's charm now. Thank you! — Steve
– Steve, Commented May 29, 2020 at 17:56

Samwise · Accepted Answer · 2020-05-29 17:51:39Z

2

A's constructor takes 3 arguments, and you're giving it 1. That's the reason for the error.

Since it doesn't use any of the 3 arguments, it should take 0 arguments, and B in turn should give it 0 arguments:

class A:
  def __init__(self):
    self.a = 10
    self.b = { 'a': 10, 'b': 20 }
    self.c = [ 1, 2, 3 ]

class B(A):
  def __init__(self):
    super().__init__()
    print(self.a)
    print(self.b)
    print(self.c)

i = B()

answered May 29, 2020 at 17:51

Samwise

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Comments

MisterMiyagi · Accepted Answer · 2020-05-29 18:11:10Z

1

Calling a method on super() already passes on the first argument (self or cls) properly. Do not pass along self explicitly.

class A:
  def __init__(self):  # receive implicit self
    self.a = 10
    self.b = { 'a': 10, 'b': 20 }
    self.c = [ 1, 2, 3 ]

class B(A):
  def __init__(self):
    super().__init__()  # pass self implicitly
    print(self.a)
    print(self.b)
    print(self.c)

i = B()

A call such as super().__init__(self) passes self to the method twice. Thus, the method must take two parameters, e.g. def __init__(self, a). Use just super().__init__() and def __init__(self) to pass along self once instead.

edited May 29, 2020 at 18:11

answered May 29, 2020 at 18:00

MisterMiyagi

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Comments

MCSH · Accepted Answer · 2020-05-29 17:53:09Z

1

When you call a method on an object, the object will be assigned to the self argument of the function, for example:

class A:
  def someFunc(self):
    pass

a = A()
a.someFunc() # self == a

When you are calling the super function, your own class will be assigned to self, you should be calling super like this:

class A:
  def __init__(self, a):
    sef.a = a

class B(A):
  def __init__(self):
    a = # something
    super(self).__init__(a)
    print(self.a)

i = B()

The problem with your code is that you are assigning a to self, and then not providing a value for b and c.

answered May 29, 2020 at 17:53

MCSH

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