Why dual init in python?

Question

I am investigating python oop style. I have seemed __init__ construction method as below. I did't see this style before. Why use dual __init__ methods as in this stuff?

ex-

class MinimumBalanceAccount(BankAccount):
    def __init__(self, minimum_balance):
        BankAccount.__init__(self)
        self.minimum_balance = minimum_balance

    def withdraw(self, amount):
        if self.balance - amount < self.minimum_balance:
            print 'Sorry, minimum balance must be maintained.'
        else:
            BankAccount.withdraw(self, amount)

This is just [Understanding Python super() with __init__() methods](http://stackoverflow.com/questions/576169/understanding-python-super-with-init-methods) in disguise. But not exactly a duplicate, since it's not obvious what's going on if the code doesn't say `super(...).__init__()`. — smci, Feb 16 '15 at 15:19

thiruvenkadam · Accepted Answer · 2017-04-12T08:49:19.463

This is an example of Class inheritance in Python. You have inherited the BankAccount Class to the MinimumBalanceAccount Class. However, by introducing an __init__ function in the MinimumBalanceAccount Class, you have overridden __init__ function of BankAccount Class. The base class might initialize some variables that are required for you. And hence it is called in the Child class' __init__ constructor to ensure that.

You can use super class to implement the same behavior. In Python 2.x, the equivalent will be

class MinimumBalanceAccount(BankAccount):
    def __init__(self, minimum_balance):
        self.minimum_balance = minimum_balance
        super(MinimumBalanceAccount, self).__init__()

Or in Python 3.x,

class MinimumBalanceAccount(BankAccount):
    def __init__(self, minimum_balance):
        super().__init__()

However, you must understand that this will just run whatever __init__ method it finds first from the base methods. So in terms of multiple inheritance, it would be difficult to call __init__ methods of various other classes, if super is not implemented in the base classes. So please avoid using multiple inheritance at all costs or implement super in all classes.

(eg)

class BankAccount(object):
    def __init__(self):
        # Some action here
        # But no super method called here

class MinimumBalanceAccount(BankAccount, LoanAccount):
    def __init__(self, minimum_value):
        super(MinimumBalanceAccount, self).__init__() # Calls BankAccount.__init__()
        super(MinimumBalanceAccount, self).__init__() # Still calls the same

If you still wish to go for Multiple Inheritance, best go with the ParentClass.__init__ approach or add the super method call to __init__ in all the base classes.

"However, you must understand that this will just run whatever init method it finds first from the base methods." => sorry but that's at least partly wrong. - as long as the parent classes `__init__` methods do a `super().__init__()` call themselves then all parent's initializers will be called in the right order (ie the `__mro__` order). — bruno desthuilliers, Apr 11 '17 at 10:06
Yes. If there is a super call in the parent class. Then with a single super statement, we will follow the entire resolution. However, if there is no super method in the base class, we could not get to the second `__init__` method other than calling by the class' name. I will update the post with more detail. — thiruvenkadam, Apr 12 '17 at 07:14

score 1 · Answer 2 · answered Feb 16 '15 at 15:13

1

The class MinimumBalanceAccount derives from the class BankAccount. Therefore, in the class' init function, it needs to call the init of the base class, which is done by BankAccount.__init__(self).

answered Feb 16 '15 at 15:13

Ludo

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6
16

score 1 · Answer 3 · edited May 23 '17 at 12:10

This is nothing more than calling ParentClass.__init__() in disguise, i.e.

super(ChildClass, self).__init__()

The preferred Python style is to explicitly use super, rather than hardcode in the name of the parent class.

(In Python 3.x: you can just say super().__init__() )

So this is really just Understanding Python super() with init() methods in disguise; here, ChildClass is MinimumBalanceAccount

Why dual __init__ in python?

3 Answers3

Why dual init in python?