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Item 40: Initialize Parent Classes with super

The old, simple way to initialize a parent class from a child class is to directly call the parent class’s __init__ method with the child instance:

class MyBaseClass:
    def __init__(self, value):
        self.value = value
class MyChildClass(MyBaseClass):
    def __init__(self):
        MyBaseClass.__init__(self, 5)

This approach works fine for basic class hierarchies but breaks in many cases.

If a class is affected by multiple inheritance (something to avoid in general; see Item 41: “Consider Composing Functionality with Mix-in Classes”), calling the superclasses’ __init__ methods directly can lead to unpredictable behavior.

One problem is that the __init__ call order isn’t specified across all subclasses. For example, here I define two parent classes that operate on the instance’s value field:

class TimesTwo:
    def __init__(self):
        self.value *= 2

class PlusFive:
    def __init__(self):
        self.value += 5

This class defines its parent classes in one ordering:

class OneWay(MyBaseClass, TimesTwo, PlusFive):
    def __init__(self, value):
        MyBaseClass.__init__(self, value)

And constructing it produces a result that matches the parent class ordering:

foo = OneWay(5)
print('First ordering value is (5 * 2) + 5 =', foo.value)

First ordering value is (5 * 2) + 5 = 15

Here’s another class that defines the same parent classes but in a different ordering (PlusFive followed by TimesTwo instead of the other way around):

class AnotherWay(MyBaseClass, PlusFive, TimesTwo):
    def __init__(self, value):
        MyBaseClass.__init__(self, value)

However, I left the calls to the parent class constructors— PlusFive.__init__ and TimesTwo.__init__—in the same order as before, which means this class’s behavior doesn’t match the order of the parent classes in its definition. The conflict here between the inheritance base classes and the __init__ calls is hard to spot, which makes this especially difficult for new readers of the code to understand:

bar = AnotherWay(5)
print('Second ordering value is', bar.value)

Second ordering value is 15

Another problem occurs with diamond inheritance. Diamond inheritance happens when a subclass inherits from two separate classes that have the same superclass somewhere in the hierarchy. Diamond inheritance causes the common superclass’s __init__ method to run multiple times, causing unexpected behavior. For example, here I define two child classes that inherit from MyBaseClass:

class TimesSeven(MyBaseClass):
    def __init__(self, value):
        MyBaseClass.__init__(self, value)
        self.value *= 7

class PlusNine(MyBaseClass):
    def __init__(self, value):
        MyBaseClass.__init__(self, value)
        self.value += 9

Then, I define a child class that inherits from both of these classes, making MyBaseClass the top of the diamond:

class ThisWay(TimesSeven, PlusNine):
    def __init__(self, value):
        TimesSeven.__init__(self, value)
        PlusNine.__init__(self, value)

foo = ThisWay(5)
print('Should be (5 * 7) + 9 = 44 but is', foo.value)

Should be (5 * 7) + 9 = 44 but is 14

The call to the second parent class’s constructor, PlusNine.__init__, causes self.value to be reset back to 5 when MyBaseClass.__init__ gets called a second time. That results in the calculation of self.value to be 5 + 9 = 14, completely ignoring the effect of the TimesSeven.__init__ constructor. This behavior is surprising and can be very difficult to debug in more complex cases.

To solve these problems, Python has the super built-in function and standard method resolution order (MRO). super ensures that common superclasses in diamond hierarchies are run only once (for another example, see Item 48: “Validate Subclasses with __init_subclass__”). The MRO defines the ordering in which superclasses are initialized, following an algorithm called C3 linearization.

Here, I create a diamond-shaped class hierarchy again, but this time I use super to initialize the parent class:

class TimesSevenCorrect(MyBaseClass):
    def __init__(self, value):
        self.value *= 7

class PlusNineCorrect(MyBaseClass):
    def __init__(self, value):
        self.value += 9

Now, the top part of the diamond, MyBaseClass.__init__, is run only a single time. The other parent classes are run in the order specified in the class statement:

class GoodWay(TimesSevenCorrect, PlusNineCorrect):
    def __init__(self, value):

foo = GoodWay(5)
print('Should be 7 * (5 + 9) = 98 and is', foo.value)

Should be 7 * (5 + 9) = 98 and is 98

This order may seem backward at first. Shouldn’t TimesSevenCorrect.__init__ have run first? Shouldn’t the result be (5 * 7) + 9 = 44? The answer is no. This ordering matches what the MRO defines for this class. The MRO ordering is available on a class method called mro:

mro_str = '\n'.join(repr(cls) for cls in GoodWay.mro())

<class '__main__.GoodWay'>
<class '__main__.TimesSevenCorrect'>
<class '__main__.PlusNineCorrect'>
<class '__main__.MyBaseClass'>
<class 'object'>

When I call GoodWay(5), it in turn calls TimesSevenCorrect.__init__, which calls PlusNineCorrect.__init__, which calls MyBaseClass.__ init__. Once this reaches the top of the diamond, all of the initialization methods actually do their work in the opposite order from how their __init__ functions were called. MyBaseClass.__init__ assigns value to 5. PlusNineCorrect.__init__ adds 9 to make value equal 14. TimesSevenCorrect.__init__ multiplies it by 7 to make value equal 98.

Besides making multiple inheritance robust, the call to super(). __init__ is also much more maintainable than calling MyBaseClass.__init__ directly from within the subclasses. I could later rename MyBaseClass to something else or have TimesSevenCorrect and PlusNineCorrect inherit from another superclass without having to update their __init__ methods to match.

The super function can also be called with two parameters: first the type of the class whose MRO parent view you’re trying to access, and then the instance on which to access that view. Using these optional parameters within the constructor looks like this:

class ExplicitTrisect(MyBaseClass):
    def __init__(self, value):
        super(ExplicitTrisect, self).__init__(value)
        self.value /= 3

However, these parameters are not required for object instance initialization. Python’s compiler automatically provides the correct parameters (__class__ and self) for you when super is called with zero arguments within a class definition. This means all three of these usages are equivalent:

class AutomaticTrisect(MyBaseClass):
    def __init__(self, value):
        super(__class__, self).__init__(value)
        self.value /= 3

class ImplicitTrisect(MyBaseClass):
    def __init__(self, value):
        self.value /= 3

assert ExplicitTrisect(9).value == 3
assert AutomaticTrisect(9).value == 3
assert ImplicitTrisect(9).value == 3

The only time you should provide parameters to super is in situations where you need to access the specific functionality of a superclass’s implementation from a child class (e.g., to wrap or reuse functionality).

Things to Remember

  • Python’s standard method resolution order (MRO) solves the problems of superclass initialization order and diamond inheritance.

  • Use the super built-in function with zero arguments to initialize parent classes.

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