Item 25: Initialize Parent Classes with super
The old 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(object): def __init__(self, value): self.value = value class MyChildClass(MyBaseClass): def __init__(self): MyBaseClass.__init__(self, 5)
This approach works fine for simple hierarchies but breaks down in many cases.
If your class is affected by multiple inheritance (something to avoid in general; see Item 26: “Use Multiple Inheritance Only for Mix-in Utility 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(object): def __init__(self): self.value *= 2 class PlusFive(object): 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) TimesTwo.__init__(self) PlusFive.__init__(self)
And constructing it produces a result that matches the parent class ordering.
foo = OneWay(5) print(‘First ordering is (5 * 2) + 5 =’, foo.value) >>> First ordering is (5 * 2) + 5 = 15
Here’s another class that defines the same parent classes but in a different ordering:
class AnotherWay(MyBaseClass, PlusFive, TimesTwo): def __init__(self, value): MyBaseClass.__init__(self, value) TimesTwo.__init__(self) PlusFive.__init__(self)
However, I left the calls to the parent class constructors PlusFive.init and TimesTwo.init in the same order as before, causing this class’s behavior not to match the order of the parent classes in its definition.
bar = AnotherWay(5) print(‘Second ordering still is’, bar.value) >>> Second ordering still 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 TimesFive(MyBaseClass): def __init__(self, value): MyBaseClass.__init__(self, value) self.value *= 5 class PlusTwo(MyBaseClass): def __init__(self, value): MyBaseClass.__init__(self, value) self.value += 2
Then, I define a child class that inherits from both of these classes, making MyBaseClass the top of the diamond.
class ThisWay(TimesFive, PlusTwo): def __init__(self, value): TimesFive.__init__(self, value) PlusTwo.__init__(self, value) foo = ThisWay(5) print(‘Should be (5 * 5) + 2 = 27 but is’, foo.value) >>> Should be (5 * 5) + 2 = 27 but is 7
The output should be 27 because (5 * 5) + 2 = 27. But the call to the second parent class’s constructor, PlusTwo.init, causes self.value to be reset back to 5 when MyBaseClass.init gets called a second time.
To solve these problems, Python 2.2 added the super built-in function and defined the method resolution order (MRO). The MRO standardizes which superclasses are initialized before others (e.g., depth-first, left-to-right). It also ensures that common superclasses in diamond hierarchies are only run once.
Here, I create a diamond-shaped class hierarchy again, but this time I use super (in the Python 2 style) to initialize the parent class:
# Python 2 class TimesFiveCorrect(MyBaseClass): def __init__(self, value): super(TimesFiveCorrect, self).__init__(value) self.value *= 5 class PlusTwoCorrect(MyBaseClass): def __init__(self, value): super(PlusTwoCorrect, self).__init__(value) self.value += 2
Now the top part of the diamond, MyBaseClass.init, is only run a single time. The other parent classes are run in the order specified in the class statement.
# Python 2 class GoodWay(TimesFiveCorrect, PlusTwoCorrect): def __init__(self, value): super(GoodWay, self).__init__(value) foo = GoodWay(5) print ‘Should be 5 * (5 + 2) = 35 and is’, foo.value >>> Should be 5 * (5 + 2) = 35 and is 35
This order may seem backwards at first. Shouldn’t TimesFiveCorrect.init have run first? Shouldn’t the result be (5 * 5) + 2 = 27? 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.
from pprint import pprint pprint(GoodWay.mro()) >>> [<class ‘__main__.GoodWay’>, <class ‘__main__.TimesFiveCorrect’>, <class ‘__main__.PlusTwoCorrect’>, <class ‘__main__.MyBaseClass’>, <class ‘object’>]
When I call GoodWay(5), it in turn calls TimesFiveCorrect.init, which calls PlusTwoCorrect.init, which calls MyBaseClass.init. Once this reaches the top of the diamond, then all of the initialization methods actually do their work in the opposite order from how their init functions were called. MyBaseClass.init assigns the value to 5. PlusTwoCorrect.init adds 2 to make value equal 7. TimesFiveCorrect.init multiplies it by 5 to make value equal 35.
The super built-in function works well, but it still has two noticeable problems in Python 2:
Its syntax is a bit verbose. You have to specify the class you’re in, the self object, the method name (usually init), and all the arguments. This construction can be confusing to new Python programmers.
You have to specify the current class by name in the call to super. If you ever change the class’s name—a very common activity when improving a class hierarchy —you also need to update every call to super.
Thankfully, Python 3 fixes these issues by making calls to super with no arguments equivalent to calling super with class and self specified. In Python 3, you should always use super because it’s clear, concise, and always does the right thing.
class Explicit(MyBaseClass): def __init__(self, value): super(__class__, self).__init__(value * 2) class Implicit(MyBaseClass): def __init__(self, value): super().__init__(value * 2) assert Explicit(10).value == Implicit(10).value
This works because Python 3 lets you reliably reference the current class in methods using the class variable. This doesn’t work in Python 2 because class isn’t defined. You may guess that you could use self.class as an argument to super, but this breaks because of the way super is implemented in Python 2.
Things to Remember
Python’s standard method resolution order (MRO) solves the problems of superclass initialization order and diamond inheritance.
Always use the super built-in function to initialize parent classes.