How to Achieve Inheritance&Polymorphism of Static Methods in Python

The aim of this page📝 is to explain how static methods behave in presence of inheritance. In other words how to modify a static method for the different requirements of a derived/child class to get a polymorphic behavior. OOP stuff…

Pavol Kutaj

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Oct 25, 2022

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• first, let’s have a little class defining a shipping container
• it has a couple of instance attributes and 1 static method for id generation
• if needed, see the details on How to Define Static Methods in Python

""" PARENT """
class ShippingContainer:

    @staticmethod
    def _make_id():
        return "foo"

    def __init__(self, owner_id, contents):
        self.owner_id = owner_id
        self.contents = contents
        self.id = ShippingContainer._make_id()

• let’s introduce a subclass of a shipping container called refrigerated shipping containers
• and change the id for this particular (bar instead of foo)
• → I’ll define another static method in the class body

""" CHILD """
class RefrigeratedShippingContainer(ShippingContainer):
    @staticmethod
    def _make_id():
        return "bar"

• however, the id above is not working as expected, the id is still foo

>>> r = RefrigeratedShippingContainer(owner_id="MPR", contents=["books"])
>>> r.id
'foo'

• in the parent class initializer you call the static method through a specific class prefix
• => ShippingContainer._make_id() <=
• to get the intended polymorphic override in your child class, we need to call the static method on an instance
• note that testing directly calling the derived class returns also a correct result (bar)

>>> RefrigeratedShippingContainer._make_id(owner_id="MPR", contents=["books"])
'bar'
>>> r2 = RefrigeratedShippingContainer(owner_id="MPR", contents=["poems"])
>>> r._make_id()
'bar'

• again, the derived class is called
• we can get polymorphic override — but only when we call a method from the instance.
• not when we call the method through the class.

>>> r = RefrigeratedShippingContainer("MAE", ["fish"])
>>> r.id
'foo'

• if we are after the polymorphic dispatch of the static method, we need to modify the _make_id() in the base class

""" BEFORE """

def __init__(self, owner_id, contents):
        self.owner_id = owner_id 
        self.contents = contents
        self.bic = ShippingContainer._make_id()
            # # ...^^^^^^^^^^^^^^^^^^ incorrect

""" FIXED """

def __init__(self, owner_id, contents):
        self.owner_id = owner_id 
        self.contents = contents
        self.bic = self._make_id( )
            # .....^^^^ switched from class-prefix to instance-prefix ('self')

>>> r = RefrigeratedShippingContainer("MRP", ["books"])
>>> r.id
'bar'
    ^ SOLVED