Python - polymorphism in instance methods vs polymorphism in static methods

Question

What is the practical difference between instance methods and static methods, since these two codes return the same results? I understand that static methods do not operate on ‘self’ and do not have direct access to attributes, but how can I verify that instance methods are truly being overridden (are truly polymorphistic), while static methods are not?

class BaseClass:
    def instance_method(self):
        return "Instance method of base class"

    @staticmethod
    def static_method():
        return "Static method of base class"

class DerivedClass(BaseClass):
    def instance_method(self):
        return "Instance method of derived class"

    @staticmethod
    def static_method():
        return "Static method of derived class"

base_object = BaseClass()
derived_object = DerivedClass()

print(base_object.instance_method())  # Displays: "Instance method of base class"
print(derived_object.instance_method())  # Displays: "Instance method of derived class"

print(BaseClass.static_method())  # Displays: "Static method of base class"
print(DerivedClass.static_method())  # Displays: "Static method of derived class"

print(base_object.static_method())  # Displays: "Static method of base class"
print(derived_object.static_method())  # Displays: "Static method of derived class"

How can I check the truly polymorphism of instance methods? Maybe I understand wrong the whole concept of polymorphism?

Answer 1

Like any programming language, you just need to write code in that language to determine what its behaviours are:

class BaseClass:
    def instance_method(self):
        return "Instance method of base class"

    @staticmethod
    def static_method():
        return "Static method of base class"
    
    def normal_base_method(self):
        return self.static_method() + " called from normal_base_method"

class DerivedClass(BaseClass):
    def instance_method(self):
        return "Instance method of derived class"

    @staticmethod
    def static_method():
        return "Static method of derived class"

base_object = BaseClass()
derived_object = DerivedClass()

print(base_object.normal_base_method())
print(derived_object.normal_base_method())

Output:

Static method of base class called from normal_base_method
Static method of derived class called from normal_base_method

The above output shows that calling a method defined solely in the base class changes behaviour because the instance is of the derived class.

Calling derived_object.normal_base_method() really calls BaseClass.normal_base_method, but because self refers to derived_object, the static_method is chosen from DerivedClass.