Conditional Inheritance in Python

It happens most of the time that given a condition we need to decide whether a particular class should inherit a class or not, for example given a person, if he/she is eligible for an admission in a university only then they should be a student otherwise they should not be a student.

Let’s consider an example, where given a condition, we want a class (say C) to dynamically inherit from either class A or class B. We need to create two different classes C_A and C_B, which inherit from A and B respectively, However if the conditional inheritance is to either inherit or not based on a condition then we can use a different approach as discussed below

Example 1: Conditional Inheritance between 2 classes:

Create two classes C_A and C_B, and based on the condition return the respective class object.

Python3

class A(object):
    def __init__(self, x):
        self.x = x

    def getX(self):
        return self.X

class B(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def getSum(self):
        return self.X + self.y

# inherits from A
class C_A(A):
    def isA(self):
        return True

    def isB(self):
        return False

# inherits from B
class C_B(B):
    def isA(self):
        return False

    def isB(self):
        return True

# return required Object of C based on cond
def getC(cond):
    if cond:
        return C_A(1)
    else:
        return C_B(1,2)

# Object of C_A
ca = getC(True)
print(ca.isA())
print(ca.isB())

# Object of C_B
cb = getC(False)
print(cb.isA())
print(cb.isB())

Output:

True
False
False
True

Example 2: For Either inheriting or not from A:

The approach is to use conditional statements while declaring the classes the given class C inherits. The below code executes and returns True

Python3

class A(object):
    def __init__(self, x):
        self.x = x

    def getX(self):
        return self.X

# Based on condition C inherits
# from A or it inherits from
# object i.e. does not inherit A
cond = True

# inherits from A or B
class C(A if cond else object):
    def isA(self):
        return True

# Object of C_A
ca = C(1)
print(ca.isA())

Output:

True

Example 3: The following code won’t run, as C does not inherit from A, thus has a default constructor that does not take any argument

Python3

class A(object):
    def __init__(self, x):
        self.x = x

    def getX(self):
        return self.X

# Based on condition C inherits from
# A or it inherits from object i.e.
# does not inherit A
cond = False

## inherits from A or B
class C(A if cond else object):
    def isA(self):
        return True

# Object of C_A
ca = C(1)
print(ca.isA())

Output:

TypeError                                 Traceback (most recent call last)
<ipython-input-16-f0efc5a814d9> in <module>
    17
    18 # Object of C_A
—> 19 ca = C(1)
    20 print(ca.isA())
    21

TypeError: object() takes no parameters